Louise Souquet
email: louise.souquet@ens-lyon.fr

Nicolas Goudemand
email: nicolas.goudemand@ens-lyon.fr

Exceptional basal-body preservation in some Early Triassic conodont elements from Oman Authors

Keywords: Vertebrate skeleton, evolution, developmental constraints, functional model, Novispathodus, apparatus

Conodont elements are composed of two main parts: the crown and the basal body. The basal body of a conodont element is a laminated structure that is less densely mineralized than the crown and, as a result, is rarely preserved. Elements with preserved basal bodies are found only in exceptional cases in the fossil record. To date, no S or M element with preserved basal body has ever been documented for post-Devonian conodonts, which had raised the question of an evolutionary trend towards unmineralized basal bodies in conodonts. Here we report the discovery, in Smithian (Early Triassic) rocks of Oman, of neospathodid conodont elements whose basal body is partly preserved. We demonstrate the presence of basal body in all elements, including S and M elements, of Novispathodus, and most likely of all gondolelloideans, thereby suggesting that the absence of basal bodies in post-Devonian conodonts was due to a preservational bias only. We further demonstrate that the morphology and extent of the basal bodies in Novispathodus are in agreement with previous predictions and do not contradict a cyclostome-like functional model of the apparatus. Based on a review of the occurrences of basal bodies in the literature, we suggest a general pattern for the morphology of the basal bodies in conodont elements that may reflect the mechanical constraints associated with feeding motions.

Introduction

Conodonts are extinct jawless animals that thrived in most marine environments from the late Cambrian to the very end of the Triassic [START_REF] Aldridge | The anatomy of conodonts[END_REF][START_REF] Sweet | The Conodonta: morphology, taxonomy, paleoecology, and evolutionary history of a long-extinct animal phylum[END_REF].

The feeding apparatus of complex euconodonts was usually composed of 15 tooth-like elements: a bilaterally symmetrical basket of so-called S and M elements located at the anterior part of the mouth was apparently used to grasp food, whereas two pairs of posteriorly-located and relatively robust elements (the so-called P elements) were used to process the food [START_REF] Goudemand | Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates[END_REF][START_REF] Purnell | Architecture and functional morphology of the skeletal apparatus of ozarkodinid conodonts[END_REF]. Conodont elements were comprised of two primary constituents made of two types of tissues: the crown, and the basal body (Fig. 1). Conodont elements and vertebrate teeth "have evolved independently and convergently" [START_REF] Murdock | The origin of conodonts and of vertebrate mineralized skeletons[END_REF] and are therefore not homologous, but considered as analogous. Crown tissue is an enamel-like tissue, whereas the basal body compares best to dentine [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF]. Both tissues are mineralized with apatite and layered with internal lamellae apposed during intermittent growth of the element [START_REF] Donoghue | Microstructural variation in conodont enamel is a functional adaptation[END_REF][START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF][START_REF] Donoghue | Growth, function, and the conodont fossil record[END_REF][START_REF] Gross | Über die Basis bei den Gattungen Palmatolepis und Polygnathus (Conodontida)[END_REF][START_REF] Gross | Über die Basis der Conodonten[END_REF][START_REF] Jeppsson | Conodont element function[END_REF][START_REF] Müller | Über den Feinbau der Conodonten[END_REF][START_REF] Sweet | The Conodonta: morphology, taxonomy, paleoecology, and evolutionary history of a long-extinct animal phylum[END_REF]. The laminated structure of the basal body is less densely mineralized than the crown because apatite crystallites in the basal body are smaller, randomly oriented, and less tightly packed [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF]. Moreover the surface between crown and basal body is one of weakness [START_REF] Sweet | The Conodonta: morphology, taxonomy, paleoecology, and evolutionary history of a long-extinct animal phylum[END_REF], leading to dissociation of the two components upon death. As a consequence basal body tissues are less likely to be preserved and are found only in instances of exceptional preservation within the fossil record. Although knowledge about the basal body is crucial for testing functional and developmental hypotheses, even in those rare cases where they are preserved, they are usually poorly described, if at all. Indeed, for many conodont workers whose goal is to use conodonts to establish biochronostratigraphic schemes or to reconstruct paleoenvironments via geochemistry, basal bodies are a bane rather than a blessing. Preserved basal bodies obstruct the basal cavity, whose outline is a major taxonomical trait, paradoxically impeding 'full' taxonomical assessment of a specimen, and intact basal bodies may also decrease the precision of some geochemical measurements [START_REF] Trotter | Did cooling oceans trigger Ordovician biodiversification? Evidence from conodont thermometry[END_REF].

Ramiform elements (usually S or M elements) with basal body preservation are particularly rare [START_REF] Jeppsson | Notes on some Upper Silurian multielement conodonts[END_REF][START_REF] Miller | A conodont, thelodont and acanthodian fauna from the Lower Přidoli (Silurian) of the Much Wenlock Area, Shropshire[END_REF][START_REF] Nicoll | The multielement genus Apatognathus from the Late Devonian of the Canning Basin, Western Australia[END_REF][START_REF] Nicoll | Conodont apparatuses in an Upper Devonian palaeoniscoid fish from the Cannig Basin, Western Australia[END_REF]. In the post-Devonian literature the basal bodies of some P elements have been illustrated occasionally [START_REF] Goel | Triassic conodonts from Spiti (Himachal Pradesh), India[END_REF][START_REF] Jiang | Revised conodont zonation and conodont evolution across the Permian-Triassic boundary at the Shangsi section, Guangyuan, Sichuan, South China[END_REF][START_REF] Nogami | Trias-Conodonten von Timor, Malaysien und Japan[END_REF][START_REF] Orchard | Conodonts from the Lower Griesbachian Otoceras latilobatum Bed of Selong, Tibet and the Position of the Permian-Triassic Boundary[END_REF], the basal bodies of S and M elements have hitherto remained unknown. However, this has fueled debates about the possible development of these conodont tissues, because, in vertebrates, enamel secretion is induced by the presence of mineralized dentine [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF][START_REF] Smith | Microstructure and evolution of enamel amongst osteichthyan fishes and early tetrapods[END_REF] and thus one would expect that the mineralization of the enamel-like crown tissue was associated with the presence of a mineralized dentine-like basal body.

