Emeric Gillet

Bertrand Lefebvre
email: bertrand.lefebvre@univ-lyon1.fr

Véronique Gardien

Emilie Steimetz

Christophe Durlet

Frédéric Marin

Veronique Gardien

Frederic Marin

Reinterpretation of the enigmatic Ordovician genus Bolboporites

Introduction

The endoskeleton of echinoderms is a complex, multi-element structure typically consisting of several thousands of individual plates bound together in life by soft tissues (including collagen fibres) and each consisting of monocrystalline calcite. Taphonomic experiments suggest that collagen decays relatively soon after the death of the organism (within days or weeks), thus leading to the collapse and rapid disarticulation of the skeleton into isolated plates and/or sometimes, more resistant modules [START_REF] Donovan | The taphonomy of echinoderms: calcareous multi-element skeletons in the marine environment[END_REF][START_REF] Brett | Echinoderm taphonomy, taphofacies, and Lagerstätten[END_REF]. The assignment of an isolated skeletal element to the phylum Echinodermata is generally straightforward and relies on the presence of a typical three-dimensional meshlike microstructure, the stereom (Smith 1980a;[START_REF] Kouchinsky | Chronology of early Cambrian biomineralization[END_REF]. However, both the potential diagenetic alteration of the stereom and the high morphological disparity of skeletal elements within a same individual (e.g., columnals, holdfast, spines) make it often difficult to identify, and sometimes interpret, such isolated plates, especially in the case of Palaeozoic taxa that have no current representatives [START_REF] Berg-Madsen | Middle Cambrian cystoid (sensu lato) columnals from Bornholm, Denmark[END_REF][START_REF] Pisera | Echinoderms from the Mójcza Limestone[END_REF][START_REF] Zamora | Cambrian echinoderm diversity and palaeobiogeography[END_REF]).

The situation is further complicated by the existence of numerous morphological convergences in echinoderms. For example, Palaeozoic deposits have yielded several relatively similar-looking, small, bowl-to cone-shaped structures (e.g., Cymbionites [START_REF] Whitehouse | The Cambrian faunas of north-eastern Australia. Part 4: Early Cambrian echinoderms similar to the larval stages of recent forms[END_REF]Oryctoconus Colchen & Ubaghs, 1969;Peridionites Whitehouse, 1941;Timorocidaris Wanner, 1920). Their nature and precise taxonomic assignment (at class level) has often been strongly debated [START_REF] Bather | Echinoid or crinoid?[END_REF][START_REF] Gislén | On the Haplozoa and the interpretation of Peridionites[END_REF][START_REF] Schmidt | Whitehouse's Ur-Echinodermen aus dem Cambrium Australiens[END_REF]Ubaghs 1968bUbaghs , 1978a;;[START_REF] Smith | The affinities of the Middle Cambrian Haplozoa (Echinodermata)[END_REF][START_REF] Alvaro | Earliest Ordovician pelmatozoan holdfasts from western Europe: the Oryctoconus problem revisited[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF][START_REF] Zamora | Pelmatozoan echinoderms from the Cambrian-Ordovician of the Iberian Chains (NE Spain): early diversification of anchoring strategies[END_REF]). However, most of them are now convincingly interpreted either as pelmatozoan holdfasts (e.g., Oryctoconus; [START_REF] Alvaro | Earliest Ordovician pelmatozoan holdfasts from western Europe: the Oryctoconus problem revisited[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF][START_REF] Zamora | Pelmatozoan echinoderms from the Cambrian-Ordovician of the Iberian Chains (NE Spain): early diversification of anchoring strategies[END_REF], highly specialized columnals (e.g., [START_REF] Sumrall | Systematics and paleoecology of late Cambrian echinoderms from the western United States[END_REF], basal thecal plates of eocrinoids (e.g., Cymbionites, Peridionites ;[START_REF] Smith | The affinities of the Middle Cambrian Haplozoa (Echinodermata)[END_REF] or highly derived crinoid calices made of few, tightly sutured ('fused') plates (e.g., Timorocidaris; [START_REF] Bather | Echinoid or crinoid?[END_REF]Ubaghs 1978a).

On the other hand, the interpretation of some other isolated echinoderm elements remains problematic and controversial. This is the case for the enigmatic Ordovician genus Bolboporites Pander, 1830, which corresponds to centimetric cone-shaped calcitic fossils, with a typical honeycomb-like ornamentation on their external lateral surface (Fig. 1B). The base of the cone is smooth, flat to strongly convex, and bears two adjoining, shallow depressions (Fig. 1A). Within this depressed area, a tiny orifice (Fig. 1A) opens into a narrow, longitudinal canal extending internally towards the apex of the cone [START_REF] Yakovlev | Bolboporites, its structure and affinity to the Hydrozoa[END_REF][START_REF] Yeltysheva | Bolboporites[END_REF][START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF]Rozhnov & Kushlina 1994a).

Bolboporites is particularly widespread and abundant in Baltica, where it is recorded from the Dapingian to the Darriwilian (e.g., Estonia, Russia;[START_REF] Pander | Beiträge zur Geognosie des Russischen Reiches[END_REF][START_REF] Eichwald | Beitrag zur geographischen Verbreitung der fossilen Thiere Russlands. Alte Period[END_REF][START_REF] Bassler | The Early Paleozoic Bryozoa of the Baltic provinces[END_REF][START_REF] Yakovlev | Bolboporites, its structure and affinity to the Hydrozoa[END_REF][START_REF] Yeltysheva | Bolboporites[END_REF][START_REF] Smith | Patterns of diversification and extinction in Early Palaeozoic echinoderms[END_REF][START_REF] Kushlina | The systematic position and composition of the genus Bolboporites (Echinodermata, Eocrinoidea)[END_REF][START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF][START_REF] Federov | Lower Ordovician mud mounds from the St. Petersburg region, northwestern Russia[END_REF]Rozhnov & Kushlina 1994a;[START_REF] Rozhnov | Echinoderms[END_REF]) and locally to the Sandbian (e.g., Norway, Sweden; [START_REF] Kjerulf | Veiviser Ved Geologiske Excursioner i Christiania Omegn. Brøgger & Christie's Bogtrykkeri[END_REF][START_REF] Lindström | Index to generic names applied to the corals of the Palaeozoic formations[END_REF][START_REF] Kushlina | The systematic position and composition of the genus Bolboporites (Echinodermata, Eocrinoidea)[END_REF]. This genus also occurs in the late Darriwilian of Laurentia (e.g., New York, Quebec, Virginia;[START_REF] Hall | Containing Descriptions of the Organic Remains of the Lower Division of the New York System (equivalent to the Lower Silurian Rocks of Europe)[END_REF][START_REF] Billings | The fossils of the Chazy Limestone with descriptions of new species[END_REF][START_REF] Logan | Report of Progress from its commencement to 1863[END_REF][START_REF] Brainerd | The original Chazy rocks[END_REF], 1896;[START_REF] Miller | North American Geology and Palaeontology for the Use of Amateurs, Students, and Scientists[END_REF][START_REF] Brainerd | The Chazy Formation in the Champlain Valley[END_REF][START_REF] Ami | Preliminary lists of the organic remains occurring in the various geological formations comprised in the southwest quartersheet map of the eastern townships of the Province of Quebec[END_REF]White & Nitecki 1990;[START_REF] Baarli | Fossilboka. Forlaget Vett & Viten AS[END_REF]) from the Sandbian of Norway showed remarkable similarities in size, morphology and external ornamentation with co-occurring specimens of Bolboporites from the same levels and localities. This observation thus questioned the echinoderm affinities of Bolboporites [START_REF] Lefebvre | Reinterpretation of the problematic Ordovician genus Bolboporites (?Echinodermata) as a calcareous alga[END_REF][START_REF] Lefebvre | Réinterprétation de Bolboporites, un fossile ordovicien énigmatique[END_REF]. As a consequence, the aims of this paper were to apply for the first time a wide range of techniques (e.g., cathodoluminescence, CTscan, FT-IR analyses, SEM, EBSD) on well-preserved specimens of Bolboporites, so as to test their putative echinoderm affinities and, if confirmed, to discuss the nature of Bolboporites (isolated skeletal element vs. body capsule), as well as its systematic position within the phylum Echinodermata.

Material and Methods

Material. This study is based on 28 specimens of Bolboporites from Norway and Russia. The ten Norwegian specimens were selected within the abundant material of Bolboporites sp.

(about 100 individuals) belonging to the collections of the Paleontologisk Museum, Oslo (acronym: PMO). All Norwegian specimens of Bolboporites were originally collected in 1975 by J.F. Bockelie in bioclastic deposits of the Fossum Formation (Sandbian), at Gravastranda, Herøya (Skien-Langesund area, Norway). These levels are generally interpreted as relatively shallow-water deposits yielding abundant and diverse benthic assemblages regularly smothered by storm deposits [START_REF] Bockelie | The Middle Ordovician of the Oslo region, Norway, 30. The eocrinoid genera Cryptocrinites, Rhipidocystis and Bockia[END_REF][START_REF] Owen | The Ordovician successions of the Oslo region, Norway. Norges Geologiske Undersøkelse[END_REF]). The fauna is dominated by brachiopods, bryozoans, echinoderms (caryocystitid and cheirocrinid rhombiferans, eocrinoids, crinoids, edrioasteroids) and trilobites, associated to rare cephalopods and graptolites [START_REF] Bockelie | The Middle Ordovician of the Oslo region, Norway, 30. The eocrinoid genera Cryptocrinites, Rhipidocystis and Bockia[END_REF][START_REF] Owen | The Ordovician successions of the Oslo region, Norway. Norges Geologiske Undersøkelse[END_REF].

Eighteen individuals of Bolboporites mitralis Pander, 1830, from Russia were made available for this study by S.V. Rozhnov, who donated them to the palaeontological collections of Lyon 1 University (acronym: UCBL). This material was collected on the banks of the Lynna river (Saint-Petersburg area) in the upper member (BIIγ, Frizy Limestone) of the Volkhov Formation. This ~3 m thick stratigraphic unit is dated as early Darriwilian, based on the occurrence of both conodonts typical of the Baltoniodus norrlandicus Zone and trilobites characteristic of the Scandinavian Megistaspis simon Zone [START_REF] Federov | Lower Ordovician mud mounds from the St. Petersburg region, northwestern Russia[END_REF][START_REF] Dronov | Introduction to the geology of the St. Petersburg region[END_REF].

The upper member of the Volkhov Formation consists predominantly of nodular, glauconitic limestones, with several intercalated levels of shales. The bioclastic limestones are generally interpreted as storm-generated deposits, in a shallow-water, temperate setting [START_REF] Dronov | Introduction to the geology of the St. Petersburg region[END_REF].

