Jian Han

Shixue Hu

Paulyn Cartwright

Fangchen Zhao

Qiang Ou

Shin Kubota

Xing Wang

Xiaoguang Yang

J I A N H A N

Ja M Yao P I N G C A I * †

S D S C H I F F B Au E R ‡, H O N G H Ua * , X I N G Wa N G * , X I Ao G Ua N G Ya N G

S U Yo

I Ko M I Ya ¶ & J I E S U N

Guoxiang Li

Junfeng Guo
email: 〈junfengg@chd.edu.cn〉

Osamu Sasaki
email: 〈sasaki@museum.tohoku.ac.jp〉

Tsuyoshi Komiya
email: 〈komiya@ea.c.u-tokyo.ac.jp〉

The earliest pelagic jellyfish with rhopalia from Cambrian Chengjiang Lagerstätte

Keywords: Pelagic Medusozoa Cambrian Chengjiang Medusa Rhopalium Cloudina, Sinotubulites, asexual reproduction, early Cambrian, Kuanchuanpu Formation early Cambrian carinachitids (Medusozoa, Cnidaria) Cnidaria, Cubozoa, Early Cambrian, embryo, Kuanchuanpu Formation, periderm

, originally described as a ctenophore, as a pelagic, predatory, crown-group medusozoan, based on the presence of rhopalia, possible radial canals and marginal tentacles. The medusae were a predatory member of the pelagic food web at the middle level of the ocean at Cambrian Stage 3.

Introduction

Cnidarian medusae are a conspicuous part of the modern marine realm. They typically exhibit a triphasic life cycle consisting of a planula larva, a sessile polyp stage and a pelagic, sexual medusa stage with sense organs, i.e. rhopalia. The rhopalia, which are in charginge of rhythmic body contraction and swimming [START_REF] Ruppert | Invertebrate Zoology: A Functional Evolutionary Approach[END_REF], are a suite of club-shaped structures suited between a pair of marginal lappets, each containing a concentration of epidermal neurons, a pair of chemosensory pits, a statocyst, and often an ocelli. Given their different modes of development, it has been hypothesized that the medusoid form in different classes of Medusozoa might have been independently derived from a polyp-like form [START_REF] Kraus | Adoption of conserved developmental genes in development and origin of the medusa body plan[END_REF][START_REF] Salvini-Plawen | On the origin and evolution of the Lower Metazoa[END_REF]. However, it is unclear when the pelagic forms arose. Molecular evidence suggests that cnidarian medusozoans originated deep within the Neoproterozoic [START_REF] Park | Estimation of divergence times in cnidarian evolution based on mitochondrial protein-coding genes and the fossil record[END_REF]. The fossil record of medusae is rather sporadic. The soft-bodied fossil Haootia quadriformis Liu et al. (2014a) with possible muscular impressions reported from the lower Fermeuse Formation of the Bonavista Peninsula of Newfoundland (approx. 560 Ma) was compared to the sessile stauromedusans, although it lacks secondary tentacles, anchors, gonads and nematocyst clusters. Many exceptionally preserved stem-group medusozoans with a diverse set of internal anatomical structures arranged in tetra-or penta-radial symmetry were found in the basal Cambrian Fortunian Stage (ca. 535 Ma) in south China (Dong et al., 2013;Han et al., 2016;Han et al., 2013); however, most of them remain at the embryonic stage and some of their polypoid forms were contained inside a tubular peridermal theca (see Steiner et al., 2014). Even if some of them have developed strong coronal muscles (Han et al., 2016), all of them, including the putative ephyra (Dong et al., 2013), lack the rhopalia necessary for pelagic life, thus calling into question whether any of these fossils represent true pelagic medusae. Hitherto, representatives of most major lineages of nektonic medusozoans with umbrellar shape, gonads, coronal muscles and elongate retractile tentacles possibly with nematocyst batteries were present in Cambrian Stage 5 in Utah [START_REF] Cartwright | Exceptionally preserved jellyfishes from the Middle Cambrian[END_REF]. In the Chengjiang fauna, pelagic forms including both diploblastic ctenophores [START_REF] Ou | A vanished history of skeletonization in Cambrian comb jellies[END_REF] and various bilaterians have been well documented in Cambrian Stage 3 [START_REF] Hu | Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian[END_REF]Vannier et al., 2009;[START_REF] Zhao | Community structure and composition of the Cambrian Chengjiang biota[END_REF]. Although some discoid Chengjiang fossils (i.e. Rotadiscus grandis, Stellostomites eumorphus, Heliomedusa orienta) were previously reported as medusae [START_REF] Sun | Early Cambrian medusae from Chengjiang, Yunnan, China[END_REF], they were later reinterpreted as organisms closely related to either lophophorates [START_REF] Dzik | Is Fossil Evidence Consistent with Traditional Views of the Early Metazoan Phylogeny[END_REF][START_REF] Zhang | Architecture and function of the lophophore in the problematic brachiopod Heliomedusa orienta (Early Cambrian, South China)[END_REF][START_REF] Zhu | Revision of the Cambrian discoidal animals Stellostomites eumorphus and Pararotadiscus guizhouensis from South China[END_REF] or stem-group deuterostomes [START_REF] Caron | Tentaculate fossils from the Cambrian of Canada (British Columbia) and China (Yunnan) interpreted as primitive deuterostomes[END_REF]. Here, we reinterpret Yunnanoascus haikouensis [START_REF] Hu | Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian[END_REF] as a tetraradial pelagic medusozoan, rather than a ctenophore, on the basis of medusozoan-like diagnostic features including 16 pairs of thick-based marginal tentacles, 16 rhopalia, 16 marginal lappets and a manubrium. The interpretation of this fossil as a crown-group medusozoan during this vital interval (520 Ma) suggests that crown-group pelagic medusa were present as far back as the early Cambrian.

Materials and methods

The holotype (No. , the unique specimen of Y. haikouensis, is deposited in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Photographs were taken using a Canon 5D Mark II camera. Backscattered electron microscopy (BSE) and energy-dispersive spectroscopy (EDS) analysis without coating were conducted on a HITACHI SU3500 scanning electron microscope with an EDS system. A cladistic analysis containing 25 taxa and 105 characters (Appendix 1) was analyzed in both PAUP* 4.0 b10 [START_REF] Swofford | PAUP*. Phylogenetic Analysis Using Parsimony (and Other Methods)[END_REF] and TNT 1.1 [START_REF] Goloboff | TNT, a free program for phylogenetic analysis[END_REF]. The data matrix (Appendix 2) derives in large part from the data in Han et al. (2016); [START_REF] Marques | Cladistic analysis of Medusozoa and cnidarian evolution[END_REF] and Van Iten et al. (2006). All 105 characters have equal weight, 8 of them are constant, 24 variable characters are parsimony-uninformative and the remaining 73 characters are parsimony-informative.

Result

Systematic palaeontology Phylum Cnidaria Verrill, 1865. Subphylum Medusozoa Peterson, 1979. Class Scyphozoa Goette, 1887. Order, Family uncertain. Genus Yunnanoascus. Type species: Y. haikouensis [START_REF] Hu | Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian[END_REF]. Revised diagnosis: Body with tetra-radial symmetry. Hemispherical central disc surrounding a wider coronal sinus. Bell margin with a ring of sixteen pairs of long thick-based tentacles with possible nematocyst batteries. Sixteen rhopalia slightly away from scalloped bell margin. Sixteen short triangular marginal lappets intercalated with tentacles. Sixteen radial canals with possible branches. Short cone-shaped manubrium without conspicuous oral lips. Occurrence: Lower Cambrian Heilinpu Formation, Kunming, Yunnan Province, China (equivalent to Cambrian Stage 3). Description:

(a) Body: The obliquely-laterally compressed body, 20 mm long and 10 mm in maximal width, consisting of a hemispherical central disc and a wide peripheral coronal sinus trailing with a ring of sixteen long strips interpreted here as tentacles (t) (Figs. 1a,2a). (b) Tentacles: On the incomplete counterpart specimen, either two of the sixteen tentacles are clearly grouped into eight prominent pairs that are distributed in different microbedding planes and more or less deviating from the body axis (Figs. 1b,2b). The tentacles measuring ca. 10 mm in maximal length and display a wider, straight basal part (0.5 mm in maximal diameter) followed by an elongate thinner distal part (Figs. 1c,2a,b). The tentacle base, comprising half of the entire tentacle, has a sharp boundary with the surrounding host rocks. Neighboring tentacles in each pair are parallel to each other but are more or less overlapping. The distal part of the tentacles taper gradually, some of them are slightly twisted or undulatory, indicative of flexibility. As seen on the tentacles of Middle Cambrian medusae [START_REF] Cartwright | Exceptionally preserved jellyfishes from the Middle Cambrian[END_REF], the entire surface of the tentacles is dark and displays dense, fine transverse striations (Fig. 1c). The distal part of some tentacles (Figs. 1a,2a) display bright zones intercalated with dense dark zones, highly reminiscent of contracted tentacles bearing nematocyst batteries. Notably, tentacles with dense dark striations were originally interpreted as characteristic comb rows of ctenophores; however, the host yellowish rocks between the tentacles (Fig. 1a) support the hypothesis that the tentacles extend from the bell rather than being part of the bell itself as found in ctenophores. (c) Coronal sinus, marginal lappets: The bell peripheral to the central dome is preserved as a wider crescent-shaped zone delimited by a conspicuous arc-shaped groove (Fig. 1a). The outer margin of the crescent zone displays a consecutive series of small serrate structures that are quite close to the tentacles roots (Figs. 1a,2a). The crescentic zone and the serrate structures probably represent the coronal sinus (cs) and the marginal lappets (ml) (Fig. 2a) along the scalloped bell margin, respectively. (d) Rhopalia: Approximately 1 mm away from the bell margin, an arched row of eight evenly spaced patches, deep brown in color and located at the spaces between the pairs of tentacles (Fig. 1a,d,e). These slightly concaved patches, round to subrectangular in outline, are ca. 0.5 mm in length and 0.3 mm in width, with the long axis parallel with the body axis. Due to weathering of the rocks, only one of them has a black carbon film (0.2 mm wide) on the surface; and the carbon film on other two patches remain as a few dark spots; the others show a naked surface that is much more reddish in color than ambient bell tissue (Fig. 1a). SEM observations and energy dispersal analysis reveal that they are composed of pyrite and organic carbon (Fig. 3), identical to that of the statoliths of co-occurring ctenophores [START_REF] Ou | A vanished history of skeletonization in Cambrian comb jellies[END_REF]. Thus, based on their disposition, morphology and diagenesis, it is reasonable to interpret these patches as rhopalia (rh), which in living medusae are composed of statoliths and pigmented ocelli. In addition, the rhopalia of the fossil specimen are consistent in size with those of scyphozoans [START_REF] Arai | A Functional Biology of Scyphozoa[END_REF] and cubozoans (ca. 0.3 mm) [START_REF] Sötje | Comparison of the statolith structures of Chironex fleckeri (Cnidaria, Cubozoa) and Periphylla periphylla (Cnidaria, Scyphozoa): a phylogenetic approach[END_REF]. As eight rhopalia are invariably aligned in an arched row parallel to the arched coronal groove and marginal lappets, they are not lined as an ellipse, we infer that the specimen was obliquely-laterally compressed and another half of the medusa was concealed at the host rocks. Therefore, the medusa most likely has a total of sixteen rhopalia and 32 tentacles (Fig. 5). (e) Manubrium: The main components of the central bell are distributed on two micro-bedding planes well separated by host muddy rocks. On the lower bedding plane, five to six short radial lines extend from the bell floor and converge toward the bell center, thus forming a cone-shaped profile with its tapered end directed adorally (Fig. 1a). The cone, half the height of the central bell, is interpreted as a manubrium (mb) inside a spacious deep subumbrellar cavity (Fig. 2a). No conspicuous oral lips are visible at the distal end of the manubrium. (f) Radial canals. In contrast to the lower bedding plane, the upper bedding plane on the right side of the central bell displays four incomplete but thick rectangular blocks (trb) delimited by five evenly-spaced deep radial lines parallel to the body axis. Due to probable taphonomic bias, the block near the body center is the widest one measuring ca. 5 mm wide. The radial lines, at an estimated number of sixteen in total, run orally and bifurcate toward the coronal groove (Fig. 2). Some of their adoral bifurcations are visible deep within the coronal sinus, and thus we interpret them as radial canals (rc). The sub-rectangle blocks which alternate with the paralleling radial canals are apparently positive relief, possibly representing a ring of thickened mesoglea (Fig. 2a). No ring canal can be confirmed at the bell margin.

Taxonomic affinity

Y. haikouensis was originally interpreted as a ctenophoran as the longitudinal dark strips resemble comb rows of extant modern ctenophores [START_REF] Hu | Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian[END_REF]. Now several lines of evidence can reject this hypothesis. First, the comb rows modern ctenophores are equally spaced meridional rows converging toward the apical sensory organs. Each row is composed of a transverse band of long, fused cilia on the body surface [START_REF] Ruppert | Invertebrate Zoology: A Functional Evolutionary Approach[END_REF]. In contrast, dark stripes on Yunnanoascus, which are separated by the host rocks, are connected to the body margin rather than distributed on the body surface; this is also supported by the twisted appearance of the distal part of dark strips, and thus, appreciate the current interpretation of strips as tentacles rather than comb rows. Second, Cambrian ctenophorans have diagnostic features of modern ctenophores such as an octamerous symmetry, prominent comb rows and an aboral sense organ with a statocyst; they are additionally characterized by a prominent aboral cone, conspicuous oral skirts and a sclerotized framework but are devoid of lateral tentacles (Conway [START_REF] Morris | Middle Cambrian Ctenophores from the Stephen Formation, British Columbia, Canada[END_REF][START_REF] Ou | A vanished history of skeletonization in Cambrian comb jellies[END_REF]. Yunnanoascus, in contrast, has a hemispherical aboral bell floor, a series of rhopalia and multiple long tentacles extending from the bell margin, thus quite different from Cambrian ctenophores. Its absence of a stalk or pedal disc excludes affinity with either stauromedusans or anthozoans. Yunnanoascus seems unlikely to be a member of Hydrozoa as hydrozoans lack complex sense organs and their statocysts are located at the base of the tentacles. In addition, the characteristic velum of hydromedusae is not seen in the current specimen.

Yunnanoascus is less likely a cubomedusan. The wide-based tentacles arranged in pairs are highly reminiscent of the pedalia of extant cubomedusae [START_REF] Conant | The Cubomedusae: A Memorial Volume[END_REF]. However, details of the gross morphology of Yunnanoascus differentiate it from extant cubomedusae. First, the cubozoan-type pedalia are smooth on the surface, and only the distal part of the tentacles bear rings of nematocyst-cells. In contrast, all tentacle surfaces of Yunnanoascus exhibits dense rings of transverse stripes similar to those of semaeostomeae scyphozoans [START_REF] Russell | The Medusae of the British Isles: 2. Pelagic Scyphozoa with a Supplement to the First Volume on Hydromedusae[END_REF]. Second, marginal tentacles with a wide base are also seen in hydrozoans (i.e. Sarsia princeps Haeckel 1879) [START_REF] Mayer | Medusae of the World: the Scyphomedusae[END_REF] and scyphozoans (i.e. Cyanea capillata in [START_REF] Russell | The Medusae of the British Isles: 2. Pelagic Scyphozoa with a Supplement to the First Volume on Hydromedusae[END_REF]). Third, the paired tentacles in Yunnanoascus appear to be independent and they did not share the same tentacle base as extant chirodropida cubomedusae. Moreover, extant cubomedusans have only four groups of pedalia and four rhopalia, inconsistent with the sixteen rhopalia seen on Yunnanoascus. Finally, Yunnanoascus lacks other diagnostic features of cubomedusans such as box-like shape, four frenulae and a velarium.

Yunnanoascus appear to have a coronal groove and a coronal sinus resembling those of scyphozoan coronates; however, its coronal sinus shows no sign of coronate-type pedalia with solid tentacles sprouting from the exumbrellar wall. Yunnanoascus has some features of the Semaeostomeae scyphozoans, including marginal tentacles alternating with rhopalia, apertural lappets and bifurcated radial canals. Notably, 16 rhopalia and 16-48 tentacles are seen in Diplulmaris and Phacellophora (Semaeostomeae), particularly, the tentacles of the latter are arranged in 16 clusters [START_REF] Mayer | Medusae of the World: the Scyphomedusae[END_REF], quite resembling 16 pairs of tentacles in Yunnanoascus.

Phylogenetic analysis

We carried out a preliminary cladistic analysis based on 25 taxa and 105 characters in PAUP* 4.0 b10 and TNT 1.1. Unweighted branch-andbound search of the data matrix in PAUP analysis yielded 189 shortest trees (tree length (TL) = 189, consistency index (CI) = 0.6243, rescaled consistency index (RC) = 0.4688). The successive weighting analysis [START_REF] Marques | Cladistic analysis of Medusozoa and cnidarian evolution[END_REF]Van Iten et al., 2006) resulted in three trees (TL = 92.90023, CI = 0.864, RC = 0.794) that differ in the position of conulariids (Appendix 3). In the TNT analysis, we acquired a consensus tree based on three best trees (Appendix 4) generated by New-Technology search. These three trees differ in the position of Kuanchuanpu fossils between ELISN83-66 and ELISn31-31, and Utah fossils presumed to be Narcomedusae (Fig. 4b, Appendix 1). Apparently, the missing data of the fossils (up to 24 parsimony-uninformative characters) are the inherent sources of the partial inconsistency of both consensus trees acquired by PAUP* 4.0 b10 and TNT 1.1 (Fig. 4); it can also account for the inconsistency with the most accepted hypothesis that cubozoans and scyphozoans are more accepted as sister groups [START_REF] Marques | Cladistic analysis of Medusozoa and cnidarian evolution[END_REF]Van Iten et al., 2006). However, all of these best trees, including the consensus trees, display Yunnanoascus as the sister taxon to the other fossil and extant Scyphozoa lineages (Fig. 4; Appendixes 3 and 4), compatible with morphological analysis mentioned above.

Life habit and the temporal constraint on the rise of medusoid stage

The rhopalia in extant medusae have been well documented as marginal centers generating rhythmic electrical impulses in the motor nerve net [START_REF] Arai | A Functional Biology of Scyphozoa[END_REF]. When all of the rhopalia are removed from the medusae, the swimming rhythm and contractions of the coronal muscles will cease; and eventually the medusae usually will sink to the bottom [START_REF] Spangenberg | Statolith differentiation in Aurelia aurita[END_REF]. The finding of 16 large rhopalia on Yunnanoascus strongly indicates a nektonic life style; and this hypothesis is further strengthened by its round aboral end devoid of stalk, and a spacious subumbrellar cavity adaptive for producing a jet of water (Fig. 5). The predatory habit is indicated by up to 32 elongate tentacles each with a broad base, and particularly a twistable distal part with possible nematocyst batteries, which, in extant medusae, contain a large amount of toxic cnidae. The swimming pattern of extant medusae varies in species [START_REF] Arai | A Functional Biology of Scyphozoa[END_REF], basically they quickly move upwards and sink slowly [START_REF] Gerritsen | Adaptive Responses to Encounter Problems[END_REF][START_REF] Strand | Predatory behavior of Phacellophora camtschatica and size-selective predation upon Aurelia aurita (Scyphozoa: Cnidaria) in Saanich Inlet, British Columbia[END_REF]. They have not been observed using sensory organs to make directed movement toward their prey; mostly during sinking they use tentacles to capture the preys by encounter [START_REF] Arai | A Functional Biology of Scyphozoa[END_REF]. Likewise, a 'density' of 32 elongate tentacles of Yunnanoascus are adaptive for a high encounter probability.