Furthermore, the most recent functional model of the conodont feeding apparatus [START_REF] Goudemand | Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates[END_REF] was based on the three-dimensional reconstruction of some Triassic apparatuses (in particular Novispathodus) for which the morphology of the basal bodies is unknown. [START_REF] Goudemand | Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates[END_REF] suggested that the feeding apparatus of conodonts function in a way that is similar, if not homologous, to that of extant cyclostomes (hagfishes and lampreys): a pulley-like mechanism whereby the S elements rotated around a presumed homologous lingual cartilage. If confirmed, this model would lend strong support to the current hypothesis that conodonts are likely stem-cyclostomes. Yet, although the model proposed by Goudemand et al. (ibid) was partially corroborated by the discovery of new Triassic natural assemblages [START_REF] Huang | Gondolelloid multielement conodont apparatus (Nicoraella) from the Middle Triassic of Yunnan Province, southwestern China[END_REF], there is still no direct evidence for the speculated lingual cartilage. Furthermore, one of the assumptions of this model was that the basal bodies of the corresponding S elements were relatively thin and did not interfere with the reconstructed movements of the elements: a hypothesis that could not be tested hitherto.

In addition, the morphology of the basal body is likely to be informative for assessing the role of functional and developmental constraints on the morphological evolution of conodont elements. Some authors have applied analytical tools such as finite element analysis (Jones et al., 2012a;2012b;[START_REF] Martínez-Pérez | Translating taxonomy into the evolution of conodont feeding ecology[END_REF]2014;[START_REF] Murdock | Functional adaptation underpinned the evolutionary assembly of the earliest vertebrate skeleton[END_REF] and geometric morphometrics [START_REF] Jones | A New Semi-Automatic Morphometric Protocol for Conodonts and Taxonomic Application[END_REF][START_REF] Renaud | Strategies of survival during extreme environmental perturbations: evolution of conodonts in response to the Kellwasser crisis (Upper Devonian)[END_REF] to quantify aspects of conodont biomechanics and patterns of intra and inter-specific variation respectively. It is uncertain whether the addition of data from intact basal bodies will affect the conclusions that have been drawn from analyses that have only used crowns.

Herein we report the recent discovery of Triassic conodont elements, including S and M elements, which have basal bodies preserved. We discuss the developmental importance of basal bodies in Triassic elements, we explore the variation and possible covariation of this structure in our samples and in the literature, and we assess the viability of the Novispathodus functional model.

Geological settings

The described elements were extracted from rock samples collected on an exotic block of marine, Lower Triassic limestone located in the Batain Plain, eastern Oman (Fig. 2).

The block, about 1.2 meters in diameter, contains abundant ammonoids and conodonts that are diagnostic of the Induan-Olenekian boundary (IOB, or Dienerian-Smithian boundary). The detailed description of this block and of its paleontological content is beyond the scope of the present paper and will be the topic of another publication. Here, we focus on conodont elements with preserved basal bodies that were recovered from the top half of the block (beds F-D, see Fig. 2). This conodont fauna is considered to be early Olenekian (Smithian) in age based on the presence of Novispathodus waageni sensu lato.

The Batain Plain, located south of the city of Sur, eastern Oman, extends over 4000 km 2 in the northeastern corner of Oman. The Batain nappes are represented by allochthonous Permian to Maastrichtian marine sediments, as well as volcanic rocks and the Eastern Oman Ophiolite Nappes, obducted onto the Oman continental margin during the transition from the Cretaceous to the Paleogene [START_REF] Hauser | The breakup of East Gondwana along the northeast coast of Oman: evidence from the Batain basin[END_REF]. They consist of several lithostratigraphic units formations, among which are the Triassic Sal and the Jurassic Guwayza Formations. The conglomerate interbeds in the Guwayza Formation contain reworked boulders of marine limestones from the Sal Formation of Early and Middle Triassic age [START_REF] Hauser | The breakup of East Gondwana along the northeast coast of Oman: evidence from the Batain basin[END_REF], of which the studied block is a typical example. These boulders are evidence of the partial destruction of the proximal Triassic Batain Basin in Late Jurassic times [START_REF] Hauser | The breakup of East Gondwana along the northeast coast of Oman: evidence from the Batain basin[END_REF].

The studied outcrop is located about 20 km northwest of the Ad Daffah village, in the northern part of the Batain Plains (GPS coordinates: 22°22'18.5"N, 59°39'46.0"E, Fig. 2: A).

The boulder was found about five meters away from the top of a small hill.

Methods

The conodonts were extracted using standard acid digestion techniques (Jeppsson et al., 1999). The discussed specimens are from four different but contiguous samples. For each sample, about one kilogram of rock was dissolved in buffered ten percent acetic acid. The insoluble residues were then treated for concentration by heavy liquid separation using Sodium-Polytungstate (Jeppsson and Anehus, 1999). The picking of the elements was done under a Motic binocular. The preservation of the specimens is pristine, with most denticles still intact, surfaces clean of debris and a color alteration index (CAI) of 1. Hence these elements were subjected to minimal post-depositional transport and minimal heating [START_REF] Epstein | Conodont color alteration: an index to organic metamorphism[END_REF]. Pictures of the elements were taken at the University of Zurich using a Jeol JSM-6010 environmental SEM, without metallic coating, which enables easier distinction of the basal body.