In these levels, faunal assemblages are dominated by brachiopods, ostracods and isolated pelmatozoan remains, associated to bryozoans, conulariids, graptolites and trilobites [START_REF] Federov | Lower Ordovician mud mounds from the St. Petersburg region, northwestern Russia[END_REF].

Methods.

As Norwegian specimens are preserved as mouldic impressions in the rock, their original external aspect was revealed by making latex casts, which were coated with ammonium chloride (NH 4 Cl) for observation and photographic purposes. External morphological features of Bolboporites sp. from Norway were observed at Lyon 1 University, with a Zeiss SteREO Discovery.V8 stereomicroscope binocular and captured with a Zeiss AxioCam MRc5 digital camera.

In contrast to the Norwegian material, the Russian specimens are preserved as threedimensional fossils, thus allowing the application of a wider range of techniques of observation and analyses. Several Russian specimens were embedded in hydrophilic acrylic resin of low viscosity (LR white resin), allowing longitudinal and transverse sections with a Leica SP 1600 saw microtome. Other Russian specimens were set in epoxy resin and cut to make polished thin sections with a thickness of 30 µm and 130 µm. Both sections were observed with a Hitachi TM-100 scanning electron microscope (SEM) at the Université de Bourgogne, Dijon, so as to document putative internal structures. Thin sections were observed under plane polarized light (PPL), cross-polarized light (XPL) and epifluorescence UV using a Nikon AZ100 microscope, equipped with a 360 nm exciting source and a Zeiss Axiocam MRc5 (Université de Bourgogne, Dijon). Polished thin sections were also observed under cathodoluminescence using a Leica MZ12 binocular microscope equipped with a Luminoscope ELM-3R device and a Zeiss Axiocam MRc5 camera (Université de Bourgogne, Dijon). Cathodoluminescence (CL) was successfully applied by [START_REF] Gorzelak | Stereom microstructures of Cambrian echinoderms revealed by cathodoluminescence (CL)[END_REF], so as to reveal the internal stereomic microstructure preserved in skeletal elements of various Cambrian echinoderms. This technique was usually shown to be efficient, even in the case of relatively strongly recrystallized specimens.

For FT-IR investigations, thin sections were used to extract in situ -with the tip of a scalpel blade -small chips of material from different sampling points located both inside the fossil and in the surrounding resin. The extracted materials were then reduced into powders with an agate mini mortar and pestle (< 10 µm) and the powder was subsequently analyzed by Fourier transform infrared (FT-IR) spectroscopy, to identify the different mineralogical phases, on a ALPHA FT-IR BRUKER device equipped with an ALPHA-P module. Data acquisition was performed in the 4000-500 cm -1 wavenumber range (12 scans at a spectral resolution of 4 cm -1 ), in ATR mode (Attenuated Total Reflectance) with a single reflection diamond crystal adapted to solids. Blank spectra were acquired on resin alone. The qualitative assignment of absorption bands was performed by comparison with known IR spectra found in the literature [START_REF] Jones | Infrared Transmission Spectra of Carbonate Minerals[END_REF].

The magnesium content of some Russian specimens of Bolboporites was checked with a Bruker S1 Titan spectrometer equipped with a collimated beam and incorporated in a laboratory console. Every measurement consisted of two successive beam phases of 60 seconds, with energies of 45 kV and 15 kV. This protocol allows measuring equivalent MgO mass concentrations above 1% in relatively small windows (2 mm in diameter) and directly in the thicker thin sections.

The internal structure of one well-preserved Bolboporites was also analyzed with electron back-scattered diffraction (EBSD), a technique that permits the characterization of complex polycrystalline materials at nanoscale. In short, it allows measuring and representing -via 2Dcoloured maps and pole figures -the crystallographic orientation of individual nanograins with respect to each other. This technique, currently used in materials science, is particularly adapted for calcium carbonate fossil and non-fossil biominerals [START_REF] Checa | Crystallographic reorganization of the calcitic prismatic layer of oysters[END_REF][START_REF] Cusack | Biomineral electron backscatter diffraction for palaeontology[END_REF]. To this end, an embedded sample was manually mirror-polished on 0.05µm aluminium oxide powder and further processed on a vibratory polisher. The sample was fixed on a sample holder and analyzed on a JEOL JSM 760 F field emission scanning electron microscope, from which coloured maps were produced, with a step of 250 nm. Measurements were performed at the periphery of the sample and also in different areas of the central zone separated from each other by a few millimeters; this allowed detection of potential crystallographic disorientations at milli-metric scale. cross-sections through some specimens, and also to reconstruct a three-dimensional model of Bolboporites. In recent years, tomography has become a routine technique of imagery, so as to reveal internal structures in various fossils and, in particular, Palaeozoic echinoderms [START_REF] Sutton | A starfish with threedimensionally preserved soft parts from the Silurian of England[END_REF][START_REF] Rahman | Re-evaluating the palaeobiology and affinities of the Ctenocystoidea (Echinodermata)[END_REF][START_REF] Rahman | The oldest cinctan carpoid (stem-group Echinodermata), and the evolution of the water-vascular system[END_REF][START_REF] Rahman | The oldest stylophoran echinoderm: a new Ceratocystis from the Middle Cambrian of Germany[END_REF][START_REF] Rahman | Early post-metamorphic, Carboniferous blastoid reveals the evolution and development of the digestive system in echinoderms[END_REF][START_REF] Briggs | An edrioateroid from the Silurian Herefordshire Lagerstätte of England reveals the nature of the water vascular system in an extinct echinoderm[END_REF]. Data acquisition was obtained at 50 kV and 800 µA. Two images per position (number of frames: 2) were obtained, each of them after an exposure time of 2500 ms. The rotation step of the sample was 0.7° and the total acquisition time was 75 minutes.

Other individuals of B. mitralis were kept intact, so as to explore minute details of their external morphology. However, in most specimens, the base of the cone and/or the honeycomb cells on the lateral walls were partly concealed by a thin layer of sedimentary rock. Consequently, fossils were placed in an ultrasonic cleaner containing a solution of dilute ethylenediaminetetraacetic acid (EDTA, 1% wt/vol, pH 8) to remove all pieces of surrounding rock and better expose the external aspect of the specimens. Once cleaned, fossils were rinsed with water, then with ethanol (C 2 H 5 OH), and finally dried with a hair-dryer. Observation of the external aspect of the specimens was made using both a Zeiss SteREO Discovery.V8 binocular stereomicroscope, equipped with a Zeiss AxioCam MRc5 digital camera, at Université Lyon 1, and a Hitachi TM-100 Scanning Electron Microscope (SEM) at the Université de Bourgogne, Dijon.

Finally, three dried specimens of the Recent asteroid Pentaceraster mammilatus [START_REF] Audouin | Explication des planches d'échinodermes de l'Egypte et de la Syrie publiées par Jules-César Savigny[END_REF] were examined and prepared for morphological comparison purposes. This material belongs to the R. Koehler collections, which are part of the zoological collections of Lyon 1 University (acronym: UCBL). The specimens were collected between 1895 and 1930 (precise date of sampling not reported on labels) from an unknown locality, possibly in the Red Sea or the western part of the Indian Ocean [START_REF] Clark | Monograph of Shallow-Water Indo-West Pacific Echinoderms[END_REF]. Dissection and extraction of some aboral spines was made with a scalpel. Photographs were made with a Nikon D5000 camera in the palaeontological collections of Lyon 1 university (CERESE).

Results

Chemical and mineralogical analyses.

The FT-IR spectroscopy performed on transverse sections of Bolboporites mitralis included in LR white resin generated two contrasting sets of infrared spectra, depending on the position of the sampling points, outside or inside the fossil (Fig. 2). The surrounding LR white resin produced a characteristic reference IR spectrum (Fig. 2B), while infrared spectra obtained in sampling points located within the fossils all showed the three absorption bands that are characteristic of calcite (Fig. 2C-E) at 711-712 cm -1 , 871 cm -1 , and 1395 cm -1 , respectively. This spectrum is clearly distinct from that of aragonite (not shown), characterized by a doublet at 700-713 cm -1 and two bands at 858 and 1477 cm -1 , in addition to a sharp one at 1083 cm -1 . No dolomite (identified by absorption bands at 729, 882 and 1441 cm -1 ) was detected in the central zone, Our data unambiguously showed that the Russian specimens of Bolboporites are entirely made of calcite.

A slightly more complex infrared spectrum was observed in sampling points located on lateral edges of the fossils, with the three absorption bands typical of calcite, but also some additional minor bands (Fig. 2C,G). Comparison with the 'control' spectrum shows that this signal corresponds to the combination of both calcite and LR white resin infrared spectra, resulting from either the irregular external morphology of the body wall (ornamentation consisting of honeycomb cells), and/or a limited penetration of the resin into micropores and microfractures of the specimens.

Finally, a large absorption band of low amplitude in the 3700-3100 cm -1 range was observed in one spectrum corresponding to a sampling spot located within the fossil (Fig. 2F). This signal did not result from any contamination from the resin, but more likely corresponded to the vibrations of OH bond, suggesting the occurrence of water, putatively in the form of small fluid inclusions within the calcite.

The XRF measurements were made in central and exterior parts of Bolboporites sections.

Magnesium was only detected, with MgO values between 1 and 5% (mass concentration), in most peripheral parts of the skeleton, where analyzed windows encroach the surrounding sedimentary rock. Close examination of these parts, under SEM and cathodoluminescence (see below) revealed the presence of small dolomite rhombs, thus driving the Mg content. In all other parts of the skeleton, Mg was never found, being below the 1% detection limit. Thus, a low magnesium calcite (LMC) is deduced for their current composition.

Microstructures.

Observation of thin sections of B. mitralis with a polarizing microscope shows that whole cones are affected by conjugate cleavage planes (Fig. 3A,C), and have a single and right crystal extinction (Fig 3B), typical of monocrystals. This important result is confirmed by our investigations using electron backscattered diffraction (EBSD), the outcome of which is synthesized in Figure 4. When performed on the periphery of the section (Fig. 4A-D), EBSD mapping shows that the crown interface is constituted of a mixture of micritic grains (2 µm or less) surrounding sparitic crystals of about tens to more than 100 microns in diameter (Fig. 4B,D) while the upper left corner of the map corresponds to the sample itself, which is symbolized by one unique colour, and is, consequently, monocrystalline. The EBSD mapping performed on the central zone of the sample (Fig. 4E-G) shows that the analyzed area is uniform, with no detectable grain limit. This demonstrates clearly that the structure is monocrystalline. Interestingly, when maps are produced in different central zones distant from each other by few millimetres, one notices a slight change of crystallographic orientation, symbolized by a minor colour change: for example, two zones 1 mm apart (either in X or Y) show a grain orientation spread of about 0.3 -0.5°. When the distance is larger between two analyzed zones (such as 4 mm between Fig. 4F andG), the disorientation is more important, around 3.73° in the present example. In summary, EBSD mapping confirms that the calcite that constitutes a specimen of Bolboporites is monocrystalline, with a very minor and gradual crystalline disorientation at millimetric scale, without any detectable grain limit.