The evidence supporting centimeter-scale, planktonic medusae at the Cambrian Stage 3 and 5 [START_REF] Cartwright | Exceptionally preserved jellyfishes from the Middle Cambrian[END_REF] is of great significance for our understanding of the evolution of the three-phase life Han et al. (2013Han et al. ( , 2016)); together with co-ocurring Olivooides and Quadrapyrgites, they come from the basal Cambrian Kuanchuanpu Formation in South China. Cormr, seamr, narmr, cubmr represent, respectively, unnamed fossils of the Coronata, Semaestomeae, Narcomedusae and Cubomedusa from the Middle Cambrian Marjum Formation, Utah [START_REF] Cartwright | Exceptionally preserved jellyfishes from the Middle Cambrian[END_REF]. cycle of medusozoans, particularly, the temporal origin of the pelagic medusoid phase. Hitherto, no reliable evidence of pelagic medusae was known from basal Cambrian or Precambrian deposits [START_REF] Young | The fossil record of cnidarian medusae[END_REF]. The earliest reliable stem group medusozoans (Dong et al., 2013;Han et al., 2013), including ephyra-like organisms (Dong et al., 2013;Han et al., 2010) were known from the early Cambrian (Fortune Stage), but these are minute forms less than 5 mm; particularly, they lack the features for swimming such as sense organs (Han et al., 2016;Han et al., 2013). In addition, many of Cambrian medusozoans possess an external cone-shaped organic periderm (i.e. Olivooides and Quadrapyrgites) (Liu et al., 2014b;Steiner et al., 2014) or biomineralized tubes (i.e. conulariids, anabaritiids, Sphenothallus) (Kouchinsky et al., 2009;Van Iten et al., 2014); thus they are suspected to be benthic or sessile forms probably related to polyps (Van Iten et al., 2006). It appears that the medusozoan communities at the Cambrian Fortune Stage were dominated in both ecology and diversity by micrometer-scale benthic or sessile forms that inhabited shallow water. From the Cambrian III onward, the fossil record of sessile medusozoans, in either ecology or diversity were rather scarce (Van Iten et al., 2014), even in the worldwide fossil Lagerstätten. It turns out that the interval between Cambrian stages 1 and 3 most likely represents a period of competition between the polypoid and medusoid phases, and finally the dominant phase of medusozoan communities most probably had undergone a transition from benthic polyps to pelagic medusae. This transition might be ascribable to the great advantages of the medusa phase in positive feeding, broader dispersal, settlement selection, escaping from harsh environments and exploiting different ecological spaces of ancient oceans.

The early Cambrian record of medusae also has implications for understanding the ecological and evolutionary history of the marine pelagic realm. The evolution of rhopalia for both gravity and light in Cambrian medusae enable them to discriminate 'up' from 'down', and to excurse vertically [START_REF] Gerritsen | Adaptive Responses to Encounter Problems[END_REF]. Thus, it is most likely a precondition for the rise of diurnal/nocturnal vertical migrations (DVM) of entire population as seen in modern epipelagic and mesopelagic medusae (see [START_REF] Arai | A Functional Biology of Scyphozoa[END_REF][START_REF] Yasuda | Ecological studies on the jelly-fish, Aurelia aurita (Linné). Urazoko Bay, Fukui prefecture-VIII. diel vertical migration of the medusa in early fall[END_REF][START_REF] Youngbluth | Distribution, abundance, behavior and metabolism of Periphylla periphylla, a mesopelagic coronate medusa in a Norwegian fjord[END_REF][START_REF] Graham | A physical context for gelatinous zooplankton aggregations: a review[END_REF]. Contrasting with other pelagic bilaterians, i.e. arthropods of Ercaia, Misszhouia, Naraoia, Pectocaris, Pisnnocaris and deuterostomes of Yunnanozoa, Haikouella, Haikouichthys, which were preserved by rapid burying as a large population [START_REF] Han | Preliminary notes on soft-bodied fossil concentrations from the Early Cambrian Chengjiang deposits[END_REF], direct evidence of medusa populations in Chengjiang have not yet been reported. Modern medusae are eaten by a variety of predators, i.e. other pelagic coelenterates, arthropods, fishes and turtles [START_REF] Arai | A Functional Biology of Scyphozoa[END_REF]. As all known Cambrian vertebrates are filter-feeders [START_REF] Shu | Lower Cambrian vertebrates from South China[END_REF][START_REF] Shu | Head and backbone of the Early Cambrian vertebrate Haikouichthys[END_REF], and only one species of medusae is reported herein, thus the medusae population in Chengjiang, if present, might have been consumed by some large predatory arthropods and anomolocarids. The evidence of nektonic medusae, together with pelagic populations mentioned above, further strengthens the hypothesis that a complex, modern-style pelagic ecosystem may have been developed by the time of early Cambrian Stage 3 [START_REF] Hu | Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian[END_REF]Vannier et al., 2009). The rise of such a highly complex pelagic ecosystem might have been accompanied, in part, by the origins of various well-developed sense organs among pelagic cnidarians and ctenophores [START_REF] Ou | A vanished history of skeletonization in Cambrian comb jellies[END_REF], as well as pelagic bilaterians [START_REF] Zhao | Community structure and composition of the Cambrian Chengjiang biota[END_REF].

Preservation of the medusae

In contrast with abundant medusae in modern ocean, the fossil record of medusae is sporadic. Apart from lacking hard tissue, the reason of their artefactual rareness is ascribable to three aspects unfavorable for fossilization: (1) Compared with sessile forms, pelagic medusa have nearly same density as seawater, and thus are difficult to bury. Generally, they would have decayed within the water column before being buried. (2) Medusae are extremely fragile organisms which probably decay extremely rapidly after death. (3) Specifically, pelagic medusae have thick mesoglea with low contents of organic materials. Most known medusae are fossilized as moulds [START_REF] Young | The fossil record of cnidarian medusae[END_REF], whereas the reason for the exceptional soft-tissue preservation of the medusae in Chengjiang and Utah [START_REF] Cartwright | Exceptionally preserved jellyfishes from the Middle Cambrian[END_REF] remains problematic. Energy-dispersive spectroscopic analysis of Y. haikouensis (Fig. 3) reveals that the soft tissue was mainly mineralized as organic carbon and pyrite, similar to the case of other Chengjiang organisms [START_REF] Gabbott | Preservation of Early Cambrian animals of the Chengjiang biota[END_REF][START_REF] Gaines | Cambrian Burgess Shale-type deposits share a common mode of fossilization[END_REF][START_REF] Zhu | Fossilization modes in the Chengjiang Lagerstätte (Cambrian of China): testing the roles of organic preservation and diagenetic alteration in exceptional preservation[END_REF]. If correct, a rapid burial caused by periodic storms would be capable of capturing both benthic and pelagic forms [START_REF] Han | Preliminary notes on soft-bodied fossil concentrations from the Early Cambrian Chengjiang deposits[END_REF][START_REF] Hu | Taphonomy and palaeoecology of the Early Cambrian Chengjiang biota from Eastern Yunnan, China[END_REF][START_REF] Hu | Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian[END_REF][START_REF] Zhao | Quantitative analysis of taphofacies and paleocommunities in the Early Cambrian Chengjiang Lagerstätte[END_REF]; a rapid burial of medusae by fine clay minerals would have inhibited microbial decay [START_REF] Butler | Experimental taphonomy of Artemia reveals the role of endogenous microbes in mediating decay and fossilization[END_REF][START_REF] Forchielli | Taphonomy of the earliest Cambrian linguliform brachiopods[END_REF]). In addition, infilling the subumbrellar cavity of the medusae by clay minerals will lead to the dehydration of the medusa mesoglea, thus the concentration of organic materials will be greatly improved.

Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.palaeo.2016.02.025.

Introduction

Although molecular clock studies estimate the origin and earliest diversification of animals within the Cryogenian Period, evidence from the fossil record reveals that crown members of nearly all animal phyla appear in a relatively rapid diversification event in early Cambrian time (the 'Cambrian explosion'). This evolutionary radiation follows after a global mass extinction of the Ediacaran fauna [START_REF] Erwin | The Cambrian conundrum: early divergence and later ecological success in the early history of animals[END_REF], a benthic community dominated by sessile, substrate-sticking organisms [START_REF] Seilacher | Biomat-related lifestyles in the Precambrian[END_REF][START_REF] Fedonkin | The Rise of Animals: Evolution and Diversification of the Kingdom Animalia[END_REF][START_REF] Yuan | An early Ediacaran assemblage of macroscopic and morphologically differentiated eukaryotes[END_REF]). In addition, geological and geochemical data suggest that the Ediacaran-Cambrian transition was not only a transition in biological diversity and ecosystem structure, but was also associated with drastic environmental change. These changes in environment and biology were likely highly intertwined; for instance, the rise of oxygen (e.g. [START_REF] Fike | Oxidation of the Ediacaran ocean[END_REF][START_REF] Canfield | Ferruginous conditions dominated later Neoproterozoic deep-water chemistry[END_REF][START_REF] Komiya | Evolution of the composition of seawater through geologic time, and its influence on the evolution of life[END_REF][START_REF] Canfield | Animal evolution, bioturbation, and the sulfate concentration of the oceans[END_REF][START_REF] Li | A stratified redox model for the Ediacaran ocean[END_REF]) has been sug- †Author for correspondence: yaopingcai@nwu.edu.cn gested to have had profound effects on the evolution and diversification of metazoans [START_REF] Sperling | Oxygen, ecology, and the Cambrian radiation of animals[END_REF]. Further, the innovation of novel ecological strategies, such as ecosystem engineering and predation, in Ediacaran communities nearing the Cambrian boundary may have served to set the stage for the impending Cambrian radiation (e.g. [START_REF] Schiffbauer | The latest Ediacaran Wormworld fauna: setting the ecological stage for the Cambrian Explosion[END_REF]. Thus, the Ediacaran-Cambrian boundary represents a revolutionary transition, wherein the combined effects of environmental, biological and ecological change impart a large influence on phylogenetic patterns for the next c. 540 million years of evolutionary history.

With several reports of the survival of a few taxa from the Ediacaran fauna into the early and middle Cambrian period (e.g. [START_REF] Jensen | Ediacaratype fossils in Cambrian sediments[END_REF][START_REF] Hagadorn | Ediacaran fossils from the southwestern Great Basin, United States[END_REF][START_REF] Shu | Lower Cambrian vendobionts from China and early diploblast evolution[END_REF]Van Iten et al. 2006), palaeontologists have sought to better understand the nature of biotic replacement at the Ediacaran-Cambrian transition [START_REF] Laflamme | The end of the Ediacara biota: extinction, biotic replacement, or Cheshire Cat?[END_REF][START_REF] Darroch | Biotic replacement and mass extinction of the Ediacara biota[END_REF], and moreover, to explore the possible extinction hold-overs and their place within this evolutionary story. One such group of possible hold-overs comprises the terminal Ediacaran tubular biomineralizing assemblage (part of the 'wormworld fauna', [START_REF] Schiffbauer | The latest Ediacaran Wormworld fauna: setting the ecological stage for the Cambrian Explosion[END_REF], exemplified by such organisms as Cloudina, Sinotubulites and Namacalathus (e.g. [START_REF] Germs | New shelly fossils from the Nama Group, South West Africa[END_REF][START_REF] Signor | A pretrilobite shelly fauna from the White-Inyo region of eastern California and western Nevada[END_REF][START_REF] Grant | Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic[END_REF][START_REF] Grotzinger | Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia[END_REF][START_REF] Hofmann | Namacalathus-Cloudina assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia: Canada's oldest shelly fossils[END_REF][START_REF] Cortijo | A new species of Cloudina from the terminal Ediacaran of Spain[END_REF][START_REF] Zhuravlev | New finds of skeletal fossils in the terminal Neoproterozoic of the Siberian Platform and Spain[END_REF][START_REF] Cortijo | Late Ediacaran skeletal body fossil assemblage from the Navalpino anticline, central Spain[END_REF]. Although the presence of these and broadly comparable genera in Cambrian rocks is rare (e.g. [START_REF] Yochelson | Discovery of early Cambrian fossils at Taylor Nunatak, 936 Antarctica[END_REF][START_REF] Mcilroy | Palaeobiology and evolution of the earliest agglutinated Foraminifera: Platysolenites, Spirosolenites and related forms[END_REF][START_REF] Rogov | Duration of the first biozone in the Siberian hypostratotype of the Vendian[END_REF], they may be evolutionarily tied to tubular forms present in small shelly fossil assemblages of the lowest Cambrian, such as the anabaritids.

The lower Cambrian Kuanchuanpu fauna (Fortunian Stage, c. 535 Ma) in South China has become increasingly significant in understanding the evolutionary history of animals in the Ediacaran-Cambrian transition. Apart from small shelly fossils, including various molluscs and protoconodonts [START_REF] Bengtson | Early Cambrian fossils from South Australia[END_REF][START_REF] Qian | Taxonomy and Biostratigraphy of Small Shelly Fossils in China[END_REF], the Kuanchuanpu Formation has yielded several extraordinary discoveries including putative arthropod embryos (Steiner et al. 2004b), scalidophorans (Liu et al. 2014b;Zhang et al. 2015) and markedly diverse types of cnidarians (Han et al. 2010(Han et al. , 2013;;Dong et al. 2013;Han et al. 2016a) with biomineralized exoskeletons. Particularly, the scalidophorans, molluscs and protoconodonts have established phylogenetic connections with the emerging complex Cambrian marine ecosystem, for example, the diversity of arthropods as represented by the Chengjiang fauna [START_REF] Vannier | Early Cambrian origin of modern food webs: evidence from predator arrow worms[END_REF](Vannier et al. , 2009)). Perhaps more importantly, the abundant millimetre-scale tubular fossils in the Kuanchuanpu fauna at least superficially resemble the terminal Ediacaran tubular ecosystem [START_REF] Fedonkin | The Rise of Animals: Evolution and Diversification of the Kingdom Animalia[END_REF][START_REF] Yang | Transitional Ediacaran-Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution[END_REF]).

Here we describe a three-dimensionally preserved tubular microfossil-Feiyanella manica gen. et sp. nov.-from the lower Cambrian Kuanchuanpu Formation, Shaanxi Province, South China. It exhibits similar funnel-in-funnel tube construction to the late Ediacaran tubular fossil Cloudina [START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF], and also shows broadly comparable dichotomous branching features posited to indicate asexual reproduction. Further, it shows a similar wrinkled/folded tube wall exterior comparable to that of Sinotubulites [START_REF] Chen | Tube structure and original composition of Sinotubulites: shelly fossils from the late Neoproterozoic in southern Shaanxi, China[END_REF][START_REF] Cai | New material of the biomineralizing tubular fossil Sinotubulites from the late Ediacaran Dengying Formation, South China[END_REF]. This newly described organism may therefore provide an important palaeobiological and palaeoecological link between tubular fossils in the latest Ediacaran Period and earliest Cambrian Period.

Stratigraphic setting, fossil material and methods

The specimens described here were recovered from Bed 31 of the lower Cambrian Kuanchuanpu Formation (Fortunian Stage, Terreneuvian Series) at the Shizhonggou section of Ningqiang County, and Bed 2 at the Zhangjiagou section, Xixiang, Shaanxi Province, South China. The Ningqiang and Xixiang areas were palaeogeographically located on the northwestern mar-gin of the Yangtze Platform during Ediacaran and Cambrian times. The fossil-bearing beds belong to the classic Anabarites trisulcatus -Protohertzina anabarica small shelly fossil biozone, indicating a stratigraphic equivalent of the Nemakit-Daldynian interval in Siberia (Steiner et al. 2004a).

Insoluble phosphatized fossils were liberated from the phosphatic limestone using 10 % acetic acid digestion. A Hyolithellus sp. specimen (ELIXX35-465) and a well-preserved specimen of Feiyanella manica gen. et sp. nov. (ELISN141-14) were imaged by scanning electron microscopy (SEM) (Fig. 1), and ELISN141-14 was three-dimensionally analysed using Synchrotron radiation X-ray tomographic microscopy (SRXTM) at SPring-8 in Hyogo, Japan (Figs 234). The 3D reconstructions of X-ray data were processed using VG Studio 2.2 Max, allowing us to document interior anatomic details of the tube structure. All specimens are reposited at the Early Life Institute (ELI), Northwest University, Xi'an, China.

Systematic palaeontology

Incertae sedis Genus Feiyanella new genus

Type species. Feiyanella manica new species, by monotypy Diagnosis. Minute, multi-layered sub-cylindrical fossil tube consisting of a number of nested funnel-shaped layers. Outermost layer strongly wrinkled/folded, inner layers ornamented with weaker transverse annulations. Tube exhibits two orders of dichotomous branching, forming three generations of tubes. Parent tube layers are fully nested in the preserved length, whereas daughter and granddaughter tube layers are partially stacked/overlapped. Feiyanella manica gen. et sp. nov.

Figures 12345Etymology. Feiyan (Feiyan Zhao), an ancient Chinese beauty famous for her slender build, similar to the slight gross appearance of the tube; manica, Latin, referring to the wrinkled outermost layer of the tube that resembles a folded shirt sleeve.

Holotype. ELISN141-14.

Type locality and horizon. The Shizhonggou section in Ningqiang County, Shaanxi Province, South China. Lower Cambrian Kuanchuanpu Formation (Fortunian Stage).

Description

The holotype specimen of Feiyanella manica gen. et sp. nov. (ELISN141-14) is three-dimensionally preserved through authigenic phosphatization. The fossil is incomplete (Fig. 1a), with both apical and apertural Incertae sedis ends not intact (Fig. 1b-d). The preserved portion of the tube is roughly conical and gently curved (Fig. 1a).

The apertural end flares with three slightly divergent, concentric sub-units (Fig. 1a,c,d). The tube is multilayered and nested, composed of a number of stacked layers with varying overlap (Figs 12345). The outermost layer of the tube is corrugated with closely spaced transverse ridges, showing stronger folds or wrinkles (Figs 1a,b,. Only weaker transverse annulations can be seen in inner layers (Fig. 4b-l). SRXTM analysis reveals that three units of tube layers with unambiguously different lengths and diameters can be identified in the holotype specimen. Although these three sets of tubes are of quite different sizes, they all show the funnel-in-funnel tube construction. They are here interpreted as representing three generations -namely the parent, the daughter and the granddaughter tubes, respectively -which are described separately below.

4.a. Parent tube

The parent tube is sub-cylindrical in gross morphology, with the apertural end slightly flaring. It consists, from exterior to interior, of four nested funnel-shaped layers (the outermost four layers of the tube of Feiyanella, marked with P1-1, P1-2, P1-3, P1-4 in all figures). The preserved length of the parent tube is c. 1379 µm (= the length of the longest third layer; P1-3). The four layers of funnels, ranging from 193 to 304 µm in diameter, are fully stacked and overlapped, reminiscent of the 'tube-in-tube' construction of late Ediacaran tubular fossil Sinotubulites [START_REF] Cai | New material of the biomineralizing tubular fossil Sinotubulites from the late Ediacaran Dengying Formation, South China[END_REF]. The first layer (P1-1) is strongly wrinkled and/or folded, forming stronger, closely spaced, irregular, transverse corrugations on the exterior surface of the tube (Fig. 1a,b), strongly distinct from those on the inner layers. Transverse corrugations often bear a few secondary transverse irregular folds (Fig. 1b), which form complex exterior corrugations and make this layer appear to be much thicker than any other layers 3b,c). The notably stronger corrugation on the outermost layer of Feiyanella is a diagnostic feature characterizing this taxon. The second, third and fourth parent layers (P1-2-P1-4) share a similar morphology with the first layer. However, layers P1-2-P1-4 are ornamented with weaker transverse annulations (Fig. 4b-d), quite different from those in layer P1-1.