Taxonomy

The described material corresponds to a very small portion of the abundant assemblages (several thousands of elements) that we retrieved. Elements of Novispathodus ex gr. waageni compose most of these assemblages. Elements of the genera ?Neospathodus, Wapitiodus, Guangxidella, Discretella, 'Cornudina', and Smithodus are also present, although much less abundant. This association is typical of the Smithian (early Olenekian).

Class Conodonta Pander, 1856

Division Prioniodontida Dzik, 1976Order Ozarkodinida Dzik, 1976Superfamily Gondolelloidae (Lindström, 1970) Family Gondolellidea Lindström, 1970 Subfamily Novispathodinae [START_REF] Orchard | Multielement conodont apparatuses of Triassic Gondolelloidea. Conodont biology and phylogeny: interpreting the fossil record[END_REF]Genus Novispathodus Orchard, 2005 For the crown part of the elements, the multielement diagnosis is as described by [START_REF] Orchard | Multielement conodont apparatuses of Triassic Gondolelloidea. Conodont biology and phylogeny: interpreting the fossil record[END_REF] and subsequently revised by [START_REF] Goudemand | Early Triassic conodont clusters from South China: revision of the architecture of the 15 element apparatuses of the superfamily Gondolelloidea[END_REF].

The M element is breviform digyrate (Fig. 5: a,b). One partly broken element has straight erect sharp denticles and a narrow angle between the two processes that is completely filled with basal body (a). The other element, which may however belong to Neospathodus, (b), likely from a different species than the latter element, possesses short discrete rounded denticles and a wide blunt angle between the two processes. A thin strip of basal body extends on the basal side along the processes, but it is unclear whether it is complete or not.

The alate S 0 element bears two symmetrical antero-lateral processes branching from a point anterior to the cusp (Fig. 5: c), and the basal body is visible on the basal side of the lateral processes. The basal body of the specimen illustrated herein seems to have been initially more extended.

The S 1 element is weakly digyrate with one much shorter anterolateral process that bears one or two small denticles (Fig. 5: h-i, p-s). Some elements bear a nearly complete basal body that fills the basal cavity of the crown, and extends along the posterior process where it divides into two lips separated by a groove. In lateral view, the outline of the basal body is smooth and subparallel to the basal margin along the larger anterolateral process.

The breviform digyrate S 2 element has two long curved anterolateral processes (which are broken in the illustrated specimens, Fig. 5: d-g, l-o). The basal body fills the narrow, acute angled space under the basal cavity.

The S 3-4 elements are bipennate elements (Fig. 5: w). The posterior process is attached but it may often be broken (as is the case in the illustrated specimen). The lower margin of the posterior process is usually sinuous. The basal body extends along the entire length of the process and its height seems to increase in the concave, posteriormost portion of the sinuous posterior process.

The P 2 angulate high-bladed elements have an anterior process twice as long as the posterior one (Fig. 5: t-v). The basal body is very reduced, forming a thin layer filling the space between the two processes. This results in a straight flat lower margin of the entire element in lateral view.

Novispathodus waageni (Sweet, 1970) (Figs. 3, 4: a, g-t) In lateral view, the P 1 element presents a subquadrate form, with a laterally compressed blade, an arched crest composed of seven to 14 slightly reclined denticles with a fan-like organization. In his original description, [START_REF] Sweet | Uppermost Permian and Lower Triassic conodonts of the Salt Range and Trans-Indus Range, West Pakistan. Stratigraphic Boundary Problems[END_REF] notes that this variable species often presents a sensibly upturned basal margin in the posterior half of the element. We observe what seems to be a morphological continuum between slightly and strongly upturned morphologies. In all variants the outline of the basal body mostly follows that of the lower margin of the crown, except at mid-length where it is U-shaped. It is not clear yet whether the variation on the outline of the basal body may correlate with any trait of the crown. The point where the basal body seems to disappear (in lateral view) within the crown is variable. Note however that the anterior part of the basal body is frequently broken. In aboral view, the lower surface of the basal body presents a slight depression at the position of the pit. From this depression in the pit a groove extends to the anterior end of the element, just as the groove of the crown does.

Novispathodus aff. waageni (Fig. 4: b-f) These short segminate P 1 elements are most similar to those of N. waageni but they bear erect and less numerous denticles and the posterior denticles do not decline in height as smoothly as in N. waageni. The basal body is much reduced.

Results

Remains of basal bodies are easily identified because they are white and opaque, while the crowns are usually creamy yellow and often translucid. Basal bodies are very common although in many cases they consist of only small patches on the aboral side of the elements. Some much less frequent elements do exhibit basal bodies that we consider to be complete, based on a superficial investigation of the surface smoothness and the apparent absence of breakage surfaces. This is particularly true for P elements, for which the basal body is also usually thicker, potentially explaining this differential preservation.

Presence of basal bodies in Novispathodus elements

The P elements of Novispathodus are the predominant form in these assemblages, as such, it is likely that the frequent S and M elements found associated with them also belong predominantly to Novispathodus. A comparison with the multi-element reconstruction of [START_REF] Orchard | Multielement conodont apparatuses of Triassic Gondolelloidea. Conodont biology and phylogeny: interpreting the fossil record[END_REF], revised by [START_REF] Goudemand | Early Triassic conodont clusters from South China: revision of the architecture of the 15 element apparatuses of the superfamily Gondolelloidea[END_REF], confirms this hypothesis (see the discussion of taxonomy above in the Taxonomy part below).