Under cathodoluminescence, both transverse and longitudinal sections of B. mitralis revealed the same mottled, orange-to-brown luminescent microstructure (Fig. 5C). Orange irregular dots and axes are aligned within a browner sealing calcite crystal, forming a CL pattern usually detected for ancient echinoderm stereoms (e.g., [START_REF] Gorzelak | Stereom microstructures of Cambrian echinoderms revealed by cathodoluminescence (CL)[END_REF].

These orange parts probably represent the former porous stereom that has been secondary cemented by a syntaxial brown-luminescent cement. Under XPL and PPL, the same stereomlike microstructure is also detected in some parts of the cones, with the tenuous occurrence of small dark spots aligned along cleavage planes (Fig. 3C). At a larger scale, the stereom of Bolboporites exhibits a slightly differentiated, darker, narrow peripheral rim in CL (Fig. 5A,B), which probably results either from a diagenetic effect or from a distinct lighter density of the former microstructure. A similar observation was made on virtual cross-sections of B. mitralis obtained by CT-scan, which show a diffuse peripheral rim, consistently lighter than the darker central part of the fossils (Fig. 6A). In CT-scan imagery, intensity restitution largely depends on the density of the materials. Consequently, the central part of Bolboporites is currently made of slightly more porous (less dense) skeleton than the periphery. Views by SEM confirm this deduction, with numerous micropores occuring everywhere in the skeleton, except in its peripheral parts (Fig. 5D), where initial micropores are probably cemented by the diagenetic brown-luminescent calcite. As shown by cathodoluminescence (Fig. 5A-C), the internal canal is also cemented by the brown syntaxial calcite.

Finally, contrary to these important observations, no structures have been detected within the cones using UV epifluorescence technique. This is probably due to the absence of fluorescent organic matter within the stereomic calcite microstructure, in probable relation with an intense thermal alteration of the organic matter during the burial history of these Palaeozoic fossils.

Internal morphological features.

Sections made through specimens of B. mitralis confirm previous reports of a narrow, straight to gently curved, longitudinal canal, extending from the base of the cone towards the apex (Fig. 5A,C ;[START_REF] Yakovlev | Bolboporites, its structure and affinity to the Hydrozoa[END_REF][START_REF] Yeltysheva | Bolboporites[END_REF][START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF]Rozhnov & Kushlina 1994a, b). The observation of this longitudinal canal in all specimens, consistently in the same position, indicates that it corresponds to an original internal structure.

In contrast, both transverse and longitudinal sections also demonstrated the presence of additional, randomly distributed tubulars and incurved holes, opening either on the base or on the lateral surface of the cone, and penetrating more or less deeply into it (Fig. 5A,B,D).

Optical observations made on thin sections of B. mitralis show that these structures are filled by a wackestone to packstone sediment, containing diverse organic fragments (e.g., shell debris, echinoderm skeletal elements) and glauconite grains (Fig. 3). In unprepared Russian specimens (i.e., without ultrasonic cleaning), the nature of this infilling appears to be similar to the surrounding matrix. Consequently, the random distribution of these holes, their variable size and depth, as well as their infilling by sedimentary rock, all indicate that they are not original internal structures, but rather correspond to borings (Trypanites isp.) made by unknown drilling organisms. They are morphologically different from the less penetrative, though superficially extensive traces already reported on the basal surface of B. mitralis [START_REF] Kushlina | Biting traces on echinoderms from the Ordovician of the St. Petersburg region (Russia)[END_REF].

Apart from the central longitudinal canal and randomly located Trypanites-like borings, no other internal morphological macrostructure was apparent in sections of B. mitralis.

Observations made with a polarizing microscope and EBSD show that each specimen of Bolboporites currently consists in a single crystal of calcite (see above). Although the calcite is recrystallized, the diverse observations of an extensive stereom-like microstructure in all parts of a sectioned specimen of Bolboporites indicates that each cone was originally a microporous, but unique, monocrystal of calcite, with a central canal forming the unique internal macro-cavity.

External morphological features.

Similar external morphological features were observed in all available Norwegian and Russian specimens of Bolboporites (Figs 6B,7,8). These features are in good agreement with previous descriptions of the external aspect of Bolboporites made on specimens from Baltica (e.g., [START_REF] Yeltysheva | Bolboporites[END_REF]Rozhnov & Kushlina 1994a;[START_REF] Kushlina | The systematic position and composition of the genus Bolboporites (Echinodermata, Eocrinoidea)[END_REF] and Laurentia (e.g., [START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF].

Cells on the lateral walls of the cones typically display rounded to hexagonal outlines (Fig. 8A) and their diameter decreases in the apical direction (Fig. 7A-C). In contrast, cells located towards the base tend to become larger and more elliptical in shape. Each cell corresponds to a shallow (less than 1 mm deep), gently concave, smooth depression delimited by raised rims, thus forming a characteristic honeycomb pattern on the lateral walls of the cone (Figs 7A-C, 8A). After ultrasonic cleaning, SEM observation of Russian specimens revealed the presence of the typical porous, stereomic microstructure on the external surface of the cells (Fig. 8A).

No specimens express an orifice at the apex of the cones, thus suggesting that their internal central canal (see above) terminated close to, though not opening at, the apex.

The base of the cone is entirely smooth, with the exception of two adjoining circular depressions (or lunules), each surrounded by a low C-shaped ridge (Figs 7D, 8B). These two shallow cavities are not in central positions, but more or less displaced laterally (Fig. 7D).

Ultrasonic cleaning of Russian specimens shows that the floor of each depression is gently concave, entirely smooth and displays a well-preserved, porous stereom microstructure (Fig. 8B). A small orifice occurs at the junction between the two depressions (Figs 7D,8B). This opening represents the basal extremity of the internal longitudinal canal (see above). No other conspicuous external morphological structure (e.g., orifice) could be observed on the basal, flat to convex surface of the cones.

Discussion

Cathodoluminescence, applied here for the first time to Bolboporites, confirms previous reports of the presence of a former stereomic microporous structure in this fossil (e.g., [START_REF] Logan | Report of Progress from its commencement to 1863[END_REF][START_REF] Lindström | Index to generic names applied to the corals of the Palaeozoic formations[END_REF][START_REF] Yeltysheva | Bolboporites[END_REF][START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF]. Currently blocked by a diagenetic syntaxial calcite, this microstructure is also detected under SEM and PPL observations. A second important result is that each specimen of Bolboporites corresponds to a single, formerly porous, monocrystal of calcite. Taken together, these two points provide definitive evidence for echinoderm affinities. Consequently, overall similarities in shape or ornamentation with algae (e.g., Coelosphaeridium), bryozoans, corals or sponges (e.g., stromatoporoids) are merely superficial.

The initial calcite stereom is now entirely cemented and recrystallized into an orange-tobrown luminescent LMC. This low Mg content does not preclude an initial low Mg content that may have been much higher during the formation of this skeletal element, but modified (and lowered) during diagenesis. Such a diagenetic pathway is well documented for most ancient echinoderm stereoms [START_REF] Gorzelak | Diagenesis of echinoderm skeletons: constraints on paleoseawater Mg/Ca reconstructions[END_REF]) and for biogenic HMCs in general [START_REF] Bischoff | Diagenetic stabilization pathways of magnesian calcites[END_REF]. Despite this recrystallization, the extensive distribution of stereomic microstructure everywhere within the cones suggests that, originally, Bolboporites did not contain any internal macro-structure, with the exception of the narrow longitudinal central canal, which opens within the two lunules, on the base of the cone. Bolboporites as a theca.

In this interpretation, Bolboporites represents either the internal mould [START_REF] Ami | Preliminary lists of the organic remains occurring in the various geological formations comprised in the southwest quartersheet map of the eastern townships of the Province of Quebec[END_REF]von Wöhrmann in Jaekel 1899;Régnell 1956) or the external wall of a blastozoan theca (Rozhnov & Kushlina 1994a, b;[START_REF] Kushlina | The systematic position and composition of the genus Bolboporites (Echinodermata, Eocrinoidea)[END_REF][START_REF] Kushlina | Biting traces on echinoderms from the Ordovician of the St. Petersburg region (Russia)[END_REF][START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]. In blastozoans, the theca was polyplated and entirely made of extraxial skeletal elements (i.e., deriving from the pre-metamorphic larva; [START_REF] David | Are homalozoans echinoderms? An answer from the extraxial-axial theory[END_REF][START_REF] Sprinkle | Growing a stalked echinoderm within the Extraxial-Axial Theory[END_REF][START_REF] Nardin | La radiation des échinodermes au Paléozoïque inférieur : l'exemple des blastozoaires[END_REF][START_REF] Nardin | Evolutionary implications of a new transitional blastozoan echinoderm from the mid Cambrian of Czech Republic[END_REF]. The theca was bearing all main body orifices (anus, hydropore, mouth) and, when present, various kinds of respiratory structures (e.g., epispires, diplopores, rhombs; [START_REF] Kesling | Cystoids[END_REF][START_REF] Sprinkle | Morphology and evolution of blastozoan echinoderms[END_REF][START_REF] David | Are homalozoans echinoderms? An answer from the extraxial-axial theory[END_REF]. In life, the theca housed the main body cavity, which contained all internal organs (e.g., gut; [START_REF] Kesling | Cystoids[END_REF][START_REF] Sprinkle | Morphology and evolution of blastozoan echinoderms[END_REF][START_REF] Rahman | Early post-metamorphic, Carboniferous blastoid reveals the evolution and development of the digestive system in echinoderms[END_REF].

The interpretation of Bolboporites as the internal mould of a blastozoan theca relies on several arguments: (1) its overall morphology is compatible with the cone-shaped aspect of the theca of various blastozoans, such as Rhopalocystis [START_REF] Ubaghs | Rhopalocystis destombesi n.g., n.sp., éocrinoïde de l'Ordovicien inférieur (Trémadocien supérieur) du Sud marocain[END_REF] (2) the honeycomb ornamentation could represent the imprint of thick polygonal thecal plates on the internal mould; (3) the smooth aspect of the basal surface could result from a more finely-plated oral surface; and (4) the lunules could correspond to the imprint left by one (or two) body opening(s). The main problem with this interpretation is that, by definition, the internal mould of a blastozoan theca corresponds to the infilling of this theca by sedimentary rock. This implies that the mineralogical composition of the internal mould should be similar to that of the surrounding rock. However, our results clearly demonstrate that (1) the mineralogical composition of Bolboporites is very distinct from that of the surrounding sedimentary rock; and (2) this fossil corresponds to a single echinoderm plate (stereom, monocrystal of calcite).