The second parent layer is c. 120 and c. 259 µm in minimum and maximum diameters, respectively. The third parent layer (P1-3) is the longest one (1379 µm) in the preserved specimen (Fig. 4c). The fourth parent layer is obviously shorter (631 µm) and smaller (77 and 108 µm in minimum and maximum diameters) than the other three parent funnels (Fig. 4d).

4.b. Daughter tube

Two daughter tubes (denoted as D1 and D2 in all figures) are discernible at the apertural end of the tube of Feiyanella (Figs 1c, 2, 3, 4), which extend from the apertural opening of the fourth parent funnel (Fig. 4d).

The two daughter tubes stand side-by-side, and the outermost funnel of each daughter tube appears nearly cemented together (Fig. 2e). The daughter tubes consist of a number of nested, funnel-shaped layers, sharing similarities in tube wall morphology and nesting patterns with the parent tube. The two daughter tubes are not equal in size. The larger one (D1) consists of two tube wall layers (Fig. 2f) and is irregular in cross-section (Fig. 2e-g). The two layers (D1-1 and D1-2) are only situated in the apertural part of Feiyanella. Its width increases greatly towards the apertural end (Fig. 4a-d). The smaller daughter tube (D2) is composed of seven tube wall layers (D2-1-D2-7 in Figs 2d-j, 3b-c, 4d-f). In contrast, ornamentations on the daughter funnels are noticeably diminished, with closely spaced transverse annulations but without complicated corrugations (e.g. funnel D2-1), as compared to the parent funnels.

4.c. Granddaughter tube

Four granddaughter tubes are identified in Feiyanella: two (G1 and G2) originated from the larger daughter tube (D1) and the other two (G3 and G4) from the smaller daughter tube (D2). The granddaughter tubes G1 and G2 are situated at the upper half of the preserved parent tube (Fig. 3b,c), whereas G3 and G4 are at the apertural end (Figs 2e,3d). Morphological details of G3 and G4 are very limited, as they can only be identified in cross-sections near the apertural end of the tube (Figs 2e, 3d). Granddaughter tubes G1 and G2 are deeply nested into the daughter tube D1 (located at the middle to apertural portion of the parent tube; Fig. 2e-k). G1 and G2 are more or less equal in size (c. 39 µm). Notably, the splitting plane along the tube between the two granddaughter tubes G1 and G2 is orientated perpendicular to the splitting plane of the daughter tubes (Figs 2e, 5). G1 contains two layers of funnels (G1-1 and G1-2) and G2 has three layers (G2-1-G2-3) (Fig. 4d-i). The innermost layer of the grand- in cross-section. The spindle exhibits a bright phase under SRXTM observation, c. 39 µm in diameter and 188 µm in length, occupying nearly one-third of the full length of granddaughter tube G1 (Fig. 4g).

Discussion

The incomplete preservation of the Feiyanella tube leaves uncertainties as to its full morphology and nesting patterns of the parent layers. Two overlapping patterns can be identified from the preserved portion, however: the outer parent tube layers (P1-1-P1-4) fully overlap (Fig. 4a-d), whereas all of the other inner tube layers only partially overlap (Fig. 4d-l). Two contrasting degrees of ornamentation are observed in Feiyanella: the outer parent tube layers (P1-1-P1-4) are visibly folded/wrinkled and form stronger transverse and/or slightly oblique corrugations (Fig. 4a-d), whereas all of the other inner tube layers are ornamented with weaker transverse annulations (Fig. 4d-l). The outermost layer of the tube in particular shows strongly folded/wrinkled corrugations (Figs 1a,b,3a,4a), making this layer appear much thicker 3bc). With regard to nesting, although the four funnelshaped parent layers (P1-1-P1-4) are completely nested in the preserved portion of the Feiyanella tube, it is uncertain whether their full morphology is funnelshaped and whether they are fully or partially stacked.

As an Örsten-type Lagerstätte, the Kuanchuanpu Formation biota is characterized by the selective preservation of refractory cuticular tissues of meiofauna (see review in [START_REF] Schiffbauer | Exceptional fossil conservation through phosphatization[END_REF]) and only fragments of larger organisms (i.e. putative grasping spines of Protohertzina anabarica; see Steiner et al. 2004a). However, Feiyanella was not likely a fragment of a more complex organism: for example, an appendage of an annelid or ecdysozoan (panarthropod cycloneuralian). Several lines of evidence are summarized here: (1) Both annelids and ecdysozoans have a cuticularized integument overlying the epidermis, particularly a tri-layered cuticle (epi-, exo-and endocuticles) in cycloneuralians [START_REF] Bereiter-Hahn | Biology of the Integument: Invertebrates[END_REF][START_REF] Peterson | Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences[END_REF]. The appendages of these animals, if fossilized, would not be preserved as loosely multi-layered, funnel-in-funnel structures. (2) The cuticle of a complex organism usually bears complex ornaments, such as sensory organs, chaetae, glands, scalids and net-like structures. These ornaments are absent in Feiyanella. (3) The branched, segmented appendages in arthropods differ from the unsegmented funnel of Feiyanella. (4) The parapodium of annelids, although varying among species, tapers apparently towards one end, in visible contrast with the sub-cylindrical shape of Feiyanella. (5) Some species of cycloneuralians (Liu et al. 2014b;Zhang et al. 2015) have been discovered from the Kuanchuanpu Formation. These fossils, together with a large number of specimens in our collection, collectively exhibit specific cuticle ornaments that are markedly different from Feiyanella. In addition, the integument of the cycloneuralian fossils is preserved as a single layer. In short, the characteristic multi-layered, funnel-in-funnel structure of Feiyanella is more appropriately interpreted as a dwelling tube of an organism, comparable to those known from the upper Ediacaran (e.g. [START_REF] Hua | Borings in Cloudina shells: complex predator-prey dynamics in the terminal Neoproterozoic[END_REF][START_REF] Cai | Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China[END_REF][START_REF] Cai | Tube growth patterns and microbial matrelated lifestyles in the Ediacaran fossil Cloudina, Gaojiashan Lagerstätte, South China[END_REF][START_REF] Cai | New material of the biomineralizing tubular fossil Sinotubulites from the late Ediacaran Dengying Formation, South China[END_REF], rather than a body fragment of a larger complex organism.

Feiyanella is distinct from other early Cambrian tubular fossils, i.e. Hyolithelminths, Byronia and Sphenothallus. Hyolithelminths, a problematic group of mineralized tubular fossils characterized by more or less regular annulations or growth lines [START_REF] Bengtson | Early Cambrian fossils from South Australia[END_REF][START_REF] Li | Early Cambrian Hyolithelminths -Torellella bisulcata sp. nov. from Zhenba, Southern Shaanxi[END_REF], exhibit a multi-laminated tube wall, and in some species, a cone-in-cone structure (see Kouchinsky et al. 2015, fig. 67). However, the micro-laminations of the tube wall are closely cemented together and each lamella (in some species) is composed of fine fibres (i.e. [START_REF] Bengtson | Early Cambrian fossils from South Australia[END_REF][START_REF] Vinn | Possible cnidarian affinities of Torellella (Hyolithelminthes, Upper Cambrian, Estonia)[END_REF][START_REF] Skovsted | Hyolithellus in life position from the Lower Cambrian of North Greenland[END_REF]. Hyolithellus from the Kuanchuanpu Formation -a common element in the Kuanchuanpu Formation in the Xixiang and Ningqiang areas Steiner et al. (2004a) -is another tubular fossil sharing a similar morphology with Feiyanella. But the tubes of Hyolithellus are much larger in size (> 2 mm in length) than Feiyanella and the exterior smooth surface bears a number of wide-spaced circular grooves (Fig. 1e). In addition, ellipse-shaped pores occasionally occur in the grooves, which may be produced by soft tissue inside the tubes. Hyolithellus may have been a more complex organism (e.g. an annelid; [START_REF] Skovsted | Hyolithellus in life position from the Lower Cambrian of North Greenland[END_REF]. The smooth tube, transverse grooves and ellipse-shaped pores indicate contrasting morphological and palaeobiological relationships with Feiyanella.

Byronia, a sessile tube-dwelling organism characterized by a lenticular cross-section and a deep split along the broader end [START_REF] Bengtson | Early Cambrian fossils from South Australia[END_REF]), differs from Feiyanella in its wide-spaced transverse ridges and longitudinal ribs (see fig. 6 in [START_REF] Skovsted | Hyolithellus in life position from the Lower Cambrian of North Greenland[END_REF]) that resemble the peridermal tube of olivooids (e.g. Olivooides and Quadrapyrgites; Liu et al. 2014a;Steiner et al. 2014) from the Kuanchuanpu Formation. Both Byronia and olivooids have been proposed to have close affinities with thecate medusozoans in the phylum Cnidaria (see [START_REF] Bengtson | Early Cambrian fossils from South Australia[END_REF][START_REF] Zhu | Occurrence of Byronia Matthew and Sphenothallus Hall in the Lower Cambrian of China[END_REF]Dong et al. 2013;Han et al. 2016b).

Sphenothallus, a cone-shaped tube characterized by a more or less elliptical cross-section and a basal holdfast and proposed as a thecate medusozoan [START_REF] Van Iten | New data on the morphology of Sphenothallus Hall: implications for its affinities[END_REF][START_REF] Zhu | Occurrence of Byronia Matthew and Sphenothallus Hall in the Lower Cambrian of China[END_REF][START_REF] Li | Early Cambrian Hyolithelminths -Torellella bisulcata sp. nov. from Zhenba, Southern Shaanxi[END_REF], differs from Feiyanella in its finely laminated and cemented tube wall consisting of alternating apatite and organic laminae.

Instead, Feiyanella shares similarities in tube morphology and construction with some late Ediacaran tubular fossils, including Cloudina [START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF][START_REF] Cortijo | A new species of Cloudina from the terminal Ediacaran of Spain[END_REF][START_REF] Cortijo | Late Ediacaran skeletal body fossil assemblage from the Navalpino anticline, central Spain[END_REF], Conotubus [START_REF] Cai | Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China[END_REF] and Sinotubulites [START_REF] Chen | Tube structure and original composition of Sinotubulites: shelly fossils from the late Neoproterozoic in southern Shaanxi, China[END_REF][START_REF] Cai | New material of the biomineralizing tubular fossil Sinotubulites from the late Ediacaran Dengying Formation, South China[END_REF]. Each of these taxa shows a nested tube construction, cylindrical or oval-shaped cross-section and the absence of transverse internal structures such as septae or tabulae.

The perceived nesting pattern and the wrinkled tubewall features of the parent layers (particularly P1-1) of Feiyanella are visibly similar to those of Sinotubulites [START_REF] Cai | New material of the biomineralizing tubular fossil Sinotubulites from the late Ediacaran Dengying Formation, South China[END_REF]. This may indicate that the parent layers of Feiyanella fully overlap, which would thus be comparable to the nesting patterns of Sinotubulites. On the other hand, the funnel-shaped tube layers of Feiyanella are strikingly similar to those of Cloudina [START_REF] Hua | Borings in Cloudina shells: complex predator-prey dynamics in the terminal Neoproterozoic[END_REF][START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF][START_REF] Cortijo | A new species of Cloudina from the terminal Ediacaran of Spain[END_REF] and Conotubus [START_REF] Cai | Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China[END_REF]. Furthermore, the dichotomous branching of Feiyanella, which is most likely indicative of an asexual reproduction strategy, is also comparable to that of Cloudina [START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF][START_REF] Cortijo | A new species of Cloudina from the terminal Ediacaran of Spain[END_REF][START_REF] Cortijo | Late Ediacaran skeletal body fossil assemblage from the Navalpino anticline, central Spain[END_REF].

While similar, Feiyanella also shows distinct differences from Cloudina, Conotubus and Sinotubulites. First, as compared to Sinotubulites, Feiyanella differs in tube layer morphology and overlapping patterns: Feiyanella is composed exclusively of funnel-shaped layers with partial overlap between adjacent layers (except for those of the parent funnels), whereas Sinotubulites is composed exclusively of cylinder-shaped layers with full overlap between two adjacent layers. The funnels of Feiyanella differ from the funnels of Cloudina and Conotubus in that the former lacks thickened rims on the apical and apertural ends of the funnels [START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF][START_REF] Cortijo | A new species of Cloudina from the terminal Ediacaran of Spain[END_REF][START_REF] Cai | Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China[END_REF]). In addition, Feiyanella differs from Cloudina in the manner of asexual reproduction. Cloudina is characterized by two manners of asexual reproduction strategies: dichotomous branching of daughter tubes in the same parent tube (see fig. 1L of [START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF] and fig. 8 of [START_REF] Cortijo | A new species of Cloudina from the terminal Ediacaran of Spain[END_REF][START_REF] Cortijo | Late Ediacaran skeletal body fossil assemblage from the Navalpino anticline, central Spain[END_REF] and budding of a daughter tube between two adjacent funnels (see fig. 1P of [START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF]. Feiyanella, however, displays a pattern of multiple-ordered dichotomous branching of the younger generation of tubes within the older generation tubes.

Most broadly, Feiyanella shares similarities in the tube morphology, nesting patterns and presence of asexual reproduction strategies with Cloudina, and in the corrugation and nesting patterns of outer layers with Sinotubulites. Considering its similarities with Cloudina [START_REF] Cai | Tube growth patterns and microbial matrelated lifestyles in the Ediacaran fossil Cloudina, Gaojiashan Lagerstätte, South China[END_REF] and Conotubus [START_REF] Cai | Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China[END_REF], Feiyanella is thus interpreted as a sessile organism with periodic growth by secretion of new funnels within the older funnels [START_REF] Grant | Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic[END_REF], with the animal presumed to have lived within the innermost, most recently secreted funnel [START_REF] Cortijo | A new species of Cloudina from the terminal Ediacaran of Spain[END_REF]. The tube of Feiyanella is hollow, indicating the soft tissue of the organism likely was able to move up and down within the tube, rather than being fixed within an isolated chamber. Although the full morphology of the tube is unknown, the apical end may have been closed, comparable to the basally closed apical end of Cloudina [START_REF] Cortijo | Late Ediacaran skeletal body fossil assemblage from the Navalpino anticline, central Spain[END_REF].

Zoological affinities and evolutionary implications

Together, Cloudina, Conotubus, Sinotubulites and Feiyanella may constitute a monophyletic group, as they are morphologically quite different from other Ediacaran and Cambrian tubular fossils (see summary in [START_REF] Cai | Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China[END_REF][START_REF] Cai | Tube construction and life mode of the late Ediacaran tubular fossil Gaojiashania cyclus from the Gaojiashan Lagerstätte[END_REF]. The zoological placement of these Cloudina-type organisms, however, has been controversial. For example, Cloudina has been compared with both serpulid annelids (e.g. [START_REF] Glaessner | Early Phanerozoic annelid worms and their geological and biological significance[END_REF][START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF] and cnidarians (e.g. [START_REF] Vinn | Inconsistencies in proposed annelid affinities of early biomineralized organism Cloudina (Ediacaran): structural and ontogenetic evidences[END_REF]Van Iten et al. 2014); the lack of preserved soft tissues in these fossils hinders convincing establishment of a palaeobiological interpretation.

While not reported in any other similar fossils, the spindle-shaped structure reported here in the innermost funnel of the granddaughter tube G1 may help provide a clue for phylogenetic placement of Feiyanella and, by extension, potentially other Cloudina-type organisms. This structure is three-dimensionally replicated by phosphate minerals, and its spindle-shaped morphology and massive size compared to the tube volume does not support an interpretation of an incompletely preserved funnel or other mineralized structure. Instead, we suggest that the spindle-shaped mass may be the phosphatized remains of the long-awaited tube dweller. Although it is difficult to depict the full morphology of the soft tissue inside the hollow tube, this spindle-shaped structure may represent the degraded organic remains of the organism that lived within these tubes. This interpretation is consistent with the morphological and palaeoecological reconstruction in Conotubus and Cloudina [START_REF] Cai | Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China[END_REF][START_REF] Cai | Tube growth patterns and microbial matrelated lifestyles in the Ediacaran fossil Cloudina, Gaojiashan Lagerstätte, South China[END_REF]; they all possess hollow tubes which allowed for the tube dwellers to move up and down freely. This interpretation is also in accordance with the periodic growth of the tubes; the funnels were not simultaneously secreted by the soft issue, but instead were episodically secreted in the innermost layers.

A cnidarian planula interpretation for Feiyanella is largely compatible, and can be supported by several lines of evidence: (1) The asexual reproduction by longitudinal and transverse fissions of the soft body is more popular in extant cnidarians than in bilaterians. In addition, transverse fission has been reported from the sea-anemone-like cnidarians in the lower Cambrian Kuanchuanpu Formation in South China (Han et al. 2010). ( 2) The corrugated outermost layer of the tube broadly resembles the periderm of medusozoan polyps (Werner, 1973;Jarms, 1991). (3) The four-lobed mor-phology seen in granddaughter tube G2 of Feiyanella shows symmetry comparable to another coeval fossil, Carinachites spinatus Qian, 1977(Qian, 1977;Conway Morris & Chen, 1992) -a Cambrian medusozoan with a phosphatized skeleton showing triradial, tetraradial and pentaradial symmetries (Conway Morris & Chen, 1992;Qian et al. 1997;Liu et al. 2011). (4) A colonial life habit proposed for Feiyanella is common in cnidarian hydropolyps [START_REF] Hyman | The Invertebrates[END_REF] (5) The funnel-in-funnel tube architecture of Feiyanella has also been observed in a few hydrotheca (e.g. Sertularelia quadrata; plates 15 and 16 of Nutting, 1900). ( 6) The proposed closed apical end of Feiyanella is compatible with a cnidarian body plan, but inconvenient for defecating of a bilateral worm with a through gut.

The ontogeny of Feiyanella indicates a possible transition from cylindrical radial symmetry to tetraradial symmetry. A similar transition from cylindrical radial symmetry to triradial symmetry was observed from a coeval fossil, Anabarites, which is also an early Cambrian tubular small shelly fossil suspected to be a cnidarian (Kouchinsky et al. 2009).

It is well known that Cambrian communities are quite different from those of the Ediacaran Period. Cambrian communities, exemplified by the Chengjiang and Burgess Shale biotas, are characterized by a complex food web with diverse types of feeding behaviours [START_REF] Vannier | Early Cambrian origin of modern food webs: evidence from predator arrow worms[END_REF][START_REF] Hou | Collective behavior in an early Cambrian arthropod[END_REF] and complex reproduction strategies [START_REF] Duan | Reproductive strategy of the bradoriid arthropod Kunmingella douvillei from the Lower Cambrian Chengjiang Lagerstätte, South China[END_REF][START_REF] Caron | Waptia and the diversification of brood care in early arthropods[END_REF] similar to modern ocean ecosystems. Ediacaran communities, however, were instead comprised of slow creeping mat feeders and sessile benthic suspension feeders and/or osmotrophic feeders (e.g. [START_REF] Fedonkin | The Rise of Animals: Evolution and Diversification of the Kingdom Animalia[END_REF][START_REF] Laflamme | Osmotrophy in modular Ediacara organisms[END_REF][START_REF] Rahman | Suspension feeding in the enigmatic Ediacaran organism Tribrachidium demonstrates complexity of Neoproterozoic ecosystems[END_REF][START_REF] Wood | Extensive metazoan reefs from the Ediacaran Nama Group, Namibia: the rise of benthic suspension feeding[END_REF]. Although the early Cambrian small shelly fossil community -exemplified by the Kuanchuanpu Formation organisms -contains mat feeders (molluscs) (Qian & Bengtson, 1989) and microscopic predators (cycloneurians) (Liu et al. 2014b;Zhang et al. 2015), this ecosystem was characterized by a high richness and diversity of sessile organisms (i.e. predominantly cnidarians). Although a zoological placement remains unresolved, Feiyanella represents a Cambrian relict containing significant similarities in tube morphology, tube construction and reproduction strategy to some terminal Ediacaran tubular fossils (e.g. Cloudina, Conotubus and Sinotubulites). The early Cambrian small shelly fossil community revealed in the Kuanchuanpu Formation exhibits palaeobiological and palaeoecological features seemingly intermixed between both late Ediacaran and early Cambrian faunas. As such, it may support the notion that the late Ediacaran tubular biotas and their descendants represent important elements in a broader evolutionary prelude of the Cambrian explosion [START_REF] Shu | Birth and early evolution of metazoans[END_REF][START_REF] Schiffbauer | The latest Ediacaran Wormworld fauna: setting the ecological stage for the Cambrian Explosion[END_REF], rather than disappearing from the fossil record in an end-Ediacaran extinction [START_REF] Smith | The end of the Ediacaran: two new exceptionally preserved body fossil assemblages from Mount Dunfee, Nevada, USA[END_REF].