All eight element types of the apparatus of Novispathodus are represented by at least one individual with at least partial basal body preservation (see Figs. 3,4,5,not all illustrated). This constitutes the first evidence of the presence of basal bodies in S or M elements younger than the Carboniferous, and hence the first evidence for the entire superfamily Gondolelloidea. The Gondolelloidea are the predominant conodont group from the mid-Permian to the end of the Triassic [START_REF] Orchard | Multielement conodont apparatuses of Triassic Gondolelloidea. Conodont biology and phylogeny: interpreting the fossil record[END_REF]. As mentioned above, besides Novispathodus, several genera also belonging to this superfamily are represented in the studied material and for most there exists at least a few individuals that show at least traces of basal body (not illustrated).

Extention of basal bodies in Gondolelloidea

Even in specimens where the basal body seems to be preserved completely, the basal body is much reduced compared to the crown. In most cases it barely extends beyond the margin of the basal cavity. In S and M elements (Fig. 5) its extent is maximal under the basal cavity, in P elements (Figs. 3,4) at element's mid-length, just in front of the cavity. On the aboral side of the processes of ramiform elements it is reduced to a thin layer.

In Novispathodus, the basal body is concave in S and M elements and convex in P elements

In S and M elements (Figs. 5,6,7), the subconical basal body fills the basal cavity and, between any two processes, the smooth curve defined by its edges suggest a hyperbola whose asymptotes are the lower margins of the processes. In other words, if we consider the lower margin of the crown as the geometrical reference, the basal body has a smooth, concave outline that converges with the crown outline along the processes.

In P elements the contrary is true, the basal body has a smooth, convex outline (Figs.

3, 4, 6, 7) and in lateral view, it forms a broad parabola with maximum curvature facing aborally. The base of the parabola is located at mid-length of the element and the sides are recurved to align with the lower margin of the crown.

Discussion

Lack of basal bodies in post-Devonian conodonts is not the result of an evolutionary loss

Because the growth lamellae are continuous between the crown and the basal body [START_REF] Müller | Über den Feinbau der Conodonten[END_REF], it is thought that the basal body and the crown developed in concert. Yet, the apparent absence of basal body in many taxa (in particular, until recently, in all late Paleozoic and Triassic S and M conodont elements) had raised the question of whether this was due to a preservational bias, or reflected an actual evolutionary trend towards unmineralized basal bodies, and, in the latter case, how this could be explained developmentally [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF].

During development of the vertebrate dermal skeleton, enamel secretion is induced by the presence of mineralized dentine [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF][START_REF] Smith | Microstructure and evolution of enamel amongst osteichthyan fishes and early tetrapods[END_REF]. This ensures outward secretion and growth of the corresponding odontodes, the basic building blocks of the vertebrate dermal skeleton (Ørvig, 1967). If an enamel-like tissue is secreted by the epithelial cells before mineralization of the underlying dentine-like tissue, as is the case with enameloid [START_REF] Smith | Heterochrony in the evolution of enamel in vertebrates[END_REF][START_REF] Smith | Microstructure and evolution of enamel amongst osteichthyan fishes and early tetrapods[END_REF], we expect growth to occur in the opposite direction: inwards instead of outwards. As a consequence, unless at least a thin layer of basal body is mineralized at the crown-basal body junction in conodont elements, the observed enamellike outward growth of the crown tissue can hardly be explained. Indeed, although the ultrastructural evolution of early conodont elements seems to exclude a homology between lamellar crown tissue and enamel [START_REF] Kemp | Hyaline tissue of thermally unaltered conodont elements and the enamel of vertebrates[END_REF][START_REF] Kemp | Hyaline tissue of thermally unaltered conodont elements and the enamel of vertebrates[END_REF]Nicoll, 1996, 1995;[START_REF] Murdock | The origin of conodonts and of vertebrate mineralized skeletons[END_REF][START_REF] Reif | Conodonts, odontodes, stem-groups, and the ancestry of enamel genes[END_REF][START_REF] Schultze | Conodont histology: An indicator of vertebrate relationship?[END_REF][START_REF] Trotter | New insights into the ultrastructure, permeability, and integrity of conodont apatite determined by transmission electron microscopy[END_REF], it is very likely that conodont elements developed in a way that is analogous, if not homologous, to vertebrate odontodes. Odontodes are formed through epithelial-ectomesenchymal interaction, via formation of a placode, invagination of the corresponding volume of epithelium into the mesenchyme, and subsequent mineralization at the epithelialmesenchymal interface. [START_REF] Smith | Heterochrony in the evolution of enamel in vertebrates[END_REF] first attempted to homologize conodont elements with odontodes. Later, [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF] who reviewed in detail the growth and patterning of conodont elements, proposed instead that each conodont element may be homologous to an odontocomplex (or polyodontode), not to a single odontode. Since conodont's crown tissue has apparently evolved independently of enamel [START_REF] Murdock | The origin of conodonts and of vertebrate mineralized skeletons[END_REF], it is not clear to what extent this putative homology may hold. Yet, a deep homology may actually exist, in terms of underlying processes, between these two organs. [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF] suggested that odontodes were "flexible enough to allow any of their component tissues (enamel, dentine, bone) […] to be present independently of the others" (1998, p. 658), which would have explained the absence of basal body in some conodont taxa. This statement may be true in general, but it seems that the biophysical, mineralization constraint mentioned above excludes the possibility of independence between enamel and dentine, and likewise between an enamel-like tissue and its dentinelike counterpart, in outward growing odontodes (or odontode-like organs), such as conodont elements.