Consequently, Bolboporites does not represent the internal mould of a blastozoan theca.

The second interpretation considers Bolboporites as the theca of an eocrinoid blastozoan, preserved either as an external mould (e.g., Norwegian specimens) or as a recrystalised threedimensional fossil (e.g., Russian material). The main argument supporting this interpretation is the observation of a partially-preserved appendage-like structure inserted onto the lunules, on the smooth basal surface of some specimens from Russia (Rozhnov & Kushlina 1994a, pl. 6;Kushlina 2007, pl. 1 fig. 3). When present, this structure occurs consistently in the same location (i.e., perfectly fitting into the lunules), and it always shows a biserial pattern, with two opposite sets of small, thick, semi-circular skeletal elements. A longitudinal groove is running on one side of the appendage, along the suture between the two series of opposite ossicles (see Rozhnov & Kushlina 1994a, pl. 6 fig. 1d). This groove is apparently leading (proximally) into the small orifice located at the junction between the two lunules.

This biserial structure was interpreted by Rozhnov & Kuslina (1994a, b) as a feeding appendage (brachiole; see also [START_REF] Kushlina | The systematic position and composition of the genus Bolboporites (Echinodermata, Eocrinoidea)[END_REF][START_REF] Kushlina | Biting traces on echinoderms from the Ordovician of the St. Petersburg region (Russia)[END_REF][START_REF] Rozhnov | Echinoderms[END_REF][START_REF] Rozhnov | New data on Ordovician eocrinoids and paracrinoids of the Baltic Region[END_REF]. This identification implies that the groove borne by brachiolar plates probably housed a single ray of the ambulacral system and that, consequently, the mouth was located at the proximal extremity of this ray. The mouth would thus correspond to the small orifice located within the lunule. In their interpretation of Bolboporites, Rozhnov & Kushlina (1994a, b) further suggested that: (1) the internal longitudinal canal of Bolboporites was probably homologous to the axial canal (lumen) of a pelmatozoan stem, thus implying that the stem was present, but entirely encased within the theca; and (2) all organs (e.g., gut) were not located within the theca, but outside of it, on the smooth surface of the cone.

No articulated appendage was preserved in our study material. However, its interpretation as a brachiole is plausible: in echinoderms, axial (ambulacral) flooring plates typically display a comparable biserial, zigzag pattern, resulting from their appearance through ontogeny, alternatively on the left and on the right of a growing ray of the ambulacral system ('ocular plate rule'; [START_REF] David | Embryology supports a new theory of skeletal homologies for the phylum Echinodermata[END_REF], 1999;[START_REF] Mooi | Skeletal homologies of echinoderms[END_REF], 1998[START_REF] Baarli | Fossilboka. Forlaget Vett & Viten AS[END_REF]. This identification has several implications: (1) the longitudinal groove borne by the flooring plates is an ambulacral food groove; (2) the orifice located at the proximal extremity of this groove is the mouth; (3) no ambulacral cover plates are apparently present (or preserved) above this groove; and (4) Bolboporites possessed one single brachiole. The reduction of the number of feeding appendages is relatively common in Palaeozoic echinoderms. It was documented both in some crinoids (e.g., Monobrachiocrinus granulatus [START_REF] Wanner | Ueber einige palaeozoische Seeigelstacheln (Timorocidaris gen. nov. und Bolboporites Pander)[END_REF][START_REF] Ausich | Crinoid form and function[END_REF] and also in various groups of vagile, epibenthic taxa, such as pleurocystitid rhombiferans [START_REF] Paul | The functional morphology and mode of life of the cystoid Pleurocystites E. Billings[END_REF][START_REF] Kesling | Cystoids[END_REF][START_REF] Parsley | Revision of the North American Pleurocystitidae (Rhombifera -Cystoidea)[END_REF], solutans [START_REF] Ubaghs | Réflexions sur la nature et la fonction de l'appendice articulé des "carpoïdes" Stylophora (Echinodermata)[END_REF][START_REF] David | Are homalozoans echinoderms? An answer from the extraxial-axial theory[END_REF]Lefebvre & Lerosey-Aubril 2018) and stylophorans (Ubaghs 1968a;[START_REF] David | Are homalozoans echinoderms? An answer from the extraxial-axial theory[END_REF][START_REF] Lefebvre | Functional morphology of stylophoran echinoderms[END_REF].

Our results demonstrate that Bolboporites corresponds to a single, massive cone-shaped echinoderm skeletal element. This observation has two major implications: if Bolboporites is interpreted as the theca of an eocrinoid, then this theca (1) is entirely made of one single plate; and (2) with the exception of the narrow central canal, it does not contain any internal structure. Reduction of the number of plates forming the body capsule is a trend described in various groups of Palaeozoic echinoderms, such as in solutans (e.g., Late Ordovician belemnocystitids from North America; [START_REF] Parsley | North American Soluta (Carpoidea, Echinodermata)[END_REF][START_REF] Caster | Homoiostelea[END_REF] In all echinoderms, including the most extreme crinoid morphologies, the body capsule always contains an internal cavity housing the viscera. However, our observations show that Bolboporites is a massive skeletal element, without any body cavity. If Bolboporites was a single-plated theca, then its mouth (i.e., the small orifice located within the lunules, at the proximal extremity of the ambulacral groove) would open into the narrow, distally closed and tapering central canal. This implies that (1) this internal canal cannot be homologous to the axial canal of a stem (in pelmatozoans, the mouth never opens into the stem canal); and (2) the absence of an internal body cavity and of any anal opening both suggest that, from a functional point of view, this interpretation is not valid. The suggestion that soft parts were lying in life over the smooth basal surface of the cone, that is, outside of the theca (see e.g., Rozhnov & Kushlina 1994a, b), is incompatible with the body plan of the phylum Echinodermata. In all echinoderms, the viscera are always housed within the body capsule, independently of whether it is loosely (e.g., holothurians) or more strongly calcified (most taxa).

In summary, our results do not confirm the identification of Bolboporites as the theca of an eocrinoid. Although most requirements of this interpretation are plausible (e.g., the biserial pattern of the appendage is similar to that of ambulacral flooring plates; echinoderms with a single feeding appendage did exist, as well as body capsules consisting of a reduced number of elements), this hypothesis has to be rejected, because the implied anatomy would be neither functional (no internal cavity, no viscera, no anus, no hydropore) nor compatible with the echinoderm body plan (extra-thecal viscera, mouth opening into the stem axial canal).

Bolboporites as an infrabasal cone.

The overall morphology of Bolboporites is strongly reminiscent of similar-looking, coneshaped, massive skeletal elements forming the aboral part of the body capsule in some blastozoans and crinoids. Such aboral (or infrabasal) cones can be made of several tightly sutured plates, as, for example, in Cymbionites and Peridionites, both from the Cambrian of Australia [START_REF] Whitehouse | The Cambrian faunas of north-eastern Australia. Part 4: Early Cambrian echinoderms similar to the larval stages of recent forms[END_REF][START_REF] Smith | The affinities of the Middle Cambrian Haplozoa (Echinodermata)[END_REF]. However, single-plated, massive infrabasal cones have been described in several Cambro-Ordovician eocrinoids [START_REF] Ubaghs | Rhopalocystis destombesi n.g., n.sp., éocrinoïde de l'Ordovicien inférieur (Trémadocien supérieur) du Sud marocain[END_REF][START_REF] Clausen | New Early Cambrian eocrinoids from the Iberian Chains (NE Spain) and their role in nonreefal benthic communities[END_REF][START_REF] Allaire | Morphological disparity analysis and systematic revision of the eocrinoid genus Rhopalocystis (Echinodermata, Blastozoa) from the Lower Ordovician of the central Anti-Atlas (Morocco)[END_REF], as well as in some Late Palaeozoic crinoids [START_REF] Ettensohn | The autecology of Agassizocrinus lobatus[END_REF][START_REF] Ettensohn | Paragassizocrinus: systematics, phylogeny and ecology[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF][START_REF] Webster | Pennsylvanian crinoids of New Mexico[END_REF]. As in Bolboporites, massive aboral pelmatozoan cones (plates) also display: (1) a very wide morphological disparity within a same assemblage (see, e.g., [START_REF] Ettensohn | Paragassizocrinus: systematics, phylogeny and ecology[END_REF][START_REF] Clausen | New Early Cambrian eocrinoids from the Iberian Chains (NE Spain) and their role in nonreefal benthic communities[END_REF]and (2) a central canal, which is tapering proximally (i.e., towards the apex of the cone) in stemless taxa (e.g., in the Pennsylvanian crinoid genus Paragassizocrinus [START_REF] Moore | Crinoids from the Upper Carboniferous and Permian strata in Texas[END_REF][START_REF] Ettensohn | Paragassizocrinus: systematics, phylogeny and ecology[END_REF].

In contrast, in stemmed taxa, the central canal extends throughout the infrabasal cone and leads proximally into the axial canal of the stem (see, e.g., [START_REF] Ubaghs | Rhopalocystis destombesi n.g., n.sp., éocrinoïde de l'Ordovicien inférieur (Trémadocien supérieur) du Sud marocain[END_REF][START_REF] Clausen | New Early Cambrian eocrinoids from the Iberian Chains (NE Spain) and their role in nonreefal benthic communities[END_REF]).

However, it seems unlikely that Bolboporites corresponds to the infrabasal cone of a stemless pelmatozoan. The main difficulty is the smooth aspect of its basal surface: in all pelmatozoans possessing an infrabasal cone, its upper (distal) surface is divided into several concave areas (facets), separated by ridges and corresponding to the insertion of the plates (i.e., basals) belonging to the overlying circlet [START_REF] Ubaghs | Rhopalocystis destombesi n.g., n.sp., éocrinoïde de l'Ordovicien inférieur (Trémadocien supérieur) du Sud marocain[END_REF][START_REF] Ettensohn | The autecology of Agassizocrinus lobatus[END_REF][START_REF] Ettensohn | Paragassizocrinus: systematics, phylogeny and ecology[END_REF][START_REF] Clausen | New Early Cambrian eocrinoids from the Iberian Chains (NE Spain) and their role in nonreefal benthic communities[END_REF][START_REF] Webster | Pennsylvanian crinoids of New Mexico[END_REF]. The absence of such facets in Bolboporites implies that its basal surface was not sutured to any overlying plates and, thus, that this fossil does not represent an infrabasal cone. This interpretation is further supported by the strongly convex and particularly high morphology of the basal surface in some specimens of Bolboporites (in particular, in B. americanus [START_REF] Billings | The fossils of the Chazy Limestone with descriptions of new species[END_REF]see Clark & Hofmann 1961), which is incompatible with the presence of a putative overlying basal circlet. Finally, the biserial appendage articulated to the basal surface of some Russian specimens of Bolboporites (Rozhnov & Kushlina 1994a;[START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]) clearly demonstrates that this surface was not in contact with overlying thecal (or calyx) plates.