Conclusions

The early Cambrian tubular fossil Feiyanella manica gen. et sp. nov. shares morphological similarities with the late Ediacaran tubular fossils Cloudina, Conotubus and Sinotubulites. Together, these tubular organisms may constitute a monophyletic group in the late Ediacaran through early Cambrian periods. The 'funnelin-funnel' tube construction and the two-ordered dichotomous branching of Feiyanella manica gen. et sp. nov. are all comparable to those reported in Cloudina. The strongly corrugated outermost funnel and the gradational variations in the degrees of folding and/or wrinkling from exterior to interior tube layers of Feiyanella manica gen. et sp. nov. resembles those of Sinotubulites. Feiyanella thus embraces evolutionary continuity of typical late Ediacaran 'funnelin-funnel' tube construction (exemplified by Cloudina and Conotubus), asexual reproduction by dichotomous branching of the tube (exemplified by Cloudina) and a gradational decrease in degrees of wrinkles and/or ornamentation from exterior to interior layers (exemplified by Sinotubulites). These continuities provide pivotal palaeobiological and palaeoecological insights into our understanding of the evolutionary history of the Ediacaran-Cambrian transition.

Introduction

The abrupt appearance of diverse small shelly fossils (SSFs) during the earliest Cambrian signals the initial stages of the Cambrian explosion (G. Li et al., 2007;[START_REF] Maloof | The earliest Cambrian record of animals and ocean geochemical change[END_REF]. It therefore seems axiomatic that SSFs are of great importance for understanding the early rise of metazoan phyla and the origins of animal skeletogeny. Paleoecological reconstruction of SSF communities is a challenging task as the majority of SSFs are fragmentary or consist of isolated sclerites. A critical exception to this rule is the set of phosphatic SSFs in Orsten-type Lagerstätten, for example the Kuanchuanpu Biota in South China (ca. 535 Ma), which together have the potential to provide unique insights into the nature and significance of these fossils thanks to their high potential for exceptional preservation of both hard parts and soft tissues.

Carinachitids are an important component of early Cambrian SSFs in South China (Conway Morris and Chen, 1992). Their gently conical skeletal tubes exhibit several (three to five) transversely ribbed faces separated from each other by wide and deep corner sulci that usually bear fine transverse wrinkles (tw) (Fig. 1). To date, three genera and six species of carinachitids-Emeiconularia trigemme Qian in Qian et al., 1997;E. amplicanalis Liu et al., 2005 (Fig. 1.1); Pentaconularia ningqiangensis Liu et al., 2011 (Fig. 2); Carinachites spinatus Qian, 1977 (Figs. 3-5); C. tetrasulcatus Jiang in [START_REF] Luo | The Sinian-Cambrian Boundary in Eastern Yunnan: Kinming, People[END_REF]and C. curvatornatus Chen, 1982-have been reported from the Kuanchuanpu Formation and equivalent horizons in South China (Qian, 1977;[START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF]Conway Morris and Chen, 1992;Qian et al., 1997;[START_REF] Liu | Two new species of protoconulariids from the early Cambrian in South Shaanxi, China[END_REF]Liu et al., , 2011)). These fossils collectively exhibit tri-, tetra-, or pentaradial symmetry in transverse sections (Liu et al., 2011), and these symmetries may have arisen independently in different lineages (Han et al., 2016a, b). In addition, the tube wall appears to have been flexible and composed of organic material and/or calcium phosphate (Conway Morris and Chen, 1992;[START_REF] Qian | Classification and early evolution of different group of SSFS[END_REF].

Carinachitids, together with co-occurring hexangulaconulariids [START_REF] Yue | Embryonic and post-embryonic development of the early Cambrian cnidarian Olivooides[END_REF][START_REF] Van Iten | Cambrian, South China): Implications for the affinities of conulariid-like small shelly fossils[END_REF] and olivooids 1 (see Steiner et al., 2014), have been assigned to the order Conulariida of the subclass Conulata [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF][START_REF] Qian | Classification and early evolution of different group of SSFS[END_REF]. Because carinachitids and hexangulaconulariids are very small (<5 mm long) and lack the facial midline and carina typical of many Ordovician and younger conulariids, they have been classified as protoconulariids [START_REF] Qian | Classification and early evolution of different group of SSFS[END_REF]. The zoological affinities of Conulata have been controversial [START_REF] Babcock | The phylum Conulariida[END_REF][START_REF] Brood | Morphology, structure, and systematics of the conulariids[END_REF], but the group is now generally assigned to the subphylum Medusozoa of the phylum Cnidaria [START_REF] Bengtson | Fossilized metazoan embryos from the earliest Cambrian[END_REF]Van Iten et al., 2006, 2010, 2014). Phylogenetic relationships among protoconulariids remain poorly understood as all previously collected specimens of carinachitids lack both the apical and apertural regions, and thus their complete morphology and growth patterns are unknown. Here we describe a tetramerous The sagittal positions of (1a-1e) are indicated respectively by '1a, ' '1b,' '1c,' '1d,' '1e,' and '1f' in (1f, 1g). The horizontal levels of (1f, 1g) are respectively indicated by '1f ' and '1g' in (1a-1f)

Materials and methods

Specimens of Carinachites spinatus were obtained from samples of phosphatic limestone collected from the Kuanchuanpu Formation in southern Shaanxi Province, South China, and digested in 10% acetic acid. Specimens ELISN148-52, ELISN93-45, ELISN93-157, ELISN19-20, ELISN23-240, and ELISN12-154 come from the Shizhonggou section in Ningqiang County, while specimen XX30-127 is from the Yangjiagou section in Xixiang County (for localities, see Steiner et al., 2007, fig.1). All specimens were coated with gold and then imaged using an FEI Quanta 400 FEG scanning electron microscope (SEM). Micro-CT data for specimen ELISN148-52 were acquired at the Tohoku University (Fig. 2) and were processed using VG Studio 2.2 Max for 3D reconstructions. The terminology used herein mostly follows that of Conway Morris and Chen (1992)

Results

Tube morphology of Carinachites spinatus.-Carinachitids are abundantly represented by tetramerous C. spinatus Qian 1977 in the Kuanchuanpu Formation in the Shizhonggou section in Shaanxi Province. The tube of this species exhibits four prominent, equidimensional convex faces separated from each other by deep corner sulci 2.6). Each face usually bears a longitudinal series of arcuate transverse ribs that range in shape from simple welts to more complex folds (Conway Morris and Chen, 1992). The distance between adjacent ribs increases slightly toward the wide or oral end of the tube (Fig. 1.3). Near the facial midline, the region between any two adjacent ribs exhibits several, mutually parallel or irregular, longitudinal striated folds that in most cases are separated from each other by an inconspicuous shallow groove (Fig. 1.4).

The transverse ribs in some specimens are arcuate near the distal end and as wide as the faces (Fig. 1.3, 1.4), while in other specimens the ribs consist of a prominent, sharp, thorn-like spine (ts) such as those exhibited by specimens ELISN93-45 (Fig. 1.5) and ELISN148-52 (Fig. 1.6). The ribs on any two neighboring faces usually are located at the same levels above the apex, but some ribs exhibit longitudinal offset (Fig. 1.3, 1.4) (Conway Morris and Chen, 1992). In addition, the ribs of some specimens are offset along the facial midline (Conway Morris and Chen, 1992, fig. 6.1). The apertural region, preserved only in the relatively large specimen ELISN148-52, is superficially dome-shaped (Fig. 1.6-1.9). The maximum diameter of the tube is approximately 1.3 mm, but near its apertural end the tube tapers rapidly, with the faces curving smoothly toward the longitudinal axis of the tube and becoming more or less perpendicular to it. Close to the longitudinal axis, the faces and intervening corner sulci are inclined toward the aboral end of the tube (Fig. 1.6, 1.7, 1.9). Present on each face, at the summit of the aperture, is a triangular tongue-shaped structure, and the distal ends of the faces almost meet near the longitudinal axis of the tube, leaving just a narrow central opening. One of the tongue-shaped structures projects much farther than the others beyond the aperture (Fig. 1.6). The tongue-shaped structures are not flat features but rather fold-like structures having two main, arched sides separated by two flanks. For this reason, we term the tongue-shaped structures 'plicate apertural lobes' (pal). The vertical abapertural side of the apertural lobes extends far into the apertural opening and exhibits a medial subtriangular groove bordered by two elevated flanks (Fig. 1.8, 1.9). The abapertural side is either flat or outwardly convex with a central ridge, and there are many longitudinal striations on the two sides. These striations converge on the tip of the lobes, and there are some oblique, irregular cracks on the striated surface (Fig. 1.11, 1.12). Situated peripheral to the apertural lobes are four longitudinal rows of nearly evenly spaced, nose-shaped, thorn-like spines aligned along the lateral sides of the faces. In addition, the distance between the apertural lobes and the marginal thorn-like spines is approximately equal to the distance between adjacent thorn-like spines. The adapertural side of the lobes, which is more or less perpendicular to the lateral faces, is concave aborally. The abapertural side, like the bridge of a nose, is inclined at approximately 30º to the lateral faces. The four corner sulci, which are substantially lower than the apertural lobes, follow the inward foldings of the adjacent faces and extend far into the tube cavity (Fig. 1.9). The external surface of the sulci is highly and irregularly folded and exhibits fine parallel striations. The summit of the four corner sulci is evidently lower than that of the faces (Fig. 1.9). Clearly, then, the tube aperture, including the inwardly folded portion, is a smooth continuation of the faces and corner sulci.

Tube wall and internal anatomy.-Micro-CT observations confirm the presence of a narrow apertural opening and the continuation of the tube walls into the inwardly and downwardly folded apertural lobes (Fig. 2.1a-e). The tube wall of carinachitids generally exhibits a prismatic inner layer of uniform thickness and a granular outer layer that is much thinner in the corner sulci than in the faces (e.g., Conway Morris and Chen, 1992, fig. 8.19;Qian et al., 1997, plate 2, 1c, 3c;Liu et al., 2005, plate 2, 1e, j;Liu et al., 2011, fig. 2f-g). The prismatic layer originally was thought to consist of overgrowths of diagenetic apatite, while the granular layer was thought to have been originally organic but later replaced by diagenetic apatite [START_REF] Qian | Classification and early evolution of different group of SSFS[END_REF]. Although we are mindful of possible preservational artefacts, Micro-CT imaging revealed that the thickness of the tube wall in specimen ELISN148-52 appears to vary, and that the apertural walls are much thinner than the lateral tube walls (Fig. 2.1a, 2.1c, 2.1e). High magnification imaging revealed a single-layered wall in the apertural region (Fig. 2.1c) and bilayered lateral tube walls (white arrows in Fig. 2.1a, 2.1g). In addition, the facial walls are thicker than those of the corner sulci (Fig. 2.1a, 2.1g). Finally, both the thorn-like spines and the apertural lobes are hollow (Fig. 2.1b).

Present within the tube is a short, subcylindrical mass measuring ~200 μm in diameter and 400 μm in length. The upper part of this feature is in direct contact with the inward folds of the faces and corner sulci (Fig. 2.1b-e), and it is connected to the lateral tube wall by numerous fine, straight filaments (mmf) (Fig. 2.1a-g). No additional details of the subcylindrical mass can be discerned.

Discussion

Relic soft-tissue of Carinachites spinatus.-Similar fine filaments commonly occur within associated fossils, including poorly preserved Carinachites (Conway Morris and Chen, 1992, fig. 7.9-7.11), other tubular microfossils (e.g., Steiner et al., 2014, figs. 7.21, 11.9, 11.12, 11.15), and egg envelopes with partly decayed embryos (e.g., Steiner et al., 2014, fig. 4.6-4.9). Because these internal filaments are generally interpreted as microbial in origin and derived from partly decomposed soft tissue [START_REF] Yue | Embryonic and post-embryonic development of the early Cambrian cnidarian Olivooides[END_REF], we interpret the subcylindrical mass surrounded by these fine filaments in specimens of C. spinatus as remains of soft tissue that underwent partial decay.

Relic soft tissue similar to that near the tube aperture of Carinachites spinatus also is present in Hexaconularia (Steiner et al., 2014, fig. 7.18), Olivooides (e.g., P. Li et al., 2007, fig. 4d;Steiner et al., 2014, figs. 10.3, 11.6, 11.12, 11.15), and Quadrapygites (e.g., Steiner et al., 2014, figs. 14.19, 15.13, 15.15). In addition, an intact, trumpet-shaped mass of relic soft tissue consisting of an upper calyx and a slender basal stalk extending to the aboral end has been documented in Olivooides (see Steiner et al., 2014, fig. 12.7-12.10). The presence of relic soft tissues in Olivooides embryos has been demonstrated convincingly by the discovery of exceptionally well-preserved, primary internal anatomy (Han et al., 2016b). All of these relic soft-tissue structures are smaller in diameter than the surrounding tube wall. Whether the soft tissues of Carinachites spinatus reached the aboral end of the tube cannot be determined at present.

Growth of the tubes of Carinachites spinatus.-The thorn-like spines on the transverse ribs of this taxon resemble the plicate apertural lobes in many respects including shape, size, surface ornament, spacing, and tip directions. Thus, it seems clear that the spines and lobes are substantially the same kind of structure and are simply located in different positions on the faces. The tube aperture was a kind of extracellular matrix that was most likely secreted by epithelial tissue at the oral end, as only in this area was the tube in direct contact with the soft body. If this hypothesis is correct, then one can make the following additional inferences: (1) the thorn-like spines most probably were derived from the plicate lobes and not vice versa. Delimited by the corner sulci, the transverse extension of the two flanks of the plicate lobes may have undergone ontogenetic transformation into the lateral face ribs. The abapertural groove of the plicate lobes was transformed into the middle groove or the central ridges between adjacent ribs. The converging striations on the Han et al.-Olivooides-like tube aperture in carinachitids plicate lobes are equivalent to the striated folds on the lateral thorn-like spines. (2) The distance between neighboring ribs on the same face is approximately equal to or less than the radius of the tube aperture and the depth of the inward portion of the plicate lobes. The displacement of the ribs reflects the displacement of the segmented faces and the corner sulcus and thus the migration of the entire tube aperture. (3) Following the previous inward portion of tube aperture, new inward portion skeleton was secreted by the epithelium of soft tissue at the oral region. The new skeleton may have been primarily attached with the epithelium, and afterward the new apertural parts may have detached with the epithelium of the oral region and been pushed onward and outward with centrifugal expansion, finally being displaced to the lateral side of the tube and becoming the lateral components of the lateral walls. (4) Periodic renewal of the tube aperture necessarily led to orally addition of iterated ribs and 'segmentation' of the faces. Together, these processes reflect the growth of the tube by apertural extension (Fig. 4.4-4.6).

The multiple thorn-like spines on specimens ELISN93-45 (Fig. 1.5) and ELISN148-52 (Fig. 1.6) indicate that the morphology of the ribs on the faces of a single individual is essentially uniform and constant, without gradual transformation from welts to arcuate ribs or other, more complex folds. Probably, this replacement began at the basal end of the tube and continued to the upper part without metamorphosis. If this hypothesis is correct, then the specimens of C. spinatus described in Conway Morris and Chen (1992) are likely a mixture of several species. Specimens with sharp, thorn-like spines or arcuate ribs as well as welts should be reinterpreted as different species rather than different developmental stages, unless these variants can be shown to co-occur in the same individual of C. spinatus.

In addition to sequential adoral addition of ribs on the faces, growth of the Carinachites tube also involved increase in the diameter of the tube and the width of the corner sulci. Along the longitudinal axis, the corner sulcus expands gradually toward the aperture (Fig. 1.5). In the most complete specimen (ELISN93-45), which however lacks the apex and apertural margin, at least 35 ribs are present on each face (Fig. 1.5). If the tube could grow up to 1.3 mm in width, as indicated by specimen ELISN148-52 (Fig. 1.6), then we infer that a single face may have contained at least 130 ribs over a total length of approximately 25 mm. With regard to the apertural extension model mentioned in the preceding, neighboring ribs at the same level indicate that the plicate apertural lobes were replaced synchronously by new ones (Fig. 4.4-4.6). In contrast, longitudinal offset of ribs along the corner sulci may reflect diachronous replacement of previously formed plicate lobes, indicating that the tube opening was always more or less partly closed. The corner sulci are generally thinner and more flexible than the faces (Qian et al., 1997), and in most fragmentary specimens, the sulci exhibit secondary breakage. Probably in life the flexible corner sulci served as buffer zones that prevented tearing of the tube during diachronous replacement of the ribs on neighboring faces. The reason for longitudinal offset of the ribs along the facial midline (Conway Morris and Chen, 1992, fig. 6.1) remains unclear, though probably each apertural lobe was formed asynchronously by two adjacent subunits of soft tissue. It is important to note that longitudinal offset or asynchronous displacement of the ribs also can be seen on hexangulaconulariids, but it has never been observed in olivooids.

Tube morphology and growth of other carinachitids.-Since the ribs most likely constitute displaced plicate lobes, the four uniform plicate lobes in tetraradiate Carinachites spinatus correspond to the four rows of lateral facial ribs. Similarly, triradiate Emeiconularia and pentamerous Pentaconularia ningqiangensis Liu et al., 2011 most likely possessed three (Fig. 4.1, 4.2) and five centripetal plicate lobes (Fig. 4.7-4.9), respectively. Similarly, specimens with arcuate ribs reflect the presence of a set of centripetal arcuate lobes in the apertural region. Nevertheless, the apertural region of Carinachites tetrasulcatus [START_REF] Chen | The new knowledge of the fossil assemblages from Maidiping section, Emei County, Sichuan with reference to the Sinian-Cambrian boundary[END_REF]) is difficult to reconstruct as its ribs are low and inconspicuous (Conway Morris and Chen, 1992, fig. 8.22). Probably the aperture of this species resembled a four-sided pyramid with deep, concave corner sulci similar to those of Hexaconularia sichuanensis [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF]Yang, 1986 (e.g., Steiner et al., 2014, fig. 7.13-7.16, 7.19-7.21). We hypothesize that this species exhibited periodical growth similar to that of Carinachites spinatus. However, such eversion probably was possible only in organic or lightly sclerotized exoskeletons and not in those with thick and rigid hard parts such as tubes of Emeiconularia trigemme (Qian et al., 1997). Thus, secondary, subsequent thickening of the lateral walls, suggested by the double-layered wall structure (Fig. 2), is proposed here to resolve conflicts between the flexibility of the primary apertural wall and the rigidity of the thick secondary tube wall. The thin primary apertural wall in Carinachites spinatus, represented by the outer layer with fine transverse wrinkles, may have been rich in organic material. The outer layer may have undergone subsequent thickening on its inner surface by a mixture of inorganic materials (represented by the smooth granular layer), thus resulting in a double-layered structure similar to that of conulariids [START_REF] Brood | Morphology, structure, and systematics of the conulariids[END_REF][START_REF] Ford | Microstructure and composition of the periderm of conulariids[END_REF]. This model could account for: (1) the flexibility of the external tube surface (as indicated by the striations and welts shown in Conway Morris and Chen, 1992) and the relative rigidity of the entire tube wall, (2) the high abundance of fragmentary specimens of carinachitids and the extremely rare preservation of their aperture, and (3) probable variation in mechanical properties between the different layers as reflected in the secondary cracks on the tube surface (Fig. 1.11, 1.12). However, because the relic soft tissue is much smaller in diameter than the host tube, the sides of the soft body may not have been in direct contact with the lateral tube wall. How the organic or inorganic material was deposited on the inner surface of the outer layer remains unknown.