Consequently the apparent absence of mineralized basal bodies in late Paleozoic and Triassic conodonts was intriguing and one could have argued that conodont elements (of that age at least) must have developed in a way that is fundamentally different from that of any other known vertebrate odontode. Although the main evidence for a vertebrate affinity of conodonts is based on the soft tissue homology and does not rest on a putative homology of their feeding elements with those of other vertebrates [START_REF] Aldridge | The anatomy of conodonts[END_REF][START_REF] Blieck | Fossils, histology, and phylogeny: why conodonts are not vertebrates[END_REF][START_REF] Donoghue | Conodont affinity and chordate phylogeny[END_REF][START_REF] Donoghue | The ins and outs of the evolutionary origin of teeth[END_REF][START_REF] Janvier | Facts and fancies about early fossil chordates and vertebrates[END_REF][START_REF] Krejsa | A neontological interpretation of conodont elements based on agnathan cyclostome tooth structire function, and development[END_REF][START_REF] Murdock | The origin of conodonts and of vertebrate mineralized skeletons[END_REF][START_REF] Purnell | Large eyes and vision in conodonts[END_REF][START_REF] Purnell | Conodonts and the first vertebrates[END_REF][START_REF] Sansom | Presence of the earliest vertebrate hard tissue in conodonts[END_REF][START_REF] Turner | False teeth: conodont-vertebrate phylogenetic relationships revisited[END_REF], this may have even been used to suggest that conodonts are not vertebrates. Based on the evidence presented here, and as far as we currently know, Novispathodus elements, and possibly all conodonts elements, still do conform to the development of vertebrate odontodes.

The extent of the basal bodies in Novispathodus agrees with the current, cyclostome-like functional model of the conodont apparatus

In the description of their functional model of the conodont apparatus, Goudemand et al., (2011 p. 8723) wrote that in "S or M elements, the basal body, when present, smoothes out the lower margin (ventral outline) of the element [...]. In Novispathodus, the lower margins of the S elements are already smooth (low 3D curvature), and we therefore assume that their respective basal bodies, if mineralized, were relatively thin and filled up the basal grooves but did not alter the shape of their lower margins substantially". They also illustrated the apparatus of Novispathodus with elements reconstructed using their hypothesis for the extent of the basal body (their additional figure S 1 ). Except for the sinuous posterior part of the posterior process in S 0 and S 3-4 elements, the observed extent of the basal body in Novispathodus is in full agreement with their predictions. Because the posterior part of the posterior process of these elements is not critical for the validity of their model, we conclude that the observed morphology of the basal body does not come into conflict with their model, which therefore still holds at least for Novispathodus.

A pattern of covariation between the morphologies of crown and basal body?

In order to discuss the universality within euconodonts of the observed morphological covariation between the crown and the basal body --that is, a convex basal body associated with a sub-flat lower margin of the crown and a concave basal body associated with a concave lower margin of the crown --we reviewed the literature for illustrated examples of basal body preservation in non-coniform elements. Note that many different names have been given to the basal body: basal filling, basal plate, basal attachment, basal structure, basal organ. Also note that in several cases the authors do not mention the presence of basal bodies even though basal bodies can be sometimes distinguished in the illustrations. Therefore, we do not claim to have identified all the published occurrences. Moreover, there were cases where the preservation of the basal bodies were too partial to be included in the following discussion.

For the ramiform elements, we were able to retrieve illustrations for twelve taxa only: the Ordovician Archeognathus primus and Iowagnathus grandis [START_REF] Liu | Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA[END_REF], Microzarkodina sp., Omanognathus sp. and Aldridgeognathus manniki [START_REF] Miller | Two new early balognathid conodont genera from the Ordovician of Oman and comments on the early evolution of prioniodontid conodonts[END_REF], the Silurian Arianagnathus jafariani and Ctenognathodus sp. [START_REF] Männik | A new early Silurian prioniodontid conodont with three P elements from Iran and associated species[END_REF], Oulodus elegans [START_REF] Jeppsson | Notes on some Upper Silurian multielement conodonts[END_REF], Ozarkodina hemensis [START_REF] Miller | A conodont, thelodont and acanthodian fauna from the Lower Přidoli (Silurian) of the Much Wenlock Area, Shropshire[END_REF], Kockelella (Slavík et al., 2010), and the Devonian Hibbardella angulata [START_REF] Nicoll | Conodont apparatuses in an Upper Devonian palaeoniscoid fish from the Cannig Basin, Western Australia[END_REF] and Apatognathus sp. [START_REF] Nicoll | The multielement genus Apatognathus from the Late Devonian of the Canning Basin, Western Australia[END_REF]. Nevertheless, the outline of the basal body is always mostly concave in lateral view. We also observe a general decrease in size (height and thickness) of the basal body over time, but this qualitative observation would deserve more attention as we may expect the extent of basal body to vary as a multivariate function of geometry (e.g., angle between processes), size and mechanical properties (e.g., elasticity of the tissues before mineralization), parameters that may in turn depend on, for instance, the position of the element within the apparatus or phylogeny.