Bolboporites as a spine . Spines are highly differentiated, mobile skeletal elements articulated to the body wall of echinoderms (see, e.g., [START_REF] Durham | Echinoids[END_REF]Smith 1980b). Although this character gave its name to the phylum Echinodermata ('spiny skin'), moveable spines indeed occur only in asterozoans, echinozoans (echinoids), edrioasteroids and stylophorans. In asterozoans, echinoids and edrioasteroids, the articulation of spines to the body wall is complex, typically consisting of (1) a concave socket (acetabulum) at the base of the spine; and (2) a corresponding convex ball (mamelon) located at the summit of a tubercle [START_REF] Durham | Echinoids[END_REF][START_REF] Spencer | Asterozoans[END_REF]Smith 1980b;[START_REF] Holloway | Silurian and Devonian edrioasteroids from Australia[END_REF][START_REF] Guensburg | Systematics, functional morphology, and life modes of Late Ordovician edrioasteroids, Orchard Creek Shale, southern Illinois[END_REF][START_REF] Lebrun | Oursins. Minéraux & Fossiles[END_REF]. Attachment and mobility of spines to the body wall are achieved by muscles and/or a ligamentary catch apparatus (Smith 1980b;[START_REF] Lebrun | Oursins. Minéraux & Fossiles[END_REF]. Spine movements are controlled by nerves, forming either a ring around the tubercle (e.g., in echinoids; [START_REF] Durham | Echinoids[END_REF]Smith 1980b;[START_REF] Lebrun | Oursins. Minéraux & Fossiles[END_REF]) and/or extending inside the spine itself (e.g., in some ophiuroids; [START_REF] Lebrun | Oursins. Minéraux & Fossiles[END_REF]). In cornute stylophorans (e.g., chauvelicystids, Thoralicystis griffei [START_REF] Ubaghs | Les Echinodermes "Carpoïdes" de l'Ordovicien Inférieur de la Montagne Noire (France)[END_REF]), spines are articulated to various parts of the body wall (proximal aulacophore, supracentral area, thecal margin) by rudimentary balls and sockets [START_REF] Ubaghs | Les Echinodermes "Carpoïdes" de l'Ordovicien Inférieur de la Montagne Noire (France)[END_REF][START_REF] Ubaghs | Echinodermata. Notes sur les Échinodermes de l'Ordovicien Inférieur de la Montagne Noire (France)[END_REF][START_REF] Lee | Latest Cambrian cornutes (Echinodermata, Stylophora) from the Taebaeksan Basin, Korea[END_REF]. In most mitrate stylophorans, the articulation of posterior spines (digital, glossal) consists of complex balls and sockets (e.g., in anomalocystitids; Ubaghs 1968a; [START_REF] Parsley | Review of selected North American mitrate stylophorans (Homalozoa: Echinodermata)[END_REF][START_REF] Ruta | A redescription of the anomalocystitid mitrate Rhenocystis latipedunculata from the Lower Devonian of Germany[END_REF]. In some other taxa (e.g., Balanocystites primus [START_REF] Barrande | Système Silurien du Centre de la Bohême. Supplément au Volume I. Trilobites[END_REF]), the posterior spine is connected to the theca by a small column of articulated plates [START_REF] Lefebvre | Stylophores (Cornuta, Mitrata) : Situation au sein du Phylum des Echinodermes et Phylogenèse[END_REF].

Spines are thus distinct from spine-shaped tubercles, which are non-articulated (fixed), protruding external structures that can be produced by various elements of the body wall (e.g., anal plates, calyx or thecal plates, columnals). Tubercles are particularly widespread at phylum-scale. In blastozoans, spine-shaped expansions of thecal plates have been described in aristocystitids (e.g., Calix sedgwicki [START_REF] Rouault | Mémoire sur le terrain paléozoïque des environs de Rennes[END_REF]Lepidocalix pulcher Termier & Termier, 1950;see Chauvel 1941;[START_REF] Makhlouf | Lepidocalix pulcher Termier and Termier, 1950 (Echinodermata, Diploporita) from the Middle Ordovician of northern Algeria: taxonomic revision and palaeoecological implications[END_REF]), blastoids (e.g., Thaumatoblastus Wanner, 1924;Pteratoblastus Wanner, 1924;see Beaver et al. 1968), eocrinoids (e.g., Rhopalocystis havliceki [START_REF] Chauvel | Compléments sur les échinodermes du Paléozoïque marocain (diploporites, éocrinoïdes, édrioastéroïdés)[END_REF]see Chauvel & Régnault 1986;[START_REF] Allaire | Morphological disparity analysis and systematic revision of the eocrinoid genus Rhopalocystis (Echinodermata, Blastozoa) from the Lower Ordovician of the central Anti-Atlas (Morocco)[END_REF]) and, to a lesser extent, in glyptocystitids (e.g., Schizocystis Jaekel, 1895;[START_REF] Kesling | Cystoids[END_REF]). Strong tubercles also occur in both cinctans (e.g., Undatacinctus quadricornuta [START_REF] Friedrich | Systematik und Funktionsmorphologie mittelkambrischer Cincta (Carpoidea, Echinodermata)[END_REF]; see [START_REF] Smith | Rooting phylogenies of problematic fossil taxa; a case study using cinctans (stem-group echinoderms)[END_REF]) and solutans (e.g., Girvanicystis batheri [START_REF] Caster | Homoiostelea[END_REF]; see [START_REF] Daley | The anatomy of the solute Girvanicystis batheri (?Chordata) from the Upper Ordovician of Scotland and a new species of Girvanicystis from the Upper Ordovician of south Wales[END_REF].

Elongate, spine-shaped elements of the body wall also occur in some edrioasteroids [START_REF] Guensburg | Systematics, functional morphology, and life modes of Late Ordovician edrioasteroids, Orchard Creek Shale, southern Illinois[END_REF]), but they are particularly widespread in Palaeozoic crinoids. In this class, such a spiny ornamentation has been described on anal plates (e.g., Stenopecrinus Strimple, 1961;Uperocrinus Meek & Worthen, 1865;see Ubaghs 1978b;[START_REF] Ausich | Crinoid form and function[END_REF]), on brachials (e.g., Eirmocrinus Strimple & Watkins, 1969;Separocrinus Knapp, 1969; see [START_REF] Moore | Inadunata[END_REF][START_REF] Ausich | Crinoid form and function[END_REF], on tegmental plates (e.g., Batocrinus [START_REF] Casseday | Beschreibung eines neuen Crinoideengeschlechts Nordamerika[END_REF] see Ubaghs 1978b), on calyx plates (e.g., Calceolispongia [START_REF] Etheridge | Western Australian Carboniferous fossils, chiefly from Mount Marmion, Lennard River, West Kimberley[END_REF]Dorycrinus Roemer, 1854; see [START_REF] Moore | Inadunata[END_REF]Ubaghs 1978b;[START_REF] Brett | Middle Devonian Windom Shale of Vincent[END_REF], and to a lesser extent, on stem elements (e.g., Aethocrinus moorei [START_REF] Ubaghs | Aethocrinus moorei Ubaghs, n. gen., n. sp., le plus ancien crinoïde dicyclique connu[END_REF]. Elongate spine-shaped tubercles occur on the proximal brachials of some cornute (e.g., Reticulocarpos hanusi [START_REF] Jefferies | A new calcichordate from the Ordovician of Bohemia and its anatomy, adaptations and relationships[END_REF]Nanocarpus milnerorum Ruta, 1999;[START_REF] Jefferies | The Ancestry of the Vertebrates[END_REF][START_REF] Lefebvre | Functional morphology of stylophoran echinoderms[END_REF]) and most mitrate stylophorans (e.g., Chinianocarpos thorali [START_REF] Ubaghs | Un échinoderme nouveau de la classe des carpoïdes dans l'Ordovicien inférieur du département de l'Hérault (France)[END_REF]Rhenocystis latipedunculata Dehm, 1932;[START_REF] Ubaghs | Les Echinodermes "Carpoïdes" de l'Ordovicien Inférieur de la Montagne Noire (France)[END_REF][START_REF] Jefferies | The Ancestry of the Vertebrates[END_REF][START_REF] Ruta | A redescription of the anomalocystitid mitrate Rhenocystis latipedunculata from the Lower Devonian of Germany[END_REF][START_REF] Lefebvre | Functional morphology of stylophoran echinoderms[END_REF]. Strong spine-shaped skeletal elements are also present on the aboral surface of some Recent oreasterid asteroids (e.g., Pentaceraster mammilatus, Protoreaster nodosus [START_REF] Linnaeus | Systema Naturae per Regna Tria Naturae[END_REF]; see [START_REF] Yeltysheva | Bolboporites[END_REF].

The cone-shaped morphology of Bolboporites is compatible with its interpretation either as an isolated spine or a tuberculated plate of an echinoderm [START_REF] Quenstedt | Petrefakten Deutschlands, Part 1[END_REF][START_REF] Lindström | Index to generic names applied to the corals of the Palaeozoic formations[END_REF][START_REF] Wanner | Ueber einige palaeozoische Seeigelstacheln (Timorocidaris gen. nov. und Bolboporites Pander)[END_REF][START_REF] Yeltysheva | Bolboporites[END_REF]Régnell 1956;Clark & Hoffman 1961). Both interpretations imply that (1) the smooth basal surface was facing towards the organism and was either articulated to it (spine) or part of its body wall (tubercle); and (2) the lateral, strongly ornamented walls of the cone were external and directed away from the organism. These two interpretations could explain the wide disparity in size and shape observed in individuals of Bolboporites from a same level [START_REF] Yeltysheva | Bolboporites[END_REF]Clark & Hoffman 1961). Both interpretations would be also in good agreement with the non-random distribution of biofilms produced by encrusting organisms on Russian specimens [START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]. When present, thick putative algal-bacterial biofilms are consistently encrusting the lateral sides of Bolboporites. They never occur on the smooth, convex surface [START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]. This pattern suggests that the biofilms formed when the organism was alive, otherwise biofilms would be present on all surfaces. The distribution of these encrusting biofilms also suggests that the lateral sides of the cones were directly in contact with sea water (i.e., lateral sides were neither buried in the substrate, nor in contact with the body wall). Conversely, the absence of biofilms on the smooth surface supports the view that the basal part of the cones was not exposed to the external medium and, thus, probably in contact with or part of the body wall.