Feeding habits of carinachitids.-Despite the absence of in situ preservation of carinachitid tubes, it was originally assumed that carinachitids were solitary sessile forms having their aboral end attached to hard or firm substrates [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF] as in extant medusozoan polyps. A pelagic habit for carinachitids is unlikely as their skeletonized tube appears to have been too dense to float in seawater. However, because the soft body was almost entirely enclosed within the tube, filter-feeding on microorganisms seems highly likely. Besides the function of supporting the growing soft tissues, periodic tube growth and thickening of carinachitids reflects competition for ecological tiering among a varied benthos. In contrast to the reduced vestigial peridermal theca of cubopolyps and most scyphistomae, the thickening of the tube wall in carinachitids, and coeval tubular fossils (anabaritiids, hyolithelminths), indicate an adaptive strategy focusing on defense against predators such as cycloneuralians (e.g., Liu et al., 2014a;Zhang et al., 2015). Interestingly, suspension feeding by the hypostome rather than normal elongate tentacles with nematocysts has also been observed in extant polyps of Eudendrium (Hydrozoa) [START_REF] Puce | Morphofunctional adaptation to suspension feeding in Eudendrium (Cnidaria, Hydrozoa)[END_REF], in which the mucous-lined gastroderm plays a major role in capturing food particles such as zooplankton. Such behavior correlates with high concentrations of food particles and intense water movement, a scenario that seems compatible with the marine shelf environment favored by Cambrian small shelly fossils [START_REF] Yin | Geological and geographical distribution of SSF, with discussion on early Cambrian geographical provinces[END_REF]Steiner et al., 2004[START_REF] Steiner | Neoproterozoic to early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China): Palaeogeography Palaeoclimatology Palaeoecology[END_REF].

The asynchronous displacement of the ribs in Carinachites spinatus indicates that the flexible tube aperture may have opened to a greater extent in this taxon than in olivooids. Relative to the radius of the tube, both the width and height of the ribs on Emeiconularia amplicanalis are smaller than in E. trigemme. This fact indicates that the oral lobes of E. amplicanalis could only partially cover the tube aperture, thus allowing continual contact of the soft body with the ambient environment. Presumably, retractile tentacles in E. amplicanalis, if present, could protrude beyond the tube opening, thus enabling limited predatorial behavior.

Comparisons

Carinachitids versus coronate polyps.-Carinachitid tubes resemble the chitinous periderm of coronate scyphozoans (i.e., Stephanoscyphus), which are sheathed in a cone-shaped tube showing well-developed longitudinal folds and horizontal annulations [START_REF] Chapman | The Cnidaria and Their Evolution[END_REF][START_REF] Werner | Stephanoscyphus (Scyphozoa, Coronatae) und seine direkte Abstammung von den fossilen Conulata: Helgoländer Wissenschaftliche Meeresuntersuchungen[END_REF]Werner, , 1973)). However, differences between them are also evident. In particular, Stephanoscyphus may be either solitary or colonial. The periderm of colonial forms is irregularly branched (Jarms, 1991), in some cases with a tube-in-tube structure (Werner, 1966, fig. 13). By contrast, carinachitid tubes are exclusively solitary. Second, whereas Stephanoscyphus has an operculum that is separate from the tube, the tube aperture and lappets of carinachitids constitute a continuous extension of the rest of the tube. Third, the scyphozoan periderm, including the peridermal teeth or cusps inside the tube of Stephanoscyphus, is secreted by ectoderm of the lateral body wall. By contrast, the external layer of the carinachitid tube, except for the apex, was generated by epithelium of the oral part, and neither teeth nor cusps are present within the carinachitid tube. Fourth, Stephanoscyphus tubes are more or less circular in transverse cross section and uniform in thickness; by contrast, carinachitid tubes are polygonal and exhibit distinct faces and corner sulci. Fifth, an operculum with triangular cusps is absent in Stephanoscyphus [START_REF] Werner | Stephanoscyphus (Scyphozoa, Coronatae) und seine direkte Abstammung von den fossilen Conulata: Helgoländer Wissenschaftliche Meeresuntersuchungen[END_REF], while oral lobes or lappets are a consistent diagnostic feature of the tube of carinachitids and co-occurring olivooids. Finally, strobilation, a characteristic of Stephanoscyphus, has not been observed in carinachitids. In short, these comparisons suggest that carinachitids may only be distantly related to extant scyphozoans.

Comparison with extant hydrothecae.-Extant, colonial thecate hydranths begin and complete their development within a small, capsule-like hydrotheca. The hydrotheca in some species, for example Sertulariella quadrata Nutting, 1900a, is square in transverse cross section and exhibits dense transverse striations or longitudinal folds (i.e., S. rugosa [START_REF] Linnaeus | Systema naturae[END_REF]) similar to those of carinachitid tubes (Nutting, 1900a). The oral end of the hydrotheca has a protective operculum with or without a set of triangular, plate-like teeth or converging cusps capable of opening and closing [START_REF] Crowell | Regression and replacement of hydranths in thecate hydroids, and the structure of hydrothecae[END_REF]. Many species of Sertulariella, for example S. quadrata, S. rugosa, and S. peculiaris (Leloup, 1935[START_REF] Galea | On a collection of shallow-water hydroids (Cnidaria : Hydrozoa) from Guadeloupe and Les Saintes, French Lesser Antilles[END_REF], have an operculum with four triangular cusps (Nutting, 1900a;[START_REF] Chapman | The Cnidaria and Their Evolution[END_REF][START_REF] Galea | On a collection of shallow-water hydroids (Cnidaria : Hydrozoa) from Guadeloupe and Les Saintes, French Lesser Antilles[END_REF] that are somewhat similar to the lobes of Carinachites spinatus. Symplectoscyphus [START_REF] Millard | Monograph on the Hydroida of Southern Africa: Cape Town[END_REF] and S. rathbuni (Nutting, 1900a) have three teeth similar to those of Emeiconularia (assuming our reconstruction is correct). Notably, the chitinous hydrotheca and operculum are secreted by glandular cells of the epidermis of the hydranth, especially the hypostome [START_REF] Berrill | Developmental analysis of Scyphomedusae[END_REF], thus supporting the previously inferred oral formation of carinachitid tubes. In rare cases, the apertural teeth of the hydrothecae are folded inward as in solitary carinachitid tubes (Nutting, 1900b, pl. 14, fig. 6). Major differences between hydrothecae and carinachitids include: (1) the colonial habit of hydrothecae; (2) the absence of triangular cusps in the lateral walls of hydrothecae; and (3) the teeth in hydrothecae, which are sheet-like, with a free adaxial end, and thus are quite different from those of carinachitids.

Comparison among carinachitids, olivooids, hexangulaconulariids, and conulariids.-Prior to conducting a cladistic analysis of relationships among extant and fossil taxa within Medusozoa, morphological comparisons among olivooids, carinachitids, hexangulaconulariids, and Paleozoic conulariids are necessary. As noted previously [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF], the similarities among the skeletons of olivooids, carinachitids, hexangulaconulariids, and Paleozoic conulariids are striking (Table 1). They include: (1) possession of a superficially cone-shaped tube that almost completely enveloped the soft tissue (Qian and Bengtson, 1989;[START_REF] Sendino | The aperture and its closure in an Ordovician conulariid[END_REF]; (2) tube with serially repeated transverse and longitudinal wrinkles (Qian and Bengtson, 1989) representing periodic growth by oral addition [START_REF] Brood | Morphology, structure, and systematics of the conulariids[END_REF]; (3) fine, regularly spaced longitudinal striations, ~5-10 μm in width, as one of typical features of Olivooides tubes (e.g., Yue and Bengtson, 1999, fig. 2D;Steiner et al., 2014, fig. 12.13, 12.14), are present also on the corner surface of Carinachites tetrasulcatus (e.g., Conway Morris and Chen, 1992, fig. 9.15, 9.16); (4) tube tapered in the apertural region (Qian and Bengtson, 1989), where the tube aperture is folded inward (e.g., [START_REF] Brood | Morphology, structure, and systematics of the conulariids[END_REF]Steiner et al., 2014); (5) all tubes exhibit distinct apical and abapical regions (Fig. 1.8) [START_REF] Van Iten | Cambrian, South China): Implications for the affinities of conulariid-like small shelly fossils[END_REF], although the carinachitid apex is unknown yet; and (6) radial symmetry, a characteristic that is a link to the medusozoans, is well represented by all four families.

Apart from the mentioned similarities, additional specific similarities between carinachitids and olivooids are remarkable:

(1) The cone-shaped tubes of these two taxa exhibit similar variation in the pattern of radial symmetry. Both of them exhibit rare pentaradial symmetry and dominant tetraradial symmetry. However, triradial symmetry is not known in olivooids. ( 2 [START_REF] Yue | Embryonic and post-embryonic development of the early Cambrian cnidarian Olivooides[END_REF]Han et al., 2016a, b;termed 'oral lobes' in Steiner et al., 2014, in Olivooides multisulcatus Qian, 1977) (Fig. 5). ( 3) Lateral ornaments on the tube wall, including plicate thorns (Fig. 5.1) (termed 'plicate cornice' in O. multisulcatus [see Han et al., 2016a, fig.2] and 'triangular thickening' by Steiner et al., 2014, fig.10), are derived from the tube aperture [START_REF] Yasui | A diploblastic radiate animal at the dawn of Cambrian diversification with a simple body plan: Distinct from Cnidaria?[END_REF]. ( 4) As mentioned in the preceding, the soft tissues are always connected to the tube aperture. ( 5) There are similar patterns of tube formation except for lateral thickening, in both cases with tube formation mediated by soft tissue at the oral end [START_REF] Yasui | A diploblastic radiate animal at the dawn of Cambrian diversification with a simple body plan: Distinct from Cnidaria?[END_REF]Liu et al., 2014b;Han et al., 2016a, b). ( 6) Rare preservation of the apertural end in olivooids and carinachitids as well as Paleozoic conulariids probably indicates that the newly secreted tube aperture was predominantly organic or weakly sclerotized and thus less resistant to decay than the lateral ribs.

Differences between carinachitids and olivooids also are evident. Although carinachitid tubes bear regular ribs, they were never compressed during diagenesis along the longitudinal axis as in olivooid tubes. This difference may be partially attributed to the presence in carinachitids of deeply concave corner sulci, outwardly bulging faces, and later ontogenetic thickening, thus providing stronger support for the soft body. The face-corner configuration in carinachitids also reflects an incipient differentiation of the meridian planes. The apertural lobes among different taxa of olivooids vary greatly in morphology. Thus, unlike Olivooides multisulcatus (Fig. 5.1-5.3), Quadrapygites and O. mirabilis Yue, 1984in Xing et al., 1984 lack clear differentiation in size between the principle apertural lobes and the adradial apertural lobes. By contrast, carinachitids exhibit only the principle apertural lobes. The corner sulci in carinachitids may correspond to the adradial apertural lobes in olivooids. In addition, carinachitids exhibit greater morphological variation on the faces than do olivooids, including variation in rib shape and height, displacement of ribs along the midline, and convergence of the striations. The facial ribs of carinachitids may represent a derived feature in comparison with the continuous transverse crests in olivooids. Finally, it has generally been accepted that the periderm of olivooids was organic and uniform in thickness. By contrast, the tubes of carinachitids, hexangulaconulariids, and conulariids, although showing some degree of flexibility, are relatively thick and slightly mineralized (e.g., [START_REF] Brood | Morphology, structure, and systematics of the conulariids[END_REF]Qian et al., 1997;[START_REF] Leme | Cladistic analysis of the suborder Conulariina Miller and Gurley, 1896 (Cnidaria, Scyphozoa; Vendian-Triassic[END_REF][START_REF] Ford | Microstructure and composition of the periderm of conulariids[END_REF].

Similarities between carinachitids and hexangulaconulariids include: (1) sclerotization of the tube wall, (2) development of faces and corner sulci, (3) transverse ornament showing displacement/offset along the midline of the faces and corner sulci, and (4) sessile benthic mode of life on firm substrates or hard parts (e.g., Van Iten et al., 2016a). The displacement mechanism of carinachitids may have also been present in hexangulaconulariids (Conway Morris and Chen, 1992, fig. 11.12;Van Iten et al., 2010, fig. 2e) and latest Ediacaran Paraconularia (e.g., Van Iten et al., 2014, fig. 3c-d;Van Iten et al., 2016b). In this connection, it should be noted that relics of small soft parts extending along the tube axis of conulariids, originally interpreted as remains of an alimentary tract [START_REF] Babcock | The enigma of conulariid affinities[END_REF], most likely represent polyps as previously suggested by [START_REF] Van Iten | Evolutionary affinities of conulariids[END_REF] and supported by currently available material of carinachitids and the internal anatomy of Olivooides (e.g., Han et al., 2016a).

Differences between carinachitids and hexangulaconulariids also are evident. In particular, the pseudohexaradial symmetry of hexangulaconulariid tubes, which exhibit a fundamental bimerous tetraradial symmetry, reflects further morphological differentiation of the meridian planes within a framework of tetraradial symmetry. Such meridian plane differentiation, oriented perpendicular to the longitudinal axis, probably indicates unknown differentiation of soft part structures such as gonads, septa, and the vascular system. Finally, the apertural lobes of hexangulaconulariids are not triangular as in carinachitids and olivooids.

Carinachitids share detailed similarities with Paleozoic conulariids (except for Cambrian Baccaconularia in [START_REF] Hughes | Late Cambrian conulariids from Wisconsin and Minnesota[END_REF] in face/corner sulcus differentiation and formation of apertural lobes (equivalent to the apertural lappets of [START_REF] Sendino | The aperture and its closure in an Ordovician conulariid[END_REF], and both taxa exhibit tri-, tetra-or pentaradial symmetry. However, biradial symmetry, common in conulariids, has not been observed in carinachitids. In addition, the corners of some conulariids, for example Eoconularia loculata (Wiman, 1895[START_REF] Jerre | Anatomy and phylogenetic significance of Eoconularia loculata, a conulariid from the Silurian of Gotland[END_REF], are much thicker than the faces [START_REF] Jerre | Anatomy and phylogenetic significance of Eoconularia loculata, a conulariid from the Silurian of Gotland[END_REF], contrasting with the relatively thickened faces of carinachitid Emeiconularia trigemme (Fig. 2.1b). Moreover, in addition to plicate apertural lobes [START_REF] Ford | Microstructure and composition of the periderm of conulariids[END_REF], conulariids Journal of Paleontology exhibit two other types of apertural lobes [START_REF] Sendino | The aperture and its closure in an Ordovician conulariid[END_REF]. Finally, the internal anatomy of the tube wall of conulariids is much more complex at the corners and midlines than in carinachitids, as summarized by [START_REF] Van Iten | Evolutionary affinities of conulariids[END_REF], 1992b). For example, there are eight types of internal midline structures [START_REF] Bischoff | Internal structures of conulariid tests and their functional significance, with special reference to Circonulariina n. suborder (Cnidaria, Scyphozoa)[END_REF][START_REF] Van Iten | Evolutionary affinities of conulariids[END_REF], 1992b;[START_REF] Jerre | Anatomy and phylogenetic significance of Eoconularia loculata, a conulariid from the Silurian of Gotland[END_REF], including:

(1) a single continuous (nonseriated) carina, and (2) a pair of continuous carinae (flanking the midline), (3) a pair of seriated carinae, (4) a single seriated carina (subsequently discovered by Hughes et al., 2000 in Baccaconularia), and ( 5) the Y-shaped continuous single carina documented by [START_REF] Jerre | Anatomy and phylogenetic significance of Eoconularia loculata, a conulariid from the Silurian of Gotland[END_REF] in Eoconularia loculata (Wiman). The corners may be: (1) nonthickened, (2) thickened without formation of a clear carina, (3) thickened and bearing a distinct nonseriated carina, or (4) thickened and bearing a seriated distinct carina. In summary, gross morphological comparisons of the skeletons of olivooids, carinachitids, hexangulaconulariids, and Paleozoic conulariids support the previous hypothesis [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF]) that these fossil taxa represent closely related lineages within the Conulata. Since the olivooid soft body exhibits a manubrium within a subumbrellar cavity, tentacles, apertural lappets, and frenula (Han et al., 2016a, b), olivooids and hence all conulatans probably were medusozoans (Van Iten et al., 2006) that were related either to extant cubozoans (Han et al., 2013;Han et al., 2016a,b) or to scyphozoans (Dong et al., 2013;Liu et al., 2014b;Van Iten et al., 2014). The proposal that Conulata constitutes an independent phylum [START_REF] Babcock | The phylum Conulariida[END_REF][START_REF] Brood | Morphology, structure, and systematics of the conulariids[END_REF] or the internal rachis of sea pens (Conway Morris and Chen, 1992) appears unlikely. Carinachitids, originally interpreted as the most primitive taxa within Conulata [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF], are interpreted here as a stock of phylogenetically intermediate forms between olivooids and hexangulaconulariids. The presence of corner sulci and faces with a median line (midline) probably represent synapomorphies of carinachitids, hexangulaconulariids, and conulariids. The general similarities shared by olivooids, hexangulaconulariids, carinachitids, and conulariids (i.e., radial symmetry), probably represent primitive conditions. Finally, the bimerous tetraradial symmetry of hexangulaconulariids may have been independently acquired in this lineage. However, these interpretations await future phylogenetical analysis.

Orientation of the radial symmetry planes in carinachitids.-Similarities in gross morphology between carinachitids, olivooids, and conulariids suggest that their peridermal apertural lobes are homologous structures. If this hypothesis is correct, then the orientation of the meridian planes of olivooids and Olivooides-like medusozoans (Han et al., 2013(Han et al., , 2016a, b) , b) may shed new light on the orientation of these planes in carinachitids and conulariids. In the soft body of Olivooides, the perradial frenula and apertural lappets, which correspond in position to the perradial pockets (e.g., Han et al., 2013, fig. 3), probably were responsible for the formation and closure of the plicate lobes of the periderm (Han et al., 2016a, figs. 3-5). Apart from the adradial frenulae and apertural lappets, no frenulae or apertural lappets are present in the interradii, where the interradial septa connect the subumbrellar and exumbrellar walls, and there is no interradial apertural lobe on the peridermal tube (Fig. 5). Similarly in carinachitids, the bulging faces and corner sulci may directly reflect the configuration of the tube aperture, and they may correspond in position, respectively, to the perradial pockets and interradial septa of the gastric cavity. This means: (1) that the midline of the facial ribs and the corner sulci were most likely located at the perradii and interradii, respectively; and (2) that the corner sulci may correspond to the former interradial septa/mesenteries (Fig. 3). This orientation may also apply to conulariids if indeed their apertural lobes are homologous with those of carinachitids and olivooids. It should be noted, however, that our suggested orientation of the interradial symmetry planes in conulariids differs from the traditional hypothesis, which is based on similarities between the conulariid and coronate periderms and between the midline carinae of Eoconularia loculata Wiman and the gastric septa of stauromedusans (Van Iten et al., 2006). According to this hypothesis, the apertural lobes and facial midlines in conulariids were situated at the interradii [START_REF] Chapman | The Cnidaria and Their Evolution[END_REF][START_REF] Werner | Stephanoscyphus (Scyphozoa, Coronatae) und seine direkte Abstammung von den fossilen Conulata: Helgoländer Wissenschaftliche Meeresuntersuchungen[END_REF]Van Iten, 1992a;[START_REF] Jerre | Anatomy and phylogenetic significance of Eoconularia loculata, a conulariid from the Silurian of Gotland[END_REF]. Confirming or disproving this hypothesis will require the discovery of additional and better-preserved relic soft tissues in conulariids.