Pectiniform elements with basal body preservation are much more frequent in the literature. We noted in particular: for the Ordovician, Archeognathus primus [START_REF] Liu | Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA[END_REF][START_REF] Mosher | A unique conodont basal structure from the Ordovician of Alabama[END_REF], Jumudontus gananda [START_REF] Pyle | Conodont fauna and biostratigraphy of the Outram, Skoki, and Owen Creek formations (Lower to Middle Ordovician), Wilcox Pass, Alberta, Canada[END_REF], Microzarkodina sp. [START_REF] Miller | Two new early balognathid conodont genera from the Ordovician of Oman and comments on the early evolution of prioniodontid conodonts[END_REF]; for the Silurian, Ozarkodina derenjalensis [START_REF] Männik | A new early Silurian prioniodontid conodont with three P elements from Iran and associated species[END_REF], Ozarkodina hemensis [START_REF] Miller | A conodont, thelodont and acanthodian fauna from the Lower Přidoli (Silurian) of the Much Wenlock Area, Shropshire[END_REF], Polygnathoides siluricus, Ozarkodina typica, Ozarkodina cf. cornidentata and Wurmiella excavata (Slavík et al., 2010;[START_REF] Slavík | Post-Lau Event (late Ludfordian, Silurian) recovery of conodont faunas of Bohemia[END_REF]; for the Devonian, Polygnathus kennettensis [START_REF] Savage | Middle Devonian species of Polygnathus from the Kennett Formation, northern California[END_REF], Palmatolepis sp. [START_REF] Uyeno | Pre-Famennian Devonian conodont biostratigraphy of selected intervals in the eastern Canadian Cordillera[END_REF], Polygnathus linguiformis linguiformis [START_REF] Sparling | Conodont biostratigraphy and biofacies of lower Middle Devonian limestones, north-central Ohio[END_REF], Hibbardella angulata [START_REF] Nicoll | Conodont apparatuses in an Upper Devonian palaeoniscoid fish from the Cannig Basin, Western Australia[END_REF]; for the Permian, Clarkina bitteri [START_REF] Kozur | Boundaries and Stage subdivision of the Mid-Permian (Guadalupian Series) in the light of new micropaleontological data[END_REF], Jinogondolella nankingensis, J. postserrata and J. shannoni [START_REF] Kozur | Guadalupian (Middle Permian) conodonts of sponge-bearing limestones from the margins of the Delaware Basin, West Texas[END_REF]; and for the Triassic, Gladigondolella malayensis [START_REF] Nogami | Trias-Conodonten von Timor, Malaysien und Japan[END_REF], Clarkina carinata [START_REF] Orchard | Conodonts from the Lower Griesbachian Otoceras latilobatum Bed of Selong, Tibet and the Position of the Permian-Triassic Boundary[END_REF] and Novispathodus waageni [START_REF] Goel | Triassic conodonts from Spiti (Himachal Pradesh), India[END_REF]. Specimens of Novispathodus waageni with preserved basal bodies presented by [START_REF] Goel | Triassic conodonts from Spiti (Himachal Pradesh), India[END_REF] are only illustrated on the latteral view. It is unclear if the observed variability of the basal bodies, especialy its extent, is due to breakages or is a real morphological feature. The visible part of the basal body are similar to the ones described in this paper.

The outline of the basal body always has a convex profile in lateral view. Except for the very early history presented in conodonts, we do not observe a temporal decrease of the basal body in pectiniform elements. This is striking when one observes the example of Archeognathus. Complex conodont elements may have evolved from simple, coniform elements via an increase of mineralization of the crown and/or the basal body and subsequent fusion of otherwise isolated (coniform) denticles and formation of a bar (compare Archeognathus in [START_REF] Liu | Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA[END_REF][START_REF] Klapper | The enigmatic Middle Ordovician fossil Archeognathus and its relations to conodonts and vertebrates[END_REF][START_REF] Klapper | The enigmatic Middle Ordovician fossil Archeognathus and its relations to conodonts and vertebrates[END_REF]. It is worth noting that this is also what is observed during the growth of some complex conodont elements [START_REF] Donoghue | Growth and patterning in the conodont skeleton[END_REF][START_REF] Murdock | The origin of conodonts and of vertebrate mineralized skeletons[END_REF].

May the morphology of the basal body inform us about the mechanical constraints acting on the elements?

Since the basal body may have served as an attachment surface for the underlying soft tissues [START_REF] Branson | Conodonts from the Jefferson City (Lower Ordovician) of Missouri[END_REF][START_REF] Mosher | A unique conodont basal structure from the Ordovician of Alabama[END_REF][START_REF] Sweet | Conodonts from the Harding Formation (Middle Ordovician) of Colorado[END_REF], it is likely that its morphology holds the answer to the mechanical constraints associated with feeding.

In some cases, especially in early conodont taxa such as the Ordovician Archeognathus [START_REF] Liu | Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA[END_REF], the basal body may actually be more informative than the crown: in Archeognathus, the crown is reduced to a 'simple' comb-like bar, whereas the relatively conspicuous basal body exhibits a large root-like extension that may be interpreted more easily in terms of biomechanics.

In that respect, the concave vs. convex outline may also reflect distinct mechanical, functional constraints on the elements: although the motion of pectiniform elements tends to be oriented mostly along an oral-aboral axis [START_REF] Jones | Testing microstructural adaptation in the earliest dental tools[END_REF], the current functional model of the ramiform S and M elements suggests pivotal motions where the main pulling forces were exerted along the processes [START_REF] Goudemand | Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates[END_REF]. Hence it might not be surprising that the basal bodies of pectiniform elements extend essentially along the oralaboral axis and the basal bodies of ramiform elements define a minimal surface at the intersection of the processes. Detailed finite element analyses (FEA) might help us to test this functional hypothesis in the future and, if it holds, then the morphology of the basal body, along with the morphology of the cusp (see [START_REF] Goudemand | Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates[END_REF][START_REF] Murdock | The origin of conodonts and of vertebrate mineralized skeletons[END_REF][START_REF] Purnell | Architecture and functional morphology of the skeletal apparatus of ozarkodinid conodonts[END_REF] might even be used someday to infer the movements of a given element.