As pointed out by [START_REF] Yeltysheva | Bolboporites[END_REF], the overall morphology of Bolboporites shows several similarities with the cone-shaped aboral elements of some oreasterid asteroids. For example, the observation and dissection of Recent specimens of Pentaceraster mammilatus (Fig. 9D,E) showed that a honeycomb pattern is present on the lateral walls of their coneshaped abaxial tubercles (Fig. 9A). This sculpture, which is similar to that observed in Bolboporites, was produced by the thick granulose membrane, which forms the aboral part of the body wall and extends over the cones. However, the dissection of Recent specimens of P. mammilatus also showed some major morphological differences between Bolboporites and oreasterid cone-shaped abaxial elements. The most important one is that the basal surface of oreasterid cones is not smooth and gently convex, but subdivided into several diverging branches (Fig. 9B), connecting the cone with surrounding plates (Fig. 9C). This situation is not unique to oreasterid abaxial cones. All echinoderm spine-shaped tubercles are borne by elements, which are part of the body wall. This implies that tuberculated plates are necessarily in contact with neighbouring skeletal elements and thus always display facets along their sutures. As the basal surface of Bolboporites is entirely smooth and does not show any evidence of facets, its interpretation as a putative oreasterid-like cone-shaped element has to be rejected.

Finally, although the interpretation of Bolboporites as a spine is plausible (cone-shaped morphology, smooth surface in contact with the body wall, wide morphological disparity, encrusting organisms restricted to lateral walls), this hypothesis has implications that can be tested: (1) skeletal evidence supporting an articulation should be present; and (2) soft parts (muscles, ligaments and/or nerves) were very likely involved, too, and should have left some traces. Clearly, the basal surface of Bolboporites is entirely smooth and does not show any skeletal evidence suggesting the presence of a socket for articulation onto a putative tubercle on the body wall. Consequently, if Bolboporites was a spine, its articulation was different from the most widespread mechanism (balls and sockets) occurring in echinoderms (see above). The only structures occurring on the smooth surface are the two lunules. However, these two concave areas were apparently the place of insertion for a biserial appendage (see discussion above; Rozhnov & Kushlina 1994a;[START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]. Consequently, if Bolboporites was a spine, the small biserial appendage would then have been directed towards the body wall. In the mitrate stylophoran Balanocystites primus, a similar-looking short appendage, made of a single column of tiny plates connects the single posterior spine (glossal) to the theca [START_REF] Lefebvre | Stylophores (Cornuta, Mitrata) : Situation au sein du Phylum des Echinodermes et Phylogenèse[END_REF]. By comparison with the situation in B. primus, the possibility that Bolboporites was a spine, which was connected to the body wall by a short, biserial, articulated appendage cannot be ruled out. If this interpretation is correct, it is thus likely that the smooth aspect of the basal surface of Bolboporites is related to the insertion of soft parts (probably muscles or ligaments) on it. Moreover, by analogy with the situation in some ophiuroids (see above; [START_REF] Lebrun | Oursins. Minéraux & Fossiles[END_REF], it is then possible to interpret the external longitudinal groove running on the small appendage and extending internally into the central canal of the cone as the probable course of a nerve.

The interpretation of Bolboporites as a spine is thus plausible and cannot be refuted on available evidence. However, its systematic position remains an open question. The frequent association of Bolboporites with skeletal remains of various blastozoans (e.g., cheirocrinids, Palaeocystites) questioned the possibility that it could represent isolated spines of one of them (Clark & Hoffman 1961). However, none of the blastozoans found in the same localities as Bolboporites in both Baltica and Laurentia shows any evidence suggesting that spines were articulated to its body wall. Indeed, spines are only known in asterozoans, echinozoans, edrioasteroids and stylophorans (see above), thus ruling out any putative blastozoan affinities for Bolboporites. The size of Bolboporites (from about 2 to 12 mm in height in both North American and Russian specimens; Clark & Hoffman 1961;Rozhnov & Kushlina 1994a) suggests that putative stylophoran affinities are highly unlikely. In this class, the size of the theca is generally comprised between 5 and 30 mm, and it rarely exceeds 30 to 40 mm (e.g., the largest known mitrate, Diamphidiocystis drepanon [START_REF] Kolata | Diamphidiocystis, a new mitrate "carpoid" from the Cincinnatian (Upper Ordovician) Maquoketa Group in southern Illinois[END_REF], is about 40 mm wide; Lefebvre 1999). The large size and cone-shaped morphology of Bolboporites seem to be also incompatible with putative edrioasteroid affinities. When preserved, edrioasteroid spines are consistently consisting of narrow, elongate elements, typically less than 3 mm in length, which were articulated to ambulacral cover plates and/or skeletal elements of the pedunculate zone [START_REF] Holloway | Silurian and Devonian edrioasteroids from Australia[END_REF][START_REF] Guensburg | Systematics, functional morphology, and life modes of Late Ordovician edrioasteroids, Orchard Creek Shale, southern Illinois[END_REF][START_REF] Guensburg | Revised phylogeny and functional interpretation of the Edrioasteroidea based on new taxa from the Early and Middle Ordovician of western Utah[END_REF].

Because of their stratigraphic range and palaeobiogeographic distribution in the Ordovician, asterozoans are more likely candidates: their oldest known representatives have been documented in Lower Ordovician deposits [START_REF] Thoral | Contribution à l'Etude Paléontologique de l'Ordovicien Inférieur de la Montagne Noire et Révision Sommaire de la Faune Cambrienne de la Montagne Noire[END_REF][START_REF] Blake | Early asterozoan (Echinodermata) diversification: a paleontologic quandary[END_REF][START_REF] Jell | A Tremadocian asterozoan from Tasmania and a late Llandovery edrioasteroid from Victoria[END_REF][START_REF] Blake | The class Somasteroidea (Echinodermata, Asterozoa): morphology and occurrence[END_REF], and their presence is recorded in Baltica at least from the Dapingian, i.e., as early as the oldest known occurrence of Bolboporites [START_REF] Pisera | Echinoderms from the Mójcza Limestone[END_REF][START_REF] Hansen | Starfish from the Ordovician of the Oslo region, Norway[END_REF][START_REF] Rozhnov | Echinoderms[END_REF][START_REF] Blake | Aspects of life mode among Ordovician asteroids: implications of new specimens from Baltica[END_REF][START_REF] Tinn | Asterozoan pedicellariae and ossicles revealed from the Middle Ordovician of Baltica[END_REF]. However, Bolboporites clearly does not show any character supporting its interpretation as an isolated asterozoan spine. The skeletal morphology of Ordovician asterozoans is strongly constrained and, even when preserved as isolated remains, their plates are highly diagnostic [START_REF] Pisera | Echinoderms from the Mójcza Limestone[END_REF][START_REF] Tinn | Asterozoan pedicellariae and ossicles revealed from the Middle Ordovician of Baltica[END_REF].

Echinozoans possibly appeared and diversified in Baltica during the Middle Ordovician, before spreading to Laurentia in Late Ordovician times [START_REF] Reich | Ordovizische und silurische Holothurien (Echinodermata)[END_REF][START_REF] Smith | Bromidechinus, a new Ordovician echinozoan (Echinodermata), and its bearing on the early history of echinoids[END_REF][START_REF] Lefebvre | Palaeobiogeography of Ordovician echinoderms[END_REF]. Baltica has yielded the oldest known occurrences of echinoids (Darriwilian; [START_REF] Bockelie | The presence of a bothriocidarid (Echinoid) in the Ordovician of Norway[END_REF][START_REF] Pisera | Echinoderms from the Mójcza Limestone[END_REF]), holothurians (Darriwilian; Reich 2010) and ophiocistioids (Dapingian; [START_REF] Reich | Linguaserra? (Echinodermata: Ophiocistoidea) aus dem Ordovizium Baltoskandiens[END_REF][START_REF] Rozhnov | Echinoderms[END_REF][START_REF] Reich | Origins and biomechanical evolution of teeth in echinoids and their relatives[END_REF]. The stratigraphic range and palaeobiogeographic distribution of echinozoans are thus compatible with those of Bolboporites. However, the presence of typical tubercles in the oldest known echinoids (including the isolated plates record; Pisera 1994) suggests the existence of mechanisms for spine articulation comparable to those occurring in younger taxa (i.e., balls and sockets). Putative echinoid affinities are thus unlikely for Bolboporites. Although spines have not been documented so far in holothurians and ophiocistioids, it cannot be entirely excluded that Bolboporites corresponds to isolated spines of a yet unknown primitive echinozoan.

Bolboporites as a columnal.

The general aspect of Bolboporites reminds in some respects the cone-shaped morphology of some late Cambrian-Early Ordovician pelmatozoan columnals from Utah [START_REF] Sumrall | Systematics and paleoecology of late Cambrian echinoderms from the western United States[END_REF] and Spain [START_REF] Zamora | Pelmatozoan echinoderms from the Cambrian-Ordovician of the Iberian Chains (NE Spain): early diversification of anchoring strategies[END_REF]), and thus questions its possible identification as a highly differentiated stem plate. This interpretation is in good agreement with (1) the existence of a longitudinal internal canal; (2) the articulation of a biserial appendage on its basal surface (Rozhnov & Kuslina 1994a;[START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]; and (3) the morphology of the lunules. The biconcave depressed area formed by the lunules, as well as the presence of a tiny orifice opening in between them are morphological features which are reminiscent of synarthrial articulations in crinoids (Ubaghs 1978a;[START_REF] Donovan | Functional morphology of synarthrial articulations in the crinoid stem[END_REF][START_REF] Ausich | Crinoid form and function[END_REF].

Interestingly, synarthrial articulations were present on the distal columnals of some Ordovician crinoids (e.g., Ristnacrinus Öpik, 1934) co-occurring with Bolboporites in Baltica (Donovan 1984).

It seems, however, difficult to interpret Bolboporites as a highly differentiated, massive columnal for several reasons. First, in all pelmatozoan echinoderms, columnals display articulatory facets on their two opposite (proximal and distal) sides, whereas Bolboporites would display only one facet (i.e., on its basal surface). A second difficulty is that columnals always display a central canal (lumen), which opens on their two opposite sides: such a canal is present in Bolboporites, but it is not in central position and, more importantly, it opens only on one side. Finally, the strongly convex morphology of the basal surface in many North American specimens of Bolboporites (see [START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF] makes their interpretation as columnals highly improbable.

Bolboporites as a holdfast.