Conclusions

A single, exceptionally well-preserved specimen of Carinachites spinatus, documented for the first time in the present paper, reveals that the apertural end of the skeletal tube of tetraradial carinachitids exhibits four plicate lobes that are similar to those of co-occurring olivooids and younger conulariids. Similarities between the lateral tube spines and the apertural lobes of carinachitids indicate that all of the transverse ribs on the faces were released adorally and were eventually displaced toward the edges of the tube, a pattern of growth similar to that of co-occurring olivooids. The internal anatomy and symmetry of Olivooides suggest a perradial and interradii disposition, respectively, for the four faces and corner sulci of carinachitids. These findings corroborate the previously proposed hypothesis that early Cambrian carinachitids, hexangulaconulariids, olivooids, and conulariids are closed related taxa within the subphylum Medusozoa, although olivooids may have retained certain primitive features.

T H E small shelly fossil (SSF) assemblages of the Kuanchuanpu Formation of South China (Shaanxi Province; Fortunian Stage, Terreneuvian Series, c. 535 Ma) contain abundant skeletal elements [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF][START_REF] Qian | Taxonomy and Biostratigraphy of Small Shelly Fossils in China[END_REF]) but also remarkably preserved soft-bodied organisms such as embryonic and larval stages of cnidarians (Conway Morris & Chen 1992;[START_REF] Bengtson | Fossilized metazoan embryos from the earliest Cambrian[END_REF]Steiner et al. 2004;Dong et al. 2013Dong et al. , 2016;;Han et al. 2013Han et al. , 2016a)), ecdysozoans (Liu et al. 2014;Zhang et al. 2015) and also possible meiobenthic deuterostomes (Han et al. 2017a ). These exceptional fossils provide key information on the early stages of animal evolution near the Ediacaran-Cambrian boundary and before the Cambrian Explosion sensu stricto.

Recent studies using scanning electron microscopy (SEM) and x-ray microtomography (XTM) provide very detailed morphological information on these early organisms (Chen & Dong 2008;Han et al. 2010Han et al. , 2013Han et al. , 2016aHan et al. , 2017b, c;, c;Dong et al. 2013). Steiner et al. (2004) described a scyphopolyp with many filamentous tentacles, Han et al. (2010) the earliest sea-anemone-like cnidarian and Dong et al. (2013) a possible ephyra-like medusa with pentaradial symmetry. The oldest representatives of Cubozoa (box jellyfish) were also discovered in the Kuanchuanpu Formation (Han et al. 2013). The exquisite preservation of their septal structures allowed unprecedented detailed comparisons with their modern counterparts (Han et al. 2013(Han et al. , 2016a, b;, b;Toshino et al. 2015). Liu et al. (2014) and Zhang et al. (2015) described two minute scalidophoran worms, which provide insight into the early evolution of ecdysozoans, a major animal clade that rapidly became dominant in the later Cambrian marine ecosystems exemplified by the Chengjiang biota.

Although the anatomy of many of these early cnidarians and ecdysozoans has been reconstructed in detail, important aspects of their palaeobiology, taphonomy, systematics and phylogeny remain poorly understood or debated, especially their cycle of development from embryo to adult, their taxonomy and their phylogenetic relationships to extant groups (Dong et al. 2013(Dong et al. , 2016;;Han et al. 2013). Numerous forms have been kept in open nomenclature in order to avoid potential problems of synonymy until additional specimens can be obtained.

One of the most studied forms, Olivooides Qian, 1977 (see [START_REF] Bengtson | Fossilized metazoan embryos from the earliest Cambrian[END_REF] for a reconstructed developmental cycle), is a spherical organism with an external stellate ornament that is assumed to represent the embryonic stage of a larger tubular animal from the same locality that has been described as Punctatus [START_REF] Bengtson | Fossilized metazoan embryos from the earliest Cambrian[END_REF]Yue & Bengtson 1999a, b;Yao et al. 2011, fig. 3b1). Recent studies (Dong et al. 2013;Han et al. 2013) have also revealed that Olivooides-like fossils had a pentaradial anatomy as evidenced by the connection of the gastric septa (mesenteries) to the double-layered body wall and the arrangement of tentacles and gonad lamellae. As a whole, this body plan is comparable to that of some extant medusozoans such as cubomedusozoans (Han et al. 2013) or scyphomedusozoans (Dong et al. 2013(Dong et al. , 2016)). The symmetry of these embryonic fossils is dominantly pentamerous, but some of them are tetraradial (Han et al. 2016b) as in the vast majority of extant cnidarians. Although the Olivooides-like cnidarians from the Kuanchuanpu biota are prehatched embryos with virtually the same overall external morphology as Olivooides, they display a greater variety of internal structures. We describe here a new genus and species, Sinaster petalon gen. et sp. nov., based on exceptionally well-preserved prehatched embryos from the Kuanchuanpu Formation. Although Sinaster resembles Olivooides-like fossils in its overall shape, its internal structures clearly distinguish it from other co-occurring embryos.

TERMINOLOGY AND ORIENTATION OF EMBRYONIC MEDUSOZOANS

The terminology used here is that of previous works on Cambrian and extant cnidarians [START_REF] Hyman | The Invertebrates[END_REF]Thiel 1966;Werner 1973;Thomas & Edwards 1990;Gershwin & Alderslade 2006;Han et al. 2013Han et al. , 2016a, b), b).

The definition of the meridian plane in these specimens follows Han et al. (2013Han et al. ( , 2016a, b), b). We follow the current classification of Cnidaria into the Class Anthozoa and the Subphylum Medusozoa which encompasses the following classes: Cubozoa (box jellyfish and sea wasps), Hydrozoa (hydroids and hydra-like animals), Scyphozoa (true jellyfish), Staurozoa (stalked jellyfish) (Daly et al. 2007).

Cubomedusa and Scyphomedusa are used here to designate the medusoid stages of Cubozoa and Scyphozoa.

Abbreviations. aal, adradial apertural lobe; af, adradial furrow; afl, adradial fold lappet; afr, adradial frenulum; as, accessory septum; cg, coronal groove; cl, claustrum; crm, circumferential muscle; cs, coronal stomach; en + pe, egg envelope + periderm; eu, exumbrella; g, gonad; gl, gonad-lamella; go, gastric ostium; icp, interradial corner pillar; if, interradial furrow; is, interradial septum; ln, lappet node; mb, manubrium; ml, marginal lappet; mp, mesogonial pockets; pal, perradial apertural lobe; pfl, perradial fold lappet; pfr, perradial frenulum; ph, phacellus; pp, perradial pocket; rc, radial canal; sf, septal funnel; sn, septal nodes; sp, suspensorium; stl, stellate ornaments; su, subumbrella; tb, tentacular bud; ve, velarium; *, perradius; ?, interradius.

MATERIAL AND METHOD

Fossil specimens were obtained by acetic acid maceration of phosphatic limestones from the Kuanchuanpu Formation [START_REF] Qian | Taxonomy and Biostratigraphy of Small Shelly Fossils in China[END_REF]) and picked under a binocular microscope. Two specimens were examined under SEM and then investigated with computed x-ray microtomography (XTM) at Tohoku University, Japan, and the synchrotron of Spring-8 in Hyogo, Japan. SEM and XTM (Tohoku University, Japan) were both used to image the external and internal features of the best preserved specimens (Wang et al. 2017, appendices 1, 2). Selected specimens were further examined using synchrotron radiation at Spring-8 in Hyogo, Japan. Thousands of images of virtual thin-sections (Figs 2,5) through each specimen were acquired at a resolution of 1004 9 1004 pixels (XTM) and 1920 9 1920 pixels (synchrotron XTM). XTM data were processed using V.G. Studio 2.2 Max in order to enhance contrast between key internal anatomical features by using artificial colours and to generate detailed three-dimensional reconstructions of the microscopic fossils (Figs 3467). Cladistic analysis was carried out using PAUP* v. 4.0 b10 [START_REF] Swofford | PAUP*. Phylogenetic Analysis Using Parsimony (and Other Methods)[END_REF]. The unweighted analysis of the data matrix (Wang et al. 2017, appendix 3) containing 104 unordered characters taken from [START_REF] Marques | Cladistic analysis of Medusozoa and cnidarian evolution[END_REF], Van Iten et al. (2006a) and Han et al. (2016c).

All specimens are deposited at the Early Life Institute (ELI), Northwest University, Xi'an, Shaanxi Province, China. This material and microtomography data are available on request via Prof. Jian Han.

SYSTEMATIC PALAEONTOLOGY

Phylum CNIDARIA Verrill, 1865 Subphylum MEDUSOZOA Petersen, 1979

Class uncertain

Family OLIVOOIDAE Steiner et al., 2014 Genus SINASTER nov.

LSID. urn:lsid:zoobank.org:act:1BDC6FE9-E8FD-4F93-AE25-ED01169B583C Derivation of name. From the Latin Sina (China) and aster (star), alluding to the star-shaped embryo in transverse cross-section and the Chinese origin of the fossil material.

Diagnosis. Ovoid shape with smooth envelope. Single polar aperture. Body with pentaradial symmetry. Periderm with smooth external surface, no stellate ornament; five perradial folded lobes and five pairs of interradial folded lobes equally distributed around the peridermal aperture. Soft-tissues: spacious subumbrella cavity with five pairs of small hollow interradial tentacles inserted on the inner wall of the subumbrella and directed toward the oralaboral axis of the animal; bell margin with perradial and adradial apertural lappets, each corresponding with overlying folded lobes and underlying frenulae; five spacious perradial pockets, separating subumbrella far from exumbrella; exumbrella and subumbrella connected by long interradial septa with no free ends; two sub-interradial accessory septa, sprouting from the exumbrella wall, relatively close to abaxial root of each interradial septum; one pair of gonad-like lamellae on each side of the middle part of the interradial septum; short cone-shaped manubrium projecting deep into the broad subumbrellar cavity.

Sinaster petalon sp. nov. Diagnosis. As for genus.

Occurrence. Lower Cambrian Kuanchuanpu Formation (equivalent to the Fortunian Stage of the Terreneuvian Series), Ningqiang County, Shaanxi Province, China.

DESCRIPTION

External morphology

The holotype of S. petalon (ELISN 89-107) has a spherical bell shape (c. 600 lm in diameter) and an inconspicuous, thin egg envelope (Fig. 1B,E). The embryonic periderm is smooth over its entire surface (Figs 1B, 3, 5A) except near the peridermal aperture. The 15 centripetal, tongue-shaped folded lobes around the periderm aperture display a pentamerous symmetry along the oral-aboral axis (Fig. 1B). Five of them are larger and folded and are designated here as perradial folded lobes. They are intercalated with five pairs of smaller, narrower folded lobes designated as adradial folded lobes. The boundary of folded lobes fades away aborally. Each folded lobe shows a set of vaulted shallow wrinkles, variable in number, arching toward the central peridermal aperture (Figs 1B,E;3A). This pattern of folded lobes (5 + 10) is relatively common in the pentamerous embryos (e.g. ELISN 31-179; Fig. 1A) from the Kuanchuanpu biota.

The paratype of S. petalon has the same size and external morphology as the holotype and lacks the typical external ornament of Olivooides. Its periderm is damaged and incomplete. It shows a pentaradial symmetry with five perradial triangular blade-like structures located near the aperture, and rooting from the inner layer (Fig. 1C). Two furrows are visible along the perradii and are designated here as an adradial furrow (af) and an interradial furrow (if) (Fig. 1F). Many thin shallow coronal grooves (cg) occur between the two furrows along the perradial on the outer layer (Fig. 5D).

Internal anatomy

XTM of the holotype reveals fine details of the internal features of S. petalon (Figs 2, 3). The egg envelope and the periderm seem to be fused with no conspicuous perivitelline space in between. However, there is a larger interspace between the periderm and the enclosed soft-tissue (Fig. 3A), designated here as the peridermal cavity. Transverse cross-sections through the holotype soft tissues show an outer circular thin layer (c. 10 lm in thickness) and an inner pentagonal thicker layer (c. 15 lm thick). The pentagonal section of the inner layer becomes rounded toward the bell margin. Apart from the medusoid rim, the outer layer is attached to the inner layer via five evenlyspaced interradial septa (Fig. 2F-I), which partition the gastric cavity into five spacious perradial pockets (pp; Han et al. 2013). Each interradial septum stretches about one-third of the height of the body (is; Fig. 3B) and lacks a free adaxial end (Fig. 2I). In the holotype, the interradial septum (c. 180 lm in maximal length and triangular in cross-section) tapers towards the aboral-oral axis, at the mid-level of the bell. The interradial septum also diminishes towards the bell rim to become invisible near to the aperture.

Oral views display five large perradial lappets and five pairs of smaller adradial lappets, which are rooted in the margin of the outer layer and seem to correspond to the perradial and adradial folded lobes of the peridermal sheath (Figs 2B-C, 3B). Five pairs of tiny buds occur at about three-quarters of the height of the bell (Fig. 2F). They are superficially rooted in the adaxial side of the inner layer and are directed horizontally toward the oral-aboral axis (Fig. 2F). Each bud is stout, being c. 45 lm long and c. 30 lm wide. A short cone-like shaft (c. 85 lm high and c. 35 lm wide) with a rounded base is present within the inner cavity below the level of the paired tiny buds (Fig. 2A,I). Above it, a large dark solid mass with a spiny appearance pierces the inner layer and extends into the perradial pockets (Fig. 2A, F-I; voids filled by minerals). This solid structure may be diagenetic in origin. Five pairs of vertical strip-like curved adradial septa (Fig. 2G) occur in the perradial pockets. They project bilaterally from the inner surface of the outer layer, and stand quite close to the proximal end of the interradial septa (Fig. 2H,I). Each adradial septum is c. 90 lm long and curves toward the perradial pockets. Its maximal length occurs at the middle level of the bell (Fig. 2G,H). Five pairs of tiny pisolitic lamellae, merely c. 15 lm long, occur in the middle of each interradial septum. They lie closer to the aboral end of the bell than to the adradial septa (Figs 2H;3B,H). Viewed in oral sections, there are 5 thin, nose-like perradial structures and 10 pairs of tiny adradial structures situated above the paired tiny buds (Fig. 2E). The oral end of these nose-like structures is connected to a distinct skirt-like circular tissue around the cavity margin of the inner layer (Figs 2,3D). This circular tissue shows a connection with the apertural lappets and its diameter (180 lm) is much less than that of the cavity of the inner layer. Five concentric striated strips occur around the skirt-like circular tissue and are interrupted at each interradius (Figs 3E,G). Each strip consists of four to five bundles of fibres, with each fibre having a maximal diameter of about c. 3 lm (Fig. 3E). The same fibres with similar bundles are also present in ELISN 83-66 (Han et al. 2016a, fig. 3D) features but its external ones are severely damaged. It has the same internal morphology and pentaradial symmetry as the holotype, but differs from it in some features. There are fewer void-filling minerals within the cavity of the inner layer (Fig. 4).

The skirt-like soft tissue of the paratype looks thinner and smaller than that of the holotype (Fig. 5C). These differences may have a taphonomic origin. In summary, x-ray microtomography of two specimens provides a detailed picture of the whole internal structure of this new early Cambrian embryonic form. A suite of ectodermic and endodermic features (see Discussion below) is recognized . Each interradial septum extends deeply into the bell while the adjacent adradial septum extends over only half of its length. The peridermal apertural lobes, apertural lappets and other structures exhibit a 5 perradial + 10 adradial pattern.

DISCUSSION

Interpretation of Sinaster

Sinaster petalon presents the following suite of morphological characters typical of modern cnidarians: 1. The bell is organized in two layers. The inner and outer layers are interpreted here as the subumbrella and the exumbrella, respectively (Figs 2, 4). 2. These two layers are connected by interradial septa (is) which divide the gastric cavity into five spacious perradial pockets (pp, see Han et al. 2013; Fig. 3B,H). 3. Each interradial septum lacks a free adaxial end.

This general organization is that of typical extant medusae [START_REF] Hyman | The Invertebrates[END_REF]. The interior of the interradial septa seems to be solid in the holotype, but has tiny hollows in the paratype (Fig. 4E-G). 4. As seen in oral view, five larger perradial lappets and five pairs of smaller adradial lappets are rooted in the ridge of the exumbrella (Figs 2-4). 5. Five paired tiny buds interpreted here as tentacular buds, are also rooted in the inner layer of the subumbrella (Fig. 3G). 6. The cone-like structure with a rounded base situated within the subumbrellar cavity and below the level of the tentacular buds, is interpreted here as the manubrium. In the holotype, the manubrium appears to be solid, which is most likely to be a diagenetic artefact. The mouth opening is indiscernible, though was probably situated at the free end of the manubrium (Figs 2, 3, 6). 7. Viewed from apertural sections, five thin nose-like structures and five pairs of smaller ones are interpreted as perradial and adradial frenula respectively (Figs 3, 5); these would have strengthened the medusa structure. 8. The circular tissue around the margin of the subumbrellar cavity, above the frenula, is interpreted here as a velarium (Figs 3,5,6), which may also have played a role in strengthening the bell of the medusa. The velarium, with the aid of coronal muscles, may conceivably have allowed the bell to contract rhythmically (Han et al. 2013). 9. These coronal muscles are represented by five bundles of concentric striated strips around the velarium. These muscles are interrupted at each interradius as in extant cnidarians (Figs 3,6). 10. Five pairs of accessory septa are rooted in the inner layer of the exumbrella, and project into the perradial pockets . 11. Five pairs of tiny pisolitic structures occur in the middle of each interradial septum (Figs 23456) and are interpreted here as gonad lamellae. These 11 characters indicate that Sinaster petalon is most probably a cnidarian.

Tentacular buds

The bud-like structures of Sinaster are intepreted here as tentacular buds (tb) (point 5 above and Figs 2345).

Although the embryonic forms described in this paper have no exact equivalent among modern cnidarians, relevant comparisons can be made with the developmental stages of tentacles in extant cubozoans and scyphozoans. In the adult medusae, tentacles generally sprout from both umbrellas, but ontogenetically originate from the subumbrella [START_REF] Conant | The Cubomedusae: A Memorial Volume[END_REF]Chapman 2001). They bud from the distal end of the blade-like gelatinous pedalium, which is rooted in the subumbrellar structure (e.g. in the adult stage of the cubozoan Tripedalia cystophora). In the younger sessile polyp stages, tentacles occur at the four corners of the oral side of the animal (Werner et al. 1971). Features interpreted as subumbrellar tentacles also occur in the sea anemone-like cnidarian Eolympia and cubozoan-like specimens (Han et al. 2010(Han et al. , 2013(Han et al. , 2016a, b) , b) and an unidentified fossil cnidarian polyp (Steiner et al. 2004) from the same horizon (Kuanchuanpu Formation) and locality where Sinaster was found. In summary, although we lack information concerning the post-embryonic development of the tentacular buds of Sinaster, we find no reason to reject the hypothesis that they originate from the subumbrellar structure and represent the early developmental stages of tentacles which would have developed at a more advanced polyp stage through ontogeny.