Conclusions

The discovery of abundant Early Triassic Novispathodus elements, including S and M elements, with preserved basal bodies has several implications for our understanding of conodont macroevolution. The apparent absence of basal bodies in post-Devonian ramiform conodonts (until now) is probably due to a preservation bias only and not the result of an evolutionary trend towards unmineralized basal bodies. The morphology and extent of the basal body in Novispathodus are in agreement with previous predictions and do not contradict a cyclostome-like functional model of the apparatus. Finally, the review of basal body occurrences in the literature suggests a general pattern for the morphology of the basal body in conodont elements that may reflect the mechanical constraints associated with the feeding motions. Although the histology of both crown and basal tissues has been studied to some extent in Paleozoic forms, much is still to be done to understand the likely couplings between the features of their respective ultrastructures, their respective sizes and morphologies, and how these couplings may originate from putative developmental and functional constraints. It would be interesting also to assess whether these couplings may or may not have evolved through conodont history. cornidentata, P 1 elements. h: Polygnathoides siluricus, P 1 element. i: Polygnathoides siluricus, P 2 element. j: ?Ozarkodina hemensis, P 1 element. k: Kockelella variabilis variabilis, P 1 element. l: Arianagnathus jafariani, S b1 dextral element. m: Oulodus elegans, ramiform element. n: Ctenognathodus sp., S 1-2 element. o: Polygnathus kennettensis, P 1 element. p: Polygnathus linguiformis linguiformis, P 1 element. q: Palmatolepis sp., P 1 element. r : Polygnathus aff. trigonicus, P 1 element. s-t: Oulodus angulatus, M element (t: enlarged view). u: Hibbardella angulata, S a element. v-w: Hibbardella angulata, "N" element (w: enlarged view). x-y: Apatognathus varians klapperi, S a element (y: enlarged view). z: Clarkina cf. bitteri, P 1 element. aa: Mesogondolella nankingensis, P 1 element. ab: Mesogondolella postserrata, P 1 element. ac: Gondolella naviculla, P 1 element in lateral and lower view. ad: Clarkina carinata, P 1 element. ae: Novispathodus waageni, P 1 element. a-d: Ordovician. e-n: Silurian. o-y: Devonian. z-ab: Permian. ac-ae: Triassic. False red color highlights the basal bodies. Scale bars 250 µm. Some scales were missing. Adapted from [START_REF] Pyle | Conodont fauna and biostratigraphy of the Outram, Skoki, and Owen Creek formations (Lower to Middle Ordovician), Wilcox Pass, Alberta, Canada[END_REF]; Miller et al., 2017 (b,d); Mosher and Bodenstein, 1969 (c);Miller and Märss, 1999 (e,j); Slavík and Carls, 2012 (f-g);Slavík et al., 2010 ( h-i,k); Männik et al., 2013 (l,n);Jeppson, 1969 (m); Savage, 1976 (o); Sparling, 1983 (p,r); Uyeno, 1991 (q); Nicoll, 1977 (s-w); Nicoll, 1980 (x-y); Kozur, 1992 (z); Kozur and Mostler, 1995 (aa-ab); Nogami, 1968 (ac); Orchard et al., 1994 (ad); Goel, 1977 (ae). The basal bodies are highlighted in false red color. Cambrian elements are coniforms. Euconodont (complex conodont) elements have been schematized, the actual morphologies differ from this simplified representation. Top: P elements; Bottom: S or M elements. Note the difference of curvature of the lower outline of the basal body between the P and S or M elements. Dark grey, continuous outline: presence of basal body already documented in the literature [START_REF] Goel | Triassic conodonts from Spiti (Himachal Pradesh), India[END_REF][START_REF] Kozur | Boundaries and Stage subdivision of the Mid-Permian (Guadalupian Series) in the light of new micropaleontological data[END_REF][START_REF] Kozur | Guadalupian (Middle Permian) conodonts of sponge-bearing limestones from the margins of the Delaware Basin, West Texas[END_REF]Männik et al., 2013;[START_REF] Miller | A conodont, thelodont and acanthodian fauna from the Lower Přidoli (Silurian) of the Much Wenlock Area, Shropshire[END_REF]Miller et al., 2017;[START_REF] Mosher | A unique conodont basal structure from the Ordovician of Alabama[END_REF][START_REF] Nicoll | Conodont apparatuses in an Upper Devonian palaeoniscoid fish from the Cannig Basin, Western Australia[END_REF][START_REF] Nicoll | The multielement genus Apatognathus from the Late Devonian of the Canning Basin, Western Australia[END_REF][START_REF] Orchard | Conodonts from the Lower Griesbachian Otoceras latilobatum Bed of Selong, Tibet and the Position of the Permian-Triassic Boundary[END_REF][START_REF] Pyle | Conodont fauna and biostratigraphy of the Outram, Skoki, and Owen Creek formations (Lower to Middle Ordovician), Wilcox Pass, Alberta, Canada[END_REF][START_REF] Savage | Middle Devonian species of Polygnathus from the Kennett Formation, northern California[END_REF]Slavík et al., 2010;[START_REF] Slavík | Post-Lau Event (late Ludfordian, Silurian) recovery of conodont faunas of Bohemia[END_REF][START_REF] Sparling | Conodont biostratigraphy and biofacies of lower Middle Devonian limestones, north-central Ohio[END_REF][START_REF] Uyeno | Pre-Famennian Devonian conodont biostratigraphy of selected intervals in the eastern Canadian Cordillera[END_REF]; soft grey, dashed outline: presence of basal body hitherto not documented but inferred. Black: presence of basal body observed in this study.