Holdfasts are anchoring structures occurring at the distalmost extremity of the stem in various blastozoans and crinoids [START_REF] Ubaghs | Le genre Balantiocystis Chauvel (Echinodermata, Eocrinoidea) dans l'Ordovicien inférieur de la Montagne Noire (France)[END_REF](Ubaghs , 1978a;;[START_REF] Brett | Terminology and functional morphology of attachment structures in pelmatozoan echinoderms[END_REF][START_REF] Ausich | Crinoid form and function[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF]. The massive, cone-shaped morphology of Bolboporites shows many similarities with similarly-shaped, isolated pelmatozoan elements (e.g., Oryctoconus), generally interpreted as holdfasts [START_REF] Colchen | Sur des restes d'échinodermes(?) du Cambro-Ordovicien de la Sierra de la Demanda (Burgos-Logrono, Espagne)[END_REF][START_REF] Alvaro | Earliest Ordovician pelmatozoan holdfasts from western Europe: the Oryctoconus problem revisited[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF][START_REF] Zamora | Pelmatozoan echinoderms from the Cambrian-Ordovician of the Iberian Chains (NE Spain): early diversification of anchoring strategies[END_REF]. The identification of Bolboporites as a putative discoidal holdfast was discussed, but rejected by Rozhnov & Kushlina (1994a).

Their main argument was that, if this fossil was a holdfast, its basal surface would then be attached (fixed) to the substrate: this orientation is incompatible with the presence of an appendage articulated to the basal surface of Bolboporites. However, it should be stressed that this base-down orientation occurs only in the case of pelmatozoan discoidal holdfasts tightly and permanently encrusted on firmgrounds and hardgrounds (Ubaghs 1978a;[START_REF] Brett | Terminology and functional morphology of attachment structures in pelmatozoan echinoderms[END_REF][START_REF] Brett | Late Cambrian hard substrate communities from Montana/Wyoming: the oldest known hardground encrusters[END_REF][START_REF] Sumrall | Systematics and paleoecology of late Cambrian echinoderms from the western United States[END_REF][START_REF] Rozhnov | Morphogenesis and evolution of crinoids and other pelmatozoan echinoderms in the Early Paleozoic[END_REF]. The opposite (base-up) orientation of the cone is observed in most pelmatozoans living on soft substrates and using their distal holdfasts as an anchor or a grapnel, as, for example, the Ordovician eocrinoid Balantiocystis [START_REF] Chauvel | Echinodermes de l'Ordovicien du Maroc[END_REF]the Devonian crinoid Ancyrocrinus Hall, 1862 ('kite strategy';[START_REF] Ubaghs | Le genre Balantiocystis Chauvel (Echinodermata, Eocrinoidea) dans l'Ordovicien inférieur de la Montagne Noire (France)[END_REF][START_REF] Brett | Terminology and functional morphology of attachment structures in pelmatozoan echinoderms[END_REF][START_REF] Le Menn | Les crinoïdes du Dévonien inférieur et moyen du Massif armoricain. Systématique, paléobiologie, évolution, biostratigraphie[END_REF][START_REF] Ausich | Crinoid form and function[END_REF][START_REF] Alvaro | Earliest Ordovician pelmatozoan holdfasts from western Europe: the Oryctoconus problem revisited[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF][START_REF] Zamora | Pelmatozoan echinoderms from the Cambrian-Ordovician of the Iberian Chains (NE Spain): early diversification of anchoring strategies[END_REF].

All above-listed arguments agreeing with the interpretation of Bolboporites as a columnal remain valid if this fossil is interpreted as a discoidal terminal holdfast (i.e., internal canal; biserial appendage inserted into the basal surface; lunules forming a facet with a synarthriallike articulation). However, if Bolboporites is a holdfast, the various issues raised for its interpretation as a columnal are no longer problematic: it then makes sense that (1) a single facet is present (on the basal surface); (2) the internal canal does not open distally into the apex of the holdfast; and (3) the inflated morphology of the basal surface in some specimens of B. americanus is not incompatible with their interpretation as distal holdfasts. Moreover, the wide morphological disparity observed between specimens of Bolboporites from a same level [START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF][START_REF] Kushlina | The systematic position and composition of the genus Bolboporites (Echinodermata, Eocrinoidea)[END_REF] is also in good agreement with its interpretation as a holdfast: similar large variabilities in shape have been reported in assemblages of, for example, Oryctoconus and grapnel-like holdfasts of Ancyrocrinus [START_REF] Le Menn | Les crinoïdes du Dévonien inférieur et moyen du Massif armoricain. Systématique, paléobiologie, évolution, biostratigraphie[END_REF][START_REF] Alvaro | Earliest Ordovician pelmatozoan holdfasts from western Europe: the Oryctoconus problem revisited[END_REF][START_REF] Zamora | Pelmatozoan echinoderms from the Cambrian-Ordovician of the Iberian Chains (NE Spain): early diversification of anchoring strategies[END_REF]. Finally, this interpretation is also compatible with the occurrence of Bolboporites in deposits corresponding to shallow, stormgenerated deposits (see above; [START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF][START_REF] Bockelie | The Middle Ordovician of the Oslo region, Norway, 30. The eocrinoid genera Cryptocrinites, Rhipidocystis and Bockia[END_REF][START_REF] Dronov | Introduction to the geology of the St. Petersburg region[END_REF]. In such environmental conditions, stemmed echinoderms and their anchoring structures are generally preserved separately [START_REF] Brett | Terminology and functional morphology of attachment structures in pelmatozoan echinoderms[END_REF]. Organisms were detached from their anchoring structures probably by autotomy rather than breakages and transported away by storm currents, whereas their holdfasts were preserved in situ [START_REF] Donovan | Was autotomy a pervasive adaptation of the crinoid stalk during the Paleozoic?[END_REF]. Apart from some rare exceptions, such as the eocrinoid Balantiocystis or the crinoid Ancyrocrinus [START_REF] Ubaghs | Le genre Balantiocystis Chauvel (Echinodermata, Eocrinoidea) dans l'Ordovicien inférieur de la Montagne Noire (France)[END_REF][START_REF] Le Menn | Les crinoïdes du Dévonien inférieur et moyen du Massif armoricain. Systématique, paléobiologie, évolution, biostratigraphie[END_REF][START_REF] Ausich | Crinoid form and function[END_REF], the distalmost part of the stem is unknown in most pelmatozoans and, conversely, most holdfasts cannot been assigned to any specific taxa (e.g., Aspidocrinus scutelliformis Hall, 1859, Oryctoconus;Ubaghs 1978a;[START_REF] Brett | Late Cambrian hard substrate communities from Montana/Wyoming: the oldest known hardground encrusters[END_REF][START_REF] Sumrall | Systematics and paleoecology of late Cambrian echinoderms from the western United States[END_REF][START_REF] Alvaro | Earliest Ordovician pelmatozoan holdfasts from western Europe: the Oryctoconus problem revisited[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF][START_REF] Zamora | Pelmatozoan echinoderms from the Cambrian-Ordovician of the Iberian Chains (NE Spain): early diversification of anchoring strategies[END_REF].

The interpretation of Bolboporites as a discoidal holdfast has also several implications, that can be tested: (1) the biserial appendage inserting on the lunules would thus probably correspond to the distal-most columnals of a pelmatozoan stem; and (2) if Bolboporites was used as an anchor, it was thus at least partly buried into the sediment. If Bolboporites was a distal holdfast, the presence of two lunules in all specimens suggests that a dimeric stem was articulated to it. This interpretation is further supported by the observation of a biserial appendage in at least some better preserved individuals from Russia (Rozhnov & Kushlina 1994a;[START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]. Although most Ordovician pelmatozoans possessed holomeric stems (i.e., formed by a single column of plates), tetra-, penta-and hexameric appendages have been also described in several crinoids (e.g., Aethocrinus moorei, Ramseyocrinus [START_REF] Bates | On 'Dendrocrinus' cambriensis Hicks, the earliest known crinoid[END_REF][START_REF] Ubaghs | Aethocrinus moorei Ubaghs, n. gen., n. sp., le plus ancien crinoïde dicyclique connu[END_REF][START_REF] Ubaghs | Echinodermata. Notes sur les Échinodermes de l'Ordovicien Inférieur de la Montagne Noire (France)[END_REF]Donovan 1984[START_REF] Donovan | Biostratigraphy and evolution of crinoid columnals from the Ordovician of Britain[END_REF], as well as in some echinosphaeritid and hemicosmitid blastozoans [START_REF] Jaekel | Stammgeschichte der Pelmatozoen. Erster Band: Thecoidea und Cystoidea[END_REF][START_REF] Bockelie | The Middle Ordovician of the Oslo region, Norway, 30. The eocrinoid genera Cryptocrinites, Rhipidocystis and Bockia[END_REF][START_REF] Bockelie | Morphology, growth and taxonomy of the Ordovician rhombiferan Caryocystites[END_REF][START_REF] Parsley | Community setting and functional morphology of Echinosphaerites infaustus (Fistuliporita: Echinodermata) from the Ordovician of Bohemia[END_REF]. Dimeric distal stems are the rule in Ordovician solutans [START_REF] Caster | Homoiostelea[END_REF][START_REF] Ubaghs | Les Echinodermes "Carpoïdes" de l'Ordovicien Inférieur de la Montagne Noire (France)[END_REF][START_REF] Lefebvre | A reinterpretation of the solutan Plasiacystis mobilis (Echinodermata) from the Middle Ordovician of Bohemia[END_REF][START_REF] Noailles | A probable case of heterochrony in the solutan Dendrocystites Barrande, 1887 (Echinodermata: Blastozoa) from the Upper Ordovician of the Prague Basin (Czech Republic) and a revision of the family Dendrocystitidae Bassler[END_REF]. Although they possibly retained an attached post-metamorphic stage, Ordovician solutans were vagile and their stem did not possess any distal discoidal holdfast.

With the exception of solutan elements, only few occurrences of tri-and dimeric columnals were documented in Ordovician deposits, and all of them have been assigned to crinoids possibly related to Ectenocrinus Miller, 1889(e.g., Donovan 1985). Consequently, the existence of Ordovician pelmatozoans with a stem comprising dimeric columnals supports the identification of Bolboporites as a possible distal holdfast articulated to a biserial appendage.

However, contrary to the situation in all echinoderm stem-like appendages, the biserial structure articulated to the basal surface of Bolboporites does not contain any lumen (internal central canal), but an external groove (Rozhnov & Kushlina 1994a). This external groove, which probably housed soft parts, communicates with the longitudinal internal canal of Bolboporites. In all stemmed echinoderms, the lumen contains coeloms associated with the extraxial part of the body wall (i.e., somatocoels) and, generally, extensions of the nervous system (Ubaghs 1978a;Heinzeller & Welsh 1994;[START_REF] David | Are homalozoans echinoderms? An answer from the extraxial-axial theory[END_REF][START_REF] Mooi | Radial symmetry, the anterior/posterior axis, and echinoderm Hox genes[END_REF]).