Reproductive organs in early and extant cnidarians

Gonads vary widely in shape, size and disposition among the four classes of cnidarians, and thus they are of great significance for the systematics and phylogeny of the group. In anthozoan anemones, the gonads occur within the gastroderm of the septa/mesenteries in the form of longitudinal band-like structures behind the septal filaments [START_REF] Russell | The Medusae of the British Isles: 2. Pelagic Scyphozoa with a Supplement to the First Volume on Hydromedusae[END_REF]. Gonads tend to be fused within one side of each septum (= mesentery) (Thomas & Edwards 1990). Thus the anthozoan septum is a combined structure with many functions such as support, digestion and body contraction and the production of gametes. The eight gastrodermal gonad bands of stauromedusae are also distributed along each side of the interradial septa, but are located closer to the distal end of the septa. As in anthozoans and staurozoans, the gonads of scyphozoans and cubozoans arise from the gastrodermis, and appear as well-delimited, paired strip-like outgrowths close to or along the interradial septa (Gershwin & Alderslade 2006). The gonad lamellae of Sinaster have a smaller size but we have no information concerning their internal complexity. However, scyphozoan gonads exhibit much greater diversity. In coronate medusae, eight gonads are derived from the subumbrellar endoderm; they are situated more or less close to the adradii and their distal end extends toward the perradii [START_REF] Russell | The Medusae of the British Isles: 2. Pelagic Scyphozoa with a Supplement to the First Volume on Hydromedusae[END_REF]; see also Fig. 8B). In Semaeostome and Rhizostomae medusae, there are only four folded gonads [START_REF] Russell | The Medusae of the British Isles: 2. Pelagic Scyphozoa with a Supplement to the First Volume on Hydromedusae[END_REF] 7A). The gonad lamellae and accessory septa, derived from exumbrellar endodermis as illustrated by Sinaster petalon, may represent the ancestral condition in cnidarians.

Comparisons with Olivooides

Sinaster petalon resembles Olivooides multisulcatus and O. mirabilis, two abundant embryonic forms from the Kuanchuanpu Formation (Hua et al. 2004;Dong 2009). However, O. mirabilis has a distinctive jar-like shape with a cap-like structure covering the aperture (Hua et al. 2004). Most of its developmental stages are characterized by a dense spiny external ornament (Steiner et al. 2014). Clearly these characteristics are not found in S. petalon. Table 1). These are: (1) pentaradial symmetry exemplified by five pairs of tentacular buds; (2) five interradial septa supporting the medusa bell; (3) five pairs of gonad-like lamellae growing bilaterally in the interradial septa; (4) five pairs of accessory lamellae projecting into each perradial pocket; (5) a short manubrium; and (6) '5 + 10 pattern' of frenula and apertural lappets. However, S. petalon differs from Olivooides-like embryos in several anatomical features: (1) a completely smooth periderm; (2) interradial septa lacking free ends; (3) smaller and shorter gonad-like lamellae without bifurcation (Fig. 7); and (4) less developed endodermic lamellae and gastric pockets. These comparisons indicate that S. petalon and the two unnamed cubozoans represented by ELISN 31-5 and ELISN 108-343 are most probably neither conspecific nor congeneric but may belong to the same family. Sinaster petalon also shares a series of common features with another unnamed embryo from the Kuanchuanpu Formation (GMPKU 3089, Dong et al. 2013; Table 1). These are: (1) interradial and adradial furrows and shallow coronal grooves on the external layer of the exumbrella (see Dong et al. 2013, fig. 3a, b); (2) spacious perradial pockets created by the straight and long interradial septa; (3) long curved accessory septa projecting into the perradial pockets (see Dong et al. 2013, fig. 3); (4) paired, tiny gonad-like lamellae rooted in the interradial septa (see Dong et al. 2013, fig. 3j); and (5) tentacular buds with inner cavity. However, GMPKU 3089 has several features which do not occur in S. petalon, such as:

(1) the lack of a periderm; (2) gonad-like lamellae in a much higher position (Fig. 7); (3) perradial pockets with a smaller volume; and (4) five pairs of depressions under the tentacular buds and five radial canals. All of these features indicate that GMPKU 3089 differs from S. petalon but may belong to Sinaster. Systematic position of Sinaster and Olivooides-like fossils Two competing hypotheses have been proposed for the systematic position of Olivooides-like fossils: Scalidophora (Ecdysozoa; Steiner et al. 2014) or Scyphozoa and Cubozoa (Cnidaria; Dong et al. 2013;Han et al. 2013;Liu et al. 2014;Steiner et al. 2014). This debate also concerns Sinaster and is summarized below.

The Scalidophora hypothesis

We think that this hypothesis can be rejected on the following grounds: 1. Olivooides-like fossils have either pentaradial or tetraradial symmetry (Dong et al. 2013;Han et al. 2013Han et al. , 2016a) ) (2013) probably starts at the aboral stalk of softtissue. 5. The body wall of extant scalidophorans such as priapulids (Vannier 2012) consists of a relatively thin cuticle lined with longitudinal and circular muscles [START_REF] Ruppert | Invertebrate Zoology: A Functional Evolutionary Approach[END_REF]) and separates the coelomic cavity from the external medium. The peridermal theca of the Olivooides embryos has no such features.

The Cnidaria option

Comparison with anthozoans. Anthozoans are characterized by bilateral symmetry, an actinopharynx, and direct and indirect mesenteries with free ends [START_REF] Hyman | The Invertebrates[END_REF]. Both extinct and extant representatives of the group lack paired tentacles, a manubrium and lappets such as those present in Sinaster petalon and other Olivooides-like embryos.

Comparison with hydrozoans

Sinaster petalon and other Olivooides-like embryos differ from hydrozoans (hydropolyps and hydromedusae). Most solitary or colonial hydropolyps are partly or entirely surrounded by a tubulous cuticle or perisac, and they possess numerous circumoral tentacles surrounding a coneshaped hypostome without oral lips or lappets [START_REF] Hyman | The Invertebrates[END_REF]Thomas & Edwards 1990). The cuticle (or perisac) and hypostome of extant hydrozoans resemble the periderm and manubrium of S. petalon. However, the hydrozoan septa are highly reduced or invisible. The gastric cavity of hydropolyps does not partition into several pouches, while the gastric pockets of S. petalon are separated by interradial septa. In addition, the manubrium/ hypostome of S. petalon are located deep within the subumbrellar cavity. Hydromedusae have four perradial canals, four primary tentacles, as well as a very distinctive velum with neither frenula nor lappets (Thiel 1966). In contrast, all known Olivooides-like embryos have a circle of tentacles close to the interradii that are derived from the subumbrellar wall.

Comparison with staurozoans

The life cycle of extant staurozoans lacks the alternation of a sessile polyp and a swimming medusa and is commonly interpreted as an attached medusa stage (Collins et al. 2006). The external morphology of extant staurozoans is similar to that of naked, juvenile Olivooides-like fossils lacking a periderm (see Steiner et al. 2014, fig. 12: 7, 8). Their spacious perradial pockets are comparable to those of S. petalon and other Olivooides-like fossils. Notably, the pedal disk of staurozoans is fastened to the substrate by a chitinous secretion [START_REF] Hyman | The Invertebrates[END_REF]) that might represent a vestigial periderm. Longitudinal muscles, usually seen in the extant staurozoans, are not present in Sinaster (Miranda et al. 2015(Miranda et al. , 2016)). The triangular marginal lappets, strip-shaped gonads and paired tentacles of Sinaster have no counterparts in modern staurozoans. For these reasons, S. petalon cannot be placed within the staurozoans.

Comparison with scyphozoans

As previously proposed for Olivooides [START_REF] He | Early Cambrian conulariids from Yangtze platform and their early evolution[END_REF], Sinaster petalon might be placed among scyphozoans and especially scyphopolyps, based on the presence of marginal lappets, paired gonads, coronal muscles and an annulated sessile periderm. However, the periderm is a common feature among anthozoans and all cnidarian polyps except staurozoans [START_REF] Ruppert | Invertebrate Zoology: A Functional Evolutionary Approach[END_REF]). In general, the periderm of extant cnidarians does not cover the entire body as it does in the Kuanchuanpu medusozoans. Also, S. petalon can hardly been placed within one of the three scyphozoan orders, namely Coronatae, Semaeostomeae and Rhizostomeae. For example, it lacks the rhopalia, complex radial canals and well-developed mouth arms of the Semaeostomeae and Rhizostomeae. Furthermore, the coronal furrow, exumbrellar pedalia, and radial canals typical of Coronatae have no equivalent in S. petalon. Marginal lappets and paired gonads are two important characters shared with scyphozoans which might indicate a common ancestry.

Comparison with cubozoans

Sinaster petalon is most similar to extant cubozoan medusae. Specifically, in that the: (1) concentration of subumbrella tentacles in the interradii is a diagnostic feature of extant cubozoans [START_REF] Conant | The Cubomedusae: A Memorial Volume[END_REF]Chapman 2001); (2) morphology and arrangement of the gonadal lamellae is similar to those of both cubozoans and scyphozoans (Figs 7, 8); (3) support of the velarium by the perradial frenula is also a distinctive feature of extant cubozoans [START_REF] Conant | The Cubomedusae: A Memorial Volume[END_REF]); (4) interradial septa, which connect the subumbrellar and exumbrellar walls, lack a free end. In these respects, S. petalon more closely resembles presentday cubozoans than do other Olivooides-like embryos (e.g. ELISN 31-5). However, Sinaster does not possess the typical pedalia and sensory organs seen in modern cubozoans and in fossil jellyfish from the Chengjiang biota (Han et al. 2016c). Finally, the marginal lappets and the adradial frenula of Sinaster are absent in the modern representatives of the group. belong to the total group Cubozoa (Fig. 9; highlighted in blue). This tentative phylogenetic analysis suggests that Sinaster and its allied embryonic forms may belong to the stem-group Cubozoa.

PHYLOGENETIC ANALYSIS

CONCLUSIONS

Sinaster petalon exhibits a mixture of features typical of polypoid and medusoid phases. Its internal anatomical structure (e.g. gonad lamellae, accessory septa, perradial and adradial frenula) appear to be more complex than those of all extant medusozoan polyps. Although it bears a well-developed subumbrellar cavity and coronal muscles, it lacks sensory organs comparable to those of extant medusae. The presence of a periderm around Sinaster and the manifest absence of sensory features would support the view that the adult stage is a sessile medusa.

Sinaster cannot be straightforwardly assigned to any crown group of extant medusozoans. However, the presence of marginal lappets and endodermal lamellae suggest that it is closer to Cubomedusa and Scyphomedusa than to any other group of modern cnidarians. The presence in Sinaster of a velarium supported by the frenulum is an important cubozoan character that would support its position in the stem-group Cubomedusae.

The morphological similarity of Sinaster with both cubozoans and scyphozoans supports the hypothesis that these fossils represent the common ancestors of Cubozoa + Scyphozoa. However, cladistic analysis suggests closer affinities with Cubozoa than with Scyphozoa. The subsequent reduction of various internal structures and of the external periderm through evolution would have led to the divergence between these two groups.

Although quite similar in their external morphology, the pentamerous embryonic medusae from the early Cambrian Kuanchuanpu biota display considerable anatomical variation, exemplified here by Sinaster petalon. Clearly, any future attempt to establish the taxonomy of these fossils should take into consideration not only their external shape and micro-ornament but also and above all their internal structures, which can be easily revealed with microtomographic techniques.

Fig. 1 .

 1 Fig. 1. Y. haikouensis Hu et al., 2007 from the early Cambrian Chengjiang Lagerstätte, China. (a) Part of the holotype; the 8th tentacle (rectangle area) likely bears nematocyst batteries. (b) Incomplete counterpart of the holotype. (c) Transverse stripes indicative of contracted tentacles with nematocyst batteries on the broad-based tentacles. (d) Close-up of the eight rhopalia (circled with white ellipses) and the scalloped bell margin. (e) Close-up of the bell margin showing the marginal lappets and four rhopalia (rh5-rh8) covered by carbon film.

Fig. 2 .

 2 Fig. 2. Camera lucida drawings of the holotype specimen of Y. haikouensis. (a) Part of the holotype. (b) Counterpart. Abbreviations: bc, bifurcated canals; bm, bell margin; cg, coronal groove; cs, coronal sinus; m, mouth; mb, manubrium (red); ml, marginal lappets; nb, nematocyst battery; pt., proximal part of tentacle; rc, radial canals (blue); rh1-rh8, sc, subumbrellar cavity; serial number of rhopalia; t1-t16, serial number of tentacles; trb, thickened rectangle block. Scale bar = 5 mm.

Fig. 3 .

 3 Fig. 3. Energy-dispersive spectroscopic analysis made at the seventh rhopalium on the holotype specimen of Y. haikouensis Hu et al., 2007. (a)-(f) Element maps of the seventh rhopalium. (g), Combined spectrum of rhopalium (spectrum 1, red rectangle in (a)) and background tissue (spectrum 2, yellow rectangle in (a)) of the bell. Note the high concentration of carbon on the rhopalium (spectrum 1). (h), Backscattered electron (BSE) image of the red rectangular area of the carbon in (a). (a)-(h) share the same scale bar in (a).

Fig. 4 .

 4 Fig. 4. Strict consensus trees for Medusozoa obtained respectively using TNT and PAUP based on morphologial phylogenetic analysis of medusozoans. (a) TNT analysis. (b) PAUP 4.0 beta 10. ELISn31-31, ELISn83-66, ELISn108-343, ELISn31-5 are undetermined taxa in Han et al. (2013, 2016); together with co-ocurring Olivooides and Quadrapyrgites, they come from the basal Cambrian Kuanchuanpu Formation in South China. Cormr, seamr, narmr, cubmr represent, respectively, unnamed fossils of the Coronata, Semaestomeae, Narcomedusae and Cubomedusa from the Middle Cambrian Marjum Formation, Utah[START_REF] Cartwright | Exceptionally preserved jellyfishes from the Middle Cambrian[END_REF].

Fig. 5 .

 5 Fig. 5. Reconstruction of Y. haikouensis Hu et al., 2007. Artwork by Mr. Dinghua Yang at Nanjing Institute of Geology and Palaeontology.

Figure 1 .

 1 Figure 1. Secondary electron photomicrographs of Feiyanella manica gen. et sp. nov. (a-d) and Hyolithellus sp. (e). Fossils were recovered from the basal Cambrian Kuanchuanpu Formation at the Shizhonggou (a-d; Ningqiang County) and the Zhangjiagou (e; Xixiang County) sections. (a) Holotype specimen (ELISN141-14). Exterior view of the tube. (b) and (c) are enlarged views of the apical and apertural part, respectively. (d) is close up of (c). The tube is generally conical in shape, with the apical end (lower left) slightly tapering and the apertural end flaring (upper right) (a). The outermost layer of the tube is ornamented with transverse corrugations (b). Two daughter tubes (D1 and D2) and two granddaughter tubes (G1 and G2) can be seen in apertural fracture (c-d). (e) Hyolithellus sp. (ELIXX35-465). Abbreviations: P -parent tube; D -daughter tube; G -granddaughter tube. One parent tube (P1), two daughter tubes (D1 and D2) and three granddaughter tubes (G1, G2, and G3) are identified. Numberings suffixed P1, D1, D2, G1, G2 and G3 represent layers of walls in the parent, daughter and granddaughter tubes, respectively.

Figure 2 .

 2 Figure 2. SRXTM virtual sections of the holotype specimen (ELISN141-14) of Feiyanella manica gen. et sp. nov. (a-d) Vertical bisections of the entire specimen; (e-m) transverse sections. Positions of the sections are indicated in (a). Abbreviations: St -soft tissue. For all other abbreviations, see Figure 1. Scale bars: 350 μm for (a-d) and 60 μm for (e-m).

Figure 3 .

 3 Figure 3. SRXTM reconstructions of the holotype specimen (ELISN141-14) of Feiyanella manica gen. et sp. nov. (a) External view of entire specimen; (b-c) vertical bisection of the specimen; (d-j) transverse sections. Positions of the sections are indicated in (c). Abbreviations: see Figure 1. Scale bars: 250 μm for (a-c) and 60 μm bar (d-j).

Figure 4 .

 4 Figure 4. SRXTM reconstruction of the holotype specimen (ELISN141-14) of Feiyanella manica gen. et sp. nov. Different tube layers are in different colours. The outer layers are stepwise removed (from a-l) to expose the inner layers. Abbreviations: see Figure 1. Scale bars: 400 μm for (a-d) and 150 μm for (e-l).

  Figure 5. (Colour online) Schematic drawing of the cross-section of Feiyanella manica gen. et sp. nov. illustrating growth patterns between parent, daughter and granddaughter tubes. Note the orthogonal splitting planes between daughter (rightward arrow) and granddaughter (upward arrow) tubes. Abbreviations: see Figure 1.

Figure 1 .

 1 Figure 1. Carinachitids from the lower Cambrian Kuanchuanpu Formation in South China. (1) Triradiate Emeiconularia amplicanalis Liu et al., 2005. (2) Pentamerous Pentaconularia ningqiangensis Liu et al., 2011. (1, 2) Courtesy of Y.H. Liu. (3-6) Tetraradiate Carinachites spinatus Qian, 1977. (3, 4) ELISN93-157, showing the displacement between neighboring arcuate ribs, which are connected in the middle by striations. Both the faces and ribs widen slightly toward the apertural end of the skeleton. (5) ELISN93-45. (6-12) ELISN148-52. (6, 7) Lateral view of the tube. (8, 9) Close-up of the tube aperture shows one of the plicate apertural lobes being elevated above the others. (10) The elevated, plicate apertural lobes with converging striated folds. (11) Single plicate apertural lobe with converging striated folds and the corner sulci with parallel striations. (12) Close-up of (11) showing secondary cracks on the striated surface. IR = interradius; PR = perradius; cs = corner sulci; pal = plicate apertural lobes; st = striations; tr = transverse ribs; ts = thorn-like spines.

Figure 2 .

 2 Figure2. Micro-CT reconstruction of specimen ELISN148-52. (1a-1e) lateral view; (1f, 1g) oral view of the virtual cross sections. The sagittal positions of (1a-1e) are indicated respectively by '1a,' '1b,' '1c,' '1d,' '1e,' and '1f' in (1f, 1g). The horizontal levels of (1f, 1g) are respectively indicated by '1f' and '1g' in (1a-1f). White arrows indicate double-layered tube wall. cs = corner sulcus; fa = face; mmf = microbial-mediated filaments; pal = plicate apertural lobes; st = soft tissue; ts = thorn-like spines; PR = perradius; IR = interradius.

  Figure2. Micro-CT reconstruction of specimen ELISN148-52. (1a-1e) lateral view; (1f, 1g) oral view of the virtual cross sections. The sagittal positions of (1a-1e) are indicated respectively by '1a,' '1b,' '1c,' '1d,' '1e,' and '1f' in (1f, 1g). The horizontal levels of (1f, 1g) are respectively indicated by '1f' and '1g' in (1a-1f). White arrows indicate double-layered tube wall. cs = corner sulcus; fa = face; mmf = microbial-mediated filaments; pal = plicate apertural lobes; st = soft tissue; ts = thorn-like spines; PR = perradius; IR = interradius.

Figure 3 .

 3 Figure 3. Cross sections and proposed orientation of the main radial symmetry planes in Cambrian carinachitids. (1, 2) Inferred triradial symmetry in Emeiconularia; (1) E. amplicanalis; (2) Emeiconularia trigemme with thickened faces sensu Qian et al., 1999; (3) tetraradial symmetry in Carinachites; (4) pentaradial symmetry in Pentaconularia (Modified from Liu et al., 2011). cs = corner sulcus; fa = face; st = soft tissue; tb = tube; PR = perradius; IR = interradius.

  , Van Iten (1992a), and Han et al., 2016a. Repository and institutional abbreviation.-The figured specimens in this study are housed in the Early Life Institute (ELI), Northwest University, Xi'an, China.