Figure captions Figure 1 :

 1 Figure captionsFigure1: Representation of a conodont P 1 element of Novispathodus waageni and its two principal parts: the crown and the basal body. A: Picture of the element. B: Drawing of the same element with basal body highlighted in grey. Figure 2: A: Location map of the studied material. The star indicates the Ad Daffah locality. B: Onsite view of the studied exotic block. The lithology of this meter-thick limestone block is subhomogeneous. The material discussed here comes from the Smithian (Olenekian) top of the block (the interval corresponding to the solid bar with a *, next to the hammer). C: Stratigraphic log with position of the studied samples, scale = 25 cm.Figure 3: Novispathodus waageni. All P 1 elements in lateral and aboral views. The basal bodies are highlighted in false red color. Scale bar 500 µm.

Figure 2 :

 2 Figure captionsFigure1: Representation of a conodont P 1 element of Novispathodus waageni and its two principal parts: the crown and the basal body. A: Picture of the element. B: Drawing of the same element with basal body highlighted in grey. Figure 2: A: Location map of the studied material. The star indicates the Ad Daffah locality. B: Onsite view of the studied exotic block. The lithology of this meter-thick limestone block is subhomogeneous. The material discussed here comes from the Smithian (Olenekian) top of the block (the interval corresponding to the solid bar with a *, next to the hammer). C: Stratigraphic log with position of the studied samples, scale = 25 cm.Figure 3: Novispathodus waageni. All P 1 elements in lateral and aboral views. The basal bodies are highlighted in false red color. Scale bar 500 µm.

Figure 3 :

 3 Figure captionsFigure1: Representation of a conodont P 1 element of Novispathodus waageni and its two principal parts: the crown and the basal body. A: Picture of the element. B: Drawing of the same element with basal body highlighted in grey. Figure 2: A: Location map of the studied material. The star indicates the Ad Daffah locality. B: Onsite view of the studied exotic block. The lithology of this meter-thick limestone block is subhomogeneous. The material discussed here comes from the Smithian (Olenekian) top of the block (the interval corresponding to the solid bar with a *, next to the hammer). C: Stratigraphic log with position of the studied samples, scale = 25 cm.Figure 3: Novispathodus waageni. All P 1 elements in lateral and aboral views. The basal bodies are highlighted in false red color. Scale bar 500 µm.

Figure 4 :

 4 Figure 4: a, g-t: Novispathodus waageni. b-f: Novispathodus aff. waageni. All P 1 elements in lateral and aboral views. The basal bodies are highlighted in false red color. Scale bar 500 µm.

Figure 5 :

 5 Figure 5: Novispathodus ramiform elements. a-b: M. c: S 0 . h-k, p-s: S 1 . d-g, l-o: S 2 . t-v: P 2 . w: S 3-4 . The basal bodies are highlighted in false red color. Scale bar 500 µm.

Figure 6 :

 6 Figure 6: Published elements with subcomplete preservation of the basal bodies. a: Jumudontus gananda, P element. b: aff. Microzarkodina sp., P element. c: ?Archeognathus primus, P element. d: Omanognathus daiqaensis, S 3 element. e: ?Ozarkodina hemensis, P 2 element. f-g: Ozarkodina cf.

Figure 7 :

 7 Figure7: Presence of basal bodies through time. Cambrian elements are coniforms. Euconodont (complex conodont) elements have been schematized, the actual morphologies differ from this simplified representation. Top: P elements; Bottom: S or M elements. Note the difference of curvature of the lower outline of the basal body between the P and S or M elements. Dark grey, continuous outline: presence of basal body already documented in the literature[START_REF] Goel | Triassic conodonts from Spiti (Himachal Pradesh), India[END_REF][START_REF] Kozur | Boundaries and Stage subdivision of the Mid-Permian (Guadalupian Series) in the light of new micropaleontological data[END_REF][START_REF] Kozur | Guadalupian (Middle Permian) conodonts of sponge-bearing limestones from the margins of the Delaware Basin, West Texas[END_REF] Männik et al., 2013;[START_REF] Miller | A conodont, thelodont and acanthodian fauna from the Lower Přidoli (Silurian) of the Much Wenlock Area, Shropshire[END_REF] Miller et al., 2017;[START_REF] Mosher | A unique conodont basal structure from the Ordovician of Alabama[END_REF][START_REF] Nicoll | Conodont apparatuses in an Upper Devonian palaeoniscoid fish from the Cannig Basin, Western Australia[END_REF][START_REF] Nicoll | The multielement genus Apatognathus from the Late Devonian of the Canning Basin, Western Australia[END_REF][START_REF] Orchard | Conodonts from the Lower Griesbachian Otoceras latilobatum Bed of Selong, Tibet and the Position of the Permian-Triassic Boundary[END_REF][START_REF] Pyle | Conodont fauna and biostratigraphy of the Outram, Skoki, and Owen Creek formations (Lower to Middle Ordovician), Wilcox Pass, Alberta, Canada[END_REF][START_REF] Savage | Middle Devonian species of Polygnathus from the Kennett Formation, northern California[END_REF]

Acknowledgments

We thank Aymon Baud (Lausane), Romain Jattiot and Hugo Bucher (Paleontological Institute and Museum, Univ. Zurich, Switzerland; PIMUZ) for their help in the field. We also thank Morgane Brosse (PIMUZ) for her help with the SEM and associated image processing, and Marc Leu (PIMUZ) for having processed some of the rocks. The editors Thomas Algeo

Funding: This work was supported by a French ANR @RAction grant (ACHN project EvoDevOdonto).

the guest-editors Annalisa Ferretti, Alyssa Bancroft and John Repetski, and the two reviewers Martyn Golding and Carlos Martínez-Pérez are thanked for their thoughtful inputs.