The topology observed in Bolboporites and its associated appendage thus strongly departs from the situation in pelmatozoan stems (external vs. internal soft parts). This implies that the biserial structure articulated to Bolboporites cannot be interpreted as (part of) a stem-like appendage and, consequently, that Bolboporites was not a distal holdfast. This conclusion is confirmed by the distribution of biofilms produced by encrusting organisms over the body wall of Bolboporites (see above; [START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF]. If this fossil was a discoidal terminal holdfast, comparison with similar structures in pelmatozoans [START_REF] Ausich | Crinoid form and function[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF] suggests that in life, a large part of the cone would have been at least partly buried into and/or in permanent contact with the substrate. This life orientation is not compatible with the observed distribution of epibionts, which produced extensive films on the lateral walls of Bolboporites, but are absent from its basal surface [START_REF] Kushlina | Possible algal-bacterial biofilms on eocrinoids from the Ordovician of the St. Petersburg region (Russia)[END_REF].

Consequently, although the interpretation of Bolboporites as a discoidal distal holdfast is plausible (e.g., massive cone-shaped morphology, wide morphological disparity, articulation to a dimeric appendage; see above), this identification has to be rejected because the structure articulated to its basal surface is not a stem-like appendage (no lumen, external groove housing soft parts). Further, the implied life orientation is not confirmed by the distribution of epibionts on the cones.

Conclusions

Our results not only confirmed the presence of stereomic microstructure in Bolboporites (and thus its echinoderm affinities), but they also showed that this fossil is a single, previously microporous, calcitic skeletal element, without any internal macrostructure, except a narrow longitudinal canal opening through a tiny orifice on the basal surface. These results combined with all previous descriptions of Bolboporites have made it possible to critically discuss several hypotheses about its nature (e.g., theca, basal cone, spine, columnal, holdfast) and its putative affinities within echinoderms (e.g., asterozoans, blastozoans, crinoids, echinozoans, stylophorans). Most interpretations could be rejected, because they comply with only part of available evidences. Although the identification of Bolboporites as a spine remains questionable, it represents the most parsimonious -and likely -interpretation. The precise affinities of Bolboporites remain difficult to assess and it is tentatively assigned here to an unknown, possibly basal echinozoan. As this was the case for other problematic fossils (e.g., conodonts, machaeridians), future discoveries of fully articulated specimens showing Bolboporites elements in connection with their host organism will probably help in revealing their actual nature and affinities within echinoderms.

  These results, combined with all available evidence obtained during this study and/or from the literature, make it possible to critically evaluate all plausible interpretations concerning the nature and systematic position of Bolboporites. Comparison with other Palaeozoic echinoderm cone-shaped elements suggests that Bolboporites can be interpreted in five different ways: (1) internal mould or body wall of a blastozoan theca (by analogy with, e.g., Timorocidaris; Ami 1896;Rozhnov & Kushlina 1994a, b;[START_REF] Kushlina | The systematic position and composition of the genus Bolboporites (Echinodermata, Eocrinoidea)[END_REF];(2) large infrabasal cone of the body capsule (calyx or theca) of a stemless pelmatozoan (by comparison with, e.g., Cymbionites); (3) isolated spine or tuberculated plate of an unknown echinoderm[START_REF] Quenstedt | Petrefakten Deutschlands, Part 1[END_REF][START_REF] Lindström | Index to generic names applied to the corals of the Palaeozoic formations[END_REF][START_REF] Wanner | Ueber einige palaeozoische Seeigelstacheln (Timorocidaris gen. nov. und Bolboporites Pander)[END_REF][START_REF] Yeltysheva | Bolboporites[END_REF][START_REF] Clark | Bolboporites americanus in the Chazy of Southern Quebec[END_REF]; (4) highly modified columnal of a pelmatozoan; and (5) distal holdfast of a stemmed echinoderm ('Oryctoconus scenario').

  or in stylophorans (e.g., Jaekelocarpus[START_REF] Kolata | The youngest carpoid: occurrence, affinities and life mode of a Pennsylvanian (Morrowan) mitrate from Oklahoma[END_REF] from the Pennsylvanian of Oklahoma;[START_REF] Dominguez | Paired gill slits in a fossil with a calcite skeleton[END_REF]. Particularly drastic examples of such a reduction in the number of plates can be documented in the calices of several derived, Late Palaeozoic, stemless crinoids, all characterized by convergent, similar-looking cone-shaped morphologies, as, for example, Agassizocrinus lobatus[START_REF] Springer | Unusual forms of fossil crinoids[END_REF], from the Mississipian of Kentucky[START_REF] Ettensohn | The autecology of Agassizocrinus lobatus[END_REF] or Edriocrinus sacculus Hall, 1859, from the Lower Devonian of New York[START_REF] Moore | Flexibilia[END_REF][START_REF] Seilacher | Crinoid anchoring strategies for soft-bottom dwelling[END_REF][START_REF] Herbert | What is Edriocrinus?[END_REF]. The stemless Permian crinoid Timorocidaris probably represents the most extreme case of reduction in the number of plates, with its bowl-shaped calyx possibly made of a single skeletal element(Ubaghs 1978a;[START_REF] Hess | Permian[END_REF]. Consequently, Bolboporites could represent a case of convergent acquisition of a single plated body capsule in blastozoans.

FIGURE 1 .

 1 FIGURE 1. External morphology of Bolboporites uncinatus Pander, 1830, Middle Ordovician (Dapingian), Saint-Petersburg area, Russia; redrawn and modified from Rozhnov & Kushlina (1994a, fig. 3b) and Kushlina (1995, pl. 5 fig. 1b). A: Base of the cone, in front view, showing the two adjoining depressed areas (lunules), and the small orifice corresponding to the outlet of the longitudinal internal canal. B: Lateral view, showing the typical honeycomb ornamentation on lateral sides of the cone.

FIGURE 2 .

 2 FIGURE 2. Fourier transform infrared (FT-IR) spectroscopy made on transverse sections of Bolboporites mitralis Pander, 1830, included in LR white resin; UCBL-FSL 712510, Middle Ordovician (Dapingian), Saint-Petersburg area, Russia. A: Location of FTIR analyses. B: IF spectrum of LR white resin (control spectrum). C, G: IF spectra of sampling points located on the external margin of the specimen (calcite). D-F: IF spectra of sampling points within the specimen (calcite). Peak values indicated in cm -1 .

FIGURE 3 .

 3 FIGURE 3. Thin section views of Bolboporites mitralis Pander, 1830 (Middle Ordovician (Dapingian), Saint-Petersburg area, Russia; UCBL-FSL 712510) included in the surrounding sedimentary rock (ss) which is a packstone-wackestone with bioclasts and glauconite grains. A: Plane-polarized optical light, showing the conjugated cleavage planes (cp) crosscutting the whole cone. B: Cross-polarized optical light, showing total extinction of the cone. C: Close up

FIGURE 4 .

 4 FIGURE 4. EBSD mapping of Bolboporites mitralis Pander, 1830; Middle Ordovician (Dapingian), Saint-Petersburg area, sample UCLB-FSL 712510, cross section, perpendicular to the cone axis. A: Periphery of the sample observed by SEM. B: corresponding EBSD map, showing the size heterogeneity of the crystals around the monocrystalline structure; the box on the down right corner gives the color code for crystallographic axes. C: another peripheral zone, at higher magnification. D: corresponding EBSD map. E: central zone of B. mitralis visualized by SEM. F and G: two EBSD maps of two areas of the central zone (E) 4 mm apart. Although the structure is fully monocrystalline on the whole cross-section (no grain limit detectable), a slight variation of the colour indicates a tiny crystallographic disorientation (< 4°).

FIGURE 5 .

 5 FIGURE 5. Internal structures of Bolboporites mitralis Pander, 1830; Middle Ordovician (Dapingian), Saint-Petersburg area, Russia. A-C: Cathodoluminescence view of sectioned Bolboporites mitralis Pander, 1830; Middle Ordovician (Dapingian), Saint-Petersburg area, Russia. A: Longitudinal section through specimen UCBL-FSL 712508, with several randomly distributed Trypanites-like borings (bor) through the body wall. B-C: Cross section through specimen UCBL-FSL 712510, showing evidence of narrow longitudinal axial canal (int. canal). B: General view of the sectioned specimen. C: Detail showing the central internal canal filled with a syntaxial blocky calcite cement (sbc). The surrounding skeleton reveals a stereom-like structure with aligned luminescent inclusions; specimen UCBL-FSL 712510 D:This SEM view shows the opening part of a tubular boring crosscutting the stereom-like microstructure of a Bolboporites cone (UCBL-FSL 712508). Note this stereom-like microstructure is highly cemented near the walls of the boring and at the periphery of the cone, whereas micropores (mp) are presents in the more internal parts. This boring is partially filled with a bioclastic and glauconitic packstone (gp) that also surround the cone. More internal parts of the boring are cemented by dolomite rhombs (dol) and by a syntaxial blocky calcite cement (sbc).

FIGURE 6 .

 6 FIGURE 6. CT-scan imagery of Bolboporites mitralis Pander, 1830; UCBL-FSL 712509 Middle Ordovician (Dapingian), Saint-Petersburg area, Russia. A: Virtual cross section showing diffuse peripheral rim, made of denser (less porous) stereom than central part of the

FIGURE 7 .

 7 FIGURE 7. External aspect and morphological disparity of Bolboporites; all specimens from same locality and level, Upper Ordovician (Sandbian), Skien-Langesund area, Norway. A: Bolboporites sp., lateral side of the cone showing honeycomb ornamentation, and almost flat base of the cone; PMO 116887. B: Bolboporites elongatus Kushlina, 1995, narrow cone in lateral view, with gently convex base; PMO 218238. C-D: Bolboporites cf. mitralis Pander, 1830; PMO 218249. C: Wide cone in lateral aspect, with convex base. D: Basal surface with two lunules in almost central position, and small orifice at their junction.

FIGURE 8 .FIGURE 9 .

 89 FIGURE 8. External morphology of Bolboporites mitralis Pander, 1830; Middle Ordovician (Dapingian), Saint-Petersburg area, Russia. A: Cells forming honeycomb ornamentation on lateral sides of the cone; SEM view of specimen UCBL-FSL 713254. B: Lunules and associated small orifice, on the base of the cone; SEM view of specimen UCBL-FSL 713250.Note also the well-preserved stereom-like microstructure that appears on the wall of these lunules.

Acknowledgments

This paper is a contribution to the International Geoscience Programme (IGCP) Project 653 -The onset of the Great Ordovician Biodiversification Event, and of the team 'Biosignatures, Vie Primitive' of UMR CNRS 5276 LGLTPE. The paper is also a contribution to the revision of volumes S and U of the Treatise on Invertebrate Paleontology. The authors are particularly grateful to Sergei V. Rozhnov (