Figure 4 .

 4 Figure 4. 3D reconstructions of Cambrian carinachitids with a hypothetical apical tip. (1-3) Lateral, oblique, and oral views, respectively, of Emeiconularia; (4-6) lateral, oblique, and oral views, respectively, of Carinachites spinatus; (7-9) lateral, oblique, and oral views, respectively, of Pentaconularia.

Figure 5 .

 5 Figure 5. Peridermal tube of Olivooides multisulcatus from the Cambrian Kuanchuanpu Formation in South China. (1) Specimen ELISN19-20, aboral view showing five longitudinal rows of plicate corners (pc); (2-4) XX30-127. Oral view of the specimen. (2) Tube aperture with five plicate lobes (pl); (3) close-up of (2). (4) Diagnostic stellate ornament of the aboral ends. PR = perradii; IR = interradii.

Figure

  Figure1B, C, E, F

  and ELISN 35-1 (Han et al. 2016b, fig. 6K). The paratype specimen (Figs 1C, F; 4; 5) has well-preserved internal External morphology of pre-hatched fossil embryos from the lower Cambrian Kuanchuanpu Formation, Shaanxi Province, China. A, D, Olivooides, ELISN 31-179 with the periderm bearing stellate ornament, general view and details of ornament. B, E, Sinaster petalon gen. et sp. nov., ELISN 89-107, holotype showing the smooth periderm, general view and details of the peridermal aperture. C, F, S. petalon gen. et sp. nov., ELISN 115-39, paratype, showing features of the outer layer. All SEM images. Abbreviations: af, adradial furrow; en, egg envelope; if, interradial furrow; pal, perradial apertural lobe; pe, periderm; pfr, perradial frenulum; stl, stellate ornaments. Scale bars represent: 200 lm (A, C); 150 lm (B); 50 lm (D); 100 lm (E, F).

  . 2 . Sinaster petalon gen. et sp. nov. from the lower Cambrian Kuanchuanpu Formation, Shaanxi Province, China; ELISN 89-107, holotype, microtomographic sections. A, axial section through the manubrium; positions of horizontal sections B-I are indicated. B-I, horizontal sections from oral to aboral part; major internal structures identified by colours generated by VG Studio 2.2 MAX. See colourless sections in Wang et al. (2017, appendix 1). Abbreviations: aal, adradial apertural lobe; afl, adradial fold lappet; afr, adradial frenulum; as, accessory septum; crm, circumferential muscle; en + pe, egg envelope + periderm; eu, exumbrella; gl, gonad-lamella; is, interradial septum; mb, manubrium; pal, perradial apertural lobe; pfl, perradial fold lappet; pfr, perradial frenulum; su, subumbrella; tb, tentacular bud; ve, velarium; *, perradius; ?, interradius. Scale bar represents 200 lm.

  . 3 . Sinaster petalon gen. et sp. nov. from the lower Cambrian Kuanchuanpu Formation, Shaanxi Province, China; ELISN 89-107, holotype, three-dimensional model obtained using VG Studio 2.2 MAX. Colours identify anatomical structures. A, oral view showing the manubrium and the perradial and adradial apertural folded lobes. B, inner-lateral view of the half body showing the arrangement of the apertural folded lobes of the periderm and the underlying perradial apertural lappets; for clarity, the subumbrellar elements are obscured. C, oral view showing the perradial lappets and the adradial lappets. D, zigzag margin of the velarium. E, five bands of coronal muscles interrupting at the interradii. F, oral view showing the perradial frenula and adradial frenula. G, inner-lateral view of the subumbrella showing the arrangement of the interradial tentacular buds and the perradial and adradial frenula; in a strict sense, each tentacle is located at the adradius. H, one-fifth of the body showing the arrangement of the interradial septum, gonad lamellae and accessory septa. Abbreviations: aal, adradial apertural lobe; afl, adradial fold lappet; afr, adradial frenulum; as, accessory septum; crm, circumferential muscle; en + pe, egg envelope + periderm; eu, exumbrella; gl, gonad-lamella; is, interradial septum; mb, manubrium; pal, perradial apertural lobe; pfl, perradial fold lappet; pfr, perradial frenulum; su, subumbrella; tb, tentacular bud; ve, velarium; *, perradius; ?, interradius. Scale bars represent: 150 lm (A-F); 75 lm (G); 150 lm (H).

  . 4 . Sinaster petalon gen. et sp. nov. from the lower Cambrian Kuanchuanpu Formation, Shaanxi Province, China; ELISN 115-39, paratype, microtomographic sections. A, axial section through the middle of the aperture; positions of horizontal sections B-I are indicated. B-I, horizontal sections from oral to aboral part. Major internal structures identified by colours generated by VG Studio 2.2 MAX; see colourless sections in Wang et al.(2017). Abbreviations: aal, adradial apertural lobe; afl, adradial fold lappet; afr, adradial frenulum; as, accessory septum; crm, circumferential muscle; en, egg envelope; eu, exumbrella; gl, gonad-lamella; is, interradial septum; mb, manubrium; pal, perradial apertural lobe; pe, periderm; pfl, perradial fold lappet; pfr, perradial frenulum; su, subumbrella; tb, tentacular bud; ve, velarium; *, perradius; ?, interradius. Scale bar represents 150 lm. W A N G E T A L . : A N A T O M Y O F N E W E M B R Y O F O S S I L S 7

  Sinaster petalon gen. et sp. nov. from the lower Cambrian Kuanchuanpu Formation, Shaanxi Province, China; ELISN 115-39, paratype, three-dimensional model obtained using VG Studio 2.2 MAX. Colours identify anatomical structures. A, oral view showing the perradial lappet, velarium, perradial and adradial frenula. B, inner-lateral view of the half body showing the subumbrellar elements and the structures along the interradial septa. C, one-fifth apertural part showing the perradial lappet underlying the velarium and the perradial frenulum. D, surface of the exumbrella showing the shallow coronal grooves. E, one-fifth subumbrellashowing the tentacular buds in the interradius and the frenula in the perradius. In a strict sense, each tentacle is located at the adradius. F, one-fifth exumbrella showing the gonad lamellae rooted into the interradial septa, and the accessory septa projecting from the interior layer of the exumbrella. Abbreviations: aal, adradial apertural lobe; af, adradial furrow; afl, adradial fold lappet; afr, adradial frenulum; as, accessory septum; cg, coronal groove; crm, circumferential muscle; eu, exumbrella; en, egg envelope; gl, gonad-lamella; if, interradial furrow; is, interradial septum; mb, manubrium; pal, perradial apertural lobe; pfl, perradial fold lappet; pfr, perradial frenulum; su, subumbrella; tb, tentacular bud; ve, velarium; *, perradius; ?, interradius. Scale bars represent: 150 lm (A, B, D, F); 50 lm (C); 75 lm (E).

  ; Tiemann & Jarms 2010). In hydrozoans, gonads develop from pouches of epidermal epithelium on either side of the manubrium or on the subumbrellar surface along the perradial canals (Thomas & Edwards 1990). The gonad lamellae and accessory septa of Sinaster petalon and other unnamed fossil embryos from the Kuanchuanpu biota (ELISN 31-5 and ELISN 108-343, Han et al. 2013; GMPKU 3089, Dong et al. 2013) are clearly derived from the exumbrellar endodermis. They closely resemble the interradial gonads of cubozoans and also display important similarities to the reproductive organs of scyphozoans (Stauromedusae, Coronatae). For example, their accessory septa recall the gonads of extant coronates. Both structures are more or less adradial and are derived from the gastric epidermis (Han et al. 2013, 2016a). The aboral part of the gonad lamellae and accessory septa of ELISN 31-5 and ELISN 108-343 are rooted in the inner layer of the exumbrella, but most of their derived tissues are fused with the exterior subumbrellar layer at the middle level of the bell (Han et al. 2013; see Figs 4,

  . 7 . Simplified reconstructions of the reproductive organs of extant (A, E) and lower Cambrian (B, F; C, G) medusozoans. Each form is represented in lateral and vertical views. Each image represents one-fifth of the animal. A, E, extant cubozoans with no accessory septa (Conant 1898). B, F, unnamed Cambrian form from the Kuanchuanpu biota with gonad lamellae and accessory septa (based on ELISN31-5; Han et al. 2013). C, G, unnamed Cambrian form from the Kuanchuanpu biota with gonad lamellae and distally branched accessory septa (based on GMPKU3089; Dong et al. 2013). D, H, Sinaster petalon gen. et sp. nov. with gonad lamellae and accessory septa (based on ELISN89-107). The subumbrellar wall and other anatomical structures are removed in order to expose the interradial septa, gonad lamellae and accessory lamellae.

  et al. 2016c), were considered for analysis using PAUP* v. 4.0 b10, yielding 999 shortest trees (tree length = 177 steps, consistency index = 0.65, rescaled consistency index = 0.50) (Wang et al. 2017, appendix 3). All characters have equal weight, 8 of them are constant, and 25 variable characters are parsimony-uninformative. This corroborates the molecular analysis of Collins et al. (2006) which found scyphozoans and cubozoans to form a monophyletic group. As shown on the consensus tree (Fig. 9), Sinaster and GMPKU 3089 are close to each other. Sinaster and other fossil embryos (e.g. GMPKU 3089, ELISN 31-5 and

  ) The Han et al.-Olivooides-like tube aperture in carinachitids tube aperture in both Carinachites spinatus and Olivooides has four or five prominent plicate apertural lobes (usually termed 'lobate folds' in

Table 1 .

 1 Morphological comparisons among olivooids, hexangulaconulariids, carinachitids, and conulariids.

	8
	Characteristics\taxa	Olivooids	Hexangulaconulariids	Carinachitids	Conulariids
	tube shape	cone	cone	cone	cone
	longitudinal folds	yes	yes	yes	yes
	longitudinal striations	yes	?	yes	no
	apical/abapical differentiation	yes	yes	?	yes
	periodical growth	yes	yes	yes	yes
	radial symmetry	4, 5	3, 4, 5	2	2, 3, 4
	oral lobes	yes	no	yes	yes
	adradial lobes	yes	no	no	no
	thorn-like spines	yes/no	no	yes	no
	face/corner	no	yes	yes	yes/no
	septa/carina	no	no	no	yes
	facial midline	no	no	no/yes	yes
	mineralization	no	yes	weak	yes
	thickening of tube wall	no	no	yes	yes
	displacement	no	yes	yes	yes
	apex ornaments	stellae	smooth	?	smooth

  Comparisons with Olivooides-like embryos Sinaster petalon shares important characters with two Olivooides-like embryos (e.g. ELISN 31-5 and ELISN 108-343 interpreted as cubozoans by Han et al. (2013; see also F I G . 6 . Simplified cross-sections showing the internal features of Sinaster gen. et sp. nov. from the lower

	Cambrian Kuanchuanpu Formation,
	Shaanxi Province, China. A, vertical
	section through the manubrium, ten-
	tacle buds. B-D, horizontal sections;
	exumbrellar surface with the internal
	furrows; B, section near oral aperture
	showing the rounded velarium and
	the coronal muscles interrupted in
	interradii; C, section under velarium
	level showing five pairs of tentacular
	buds; D, section showing five pairs of
	the accessory septa and five pairs of
	the gonad lamellae. Abbreviations: as,
	accessory septum; crm, circumferen-
	tial muscle; eu, exumbrella; gl, gonad
	lamellae; if, interradial furrow; is,
	interradial septum; mb, manubrium;
	pe, periderm; su, subumbrella; tb,
	tentacular buds; ve, velarium; *, per-
	radius; ?, interradius.

  Morphological comparisons between Sinaster petalon gen. et sp. nov. and Olivooides embryos, all from the lower Cambrian Kuanchuanpu Formation.

		Periderm	Interradial septa /	Gonad lamellae /	Accessory septa /	Bell texture /
			radial wall	unnamed	recurved wall	unnamed
	ELISN 89-107	Complete,	One third of the	Near to the aboral	One sixth of the height	Unknown
		smooth	height of the bell	section of the bell,	of the bell
				continuous
	ELISN 115-39	Incomplete,	One third of the	Near to the aboral	One sixth of the height	With adradial
		smooth	height of the bell	section of the bell,	of the bell	furrow, interradial
				continuous		furrow and
						coronal groove
	ELISN 31-5	Unknown, with	Half the height of	Middle of the bell,	One sixth of the height	Same as ELISN
		an envelope	the bell	discontinuous	of the bell	115-39
	ELISN 108-343 Incomplete, with	Half the height of	Middle of the bell,	One sixth of the height	Same as ELISN
		an envelope	the bell	discontinuous	of the bell	115-39
	GMPKU 3089	Unknown	Half the height of	Near to the oral	One eighth of the	Same as ELISN115-
			the bell	section, tiny;	height of the bell	39; unnamed
				unnamed

T A B L E . 1 .

  and thus do not have a bilateral body plan. Modern Scalidophora such as priapulids and their assumed early Cambrian ancestors (e.g. Chengjiang biota;Huang et al. 2004; Vannier 2012) are bilaterians with an antero-posterior polarity and a ventral cord (Nielsen 1995; Adrianov & Malakhov 2001). No such bilateral arrangement occurs in Olivooides-like fossils. 2. No eubilaterian has a double-layered body wall connected by pentaradial septa. 3. Representatives of Olivooides-like fossils lack the annulated trunk and swollen introvert with scalid rows that characterizes extinct and extant scalidophorans. Eopriapulites sphinx, which co-occurs with Olivooides embryos in the Kuanchuanpu Formation, is the oldest known scalidophoran. Its 18 longitudinal rows of scalids (Ruppert et al. 2004; Liu et al. 2014) have no equivalent in Olivooides. 4. Both extinct (e.g. Ottoia prolifica; Vannier 2012) and extant scalidophorans have a cylindrical gut linking the anterior (mouth) and posterior (anus) ends of the body. The single apical aperture of Olivooides embryos superficially resembles the mouth opening of scalidophorans (Steiner et al. 2014), but it is really just an invagination of the theca. The real 'mouth' is located at the end of the manubrium (Han et al. 2013; Figs 2, 3) whereas the 'gut' sensu Dong et al.

J.Han et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 449 (2016) 166-173

W A N G E T A L . : A N A T O M Y O F N E W E M B R Y O F O S S I L S 3

W A N G E T A L . : A N A T O M Y O F N E W E M B R Y O F O S S I L S 1 1

Authors' contributions. JH designed the research; XW and JH developed the interpretation; KU, OS and TK conducted the synchrotron and micro-CT. XW reconstructed the micro-CT data. XW, JH, JV and QO wrote the paper with input with the other authors, all of whom read and approved the final manuscript.

W A N G E T A L . : A N A T O M Y O F N E W E M B R Y O F O S S I L S 1 5

Acknowledgments

We thank Dinghua Yang for his colorful reconstruction of the specimen, and J. Sun. and M.R. Cheng (State Key Laboratory for Continental Dynamics, Northwest University, Xi'an, China) for their assistance in both field and lab work. This study was supported by the Natural Science Foundation of China (NSFC grants: 41272019, 41472012), the, 41102012, 41572017 National Basic Research Program of China (2013CB835002, 2013CB835006, 2013CB837100), the Programs of Introducing Talents of Discipline to Universities (No: W20136100061). We are grateful to Prof. Jean Vannier (France) and an anonymous reviewer for invaluable suggestions and linguistic improvement.

Acknowledgements. We thank H. Gong, J. Luo, N. Liu and M. Cheng (State Key Laboratory for Continental Dynamics, Northwest University (NWU), Xi'an, China) for their assistance in both field and lab works. We are grateful to Prof. Li Guoxiang (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences) for invaluable advice. Current work was supported by the Natural Science Foundation of China (Nos. 41621003, 41272019 and 41572012), the '973 project' of the Ministry of Science and Technology of China (2013CB835002), and the State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (No. 163107). Acknowledgments We thank Drs. H. Van Iten (Hanover College, USA), C.B. Skovsted (Swedish Museum of Natural History), and G.A. Brock (Macquarie University) for their suggestions and linguistic improvement of the manuscript. We also thank H.J. Gong, J. Sun, J. Luo, and M.R. Cheng (Northwest University, Xi'an, China) for their assistance in the field and with lab work. X. Han prepared the 3D drawings of the carinachitid specimens, and Y.H. Liu (Chang'an University) provided two carinachitid photos. This work was supported by the Natural Science Foundation of China (NSFC grant 41272019, 41621003, 41372021, 41472015), the '973 project' of the Ministry of Science and Technology of China (grant 2013CB835002, 2013CB837100), the Chinese Academy of Sciences (XDB10010101), and the State Key Laboratory of Palaeobiology and Stratigraphy (No. 163107).

Han et al.-Olivooides-like tube aperture in carinachitids

Acknowledgements. We thank Gong, H. J., Sun, J., Luo, J., Han, X. and Cheng M. R. (State Key Laboratory for Continental Dynamics, Northwest University, Xi'an, China) for their F I G . 9 . Consensus tree for Medusozoa acquired using PAUP* v. 4.0 b10. ELISN 31-5, ELISN 108-343 and GMPKU 3089 are uncertain taxa in Han et al. (2013) and Dong et al. (2013) respectively. The co-occurring Quadrapyrgites and Punctatus also were found in the Kuanchuanpu Formation. Cormr, semamr, narmr, cubomr represent, respectively, unnamed fossils of the Coronata, Semaestomeae, Narcomedusae and Cubomedusa from the Middle Cambrian Marjum Formation, Utah (Cartwright et al. 2007). W A N G E T A L . : A N A T O M Y O F N E W E M B R Y O F O S S I L S 1 3 assistance in both field and laboratory work, Yin, Z. J. (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China) for technical assistance with 3D-reconstructions. We thank H. Van Iten (Department of Geology, Hanover College, Indiana) and an anonymous referees for correcting and improving our manuscript. Current work is supported by the National Natural Science Foundation of China (NSFC grant 41272019), the '973 project' of the Ministry of Science and Technology of China (2013CB835002, 2013CB837100), the '111 project' of the Programs of Introducing Talents of Discipline to Universities (W20136100061) and the Most Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China.

DATA ARCHIVING STATEMENT

This published work and the nomenclatural acts it contains, have been registered in ZooBank: http://zoobank.org/References/3F910127-DEE5-406F-9808-B32C2FB177DB

Data for this study are available in the Dryad Digital Repository: https://doi.org/10.5061/dryad.ng76g

Editor. George Sevastopulo

Competing interests

The authors declare that they have no competing interests.

Author Contributions

J. H. conceived and designed the work; S.X. H. and F.C. Z. acquired the data; All authors analyzed and interpreted the data; J. H., P. C., S.X., H and Q, O. wrote the paper. Abstract: We describe here Sinaster petalon gen. et sp. nov., a new embryonic form from the c. 535 million-yearold Kuanchuanpu Formation of South China (Ningqiang, Shaanxi Province). The excellent three-dimensional, phosphatic preservation of these microfossils allowed us to use x-ray microtomographic techniques to make accurate reconstructions of their internal structures and to compare their anatomy point-by-point with that of extant cnidarians and other animal groups. Sinaster petalon has anatomical features typical of extant Medusozoa (Cnidaria), such as coronal muscles, perradial and adradial frenula, interradial septa, accessory septa, gonad-lamellae, tentacle buds and perradial pockets. Although Sinaster cannot be straightforwardly assigned to any crown-group within Medusozoa, the presence of marginal lappets and endodermal lamellae suggests that it is closer to Cubozoa and Scyphozoa than to any other group of modern cnidarians. The tentative placement of Sinaster within the stem-group Cubozoa is justified by the presence of a velarium supported by a frenulum. The cubozoan affinities of Sinaster are also supported by cladistic analysis.

ANATOMY AND AFFINITIES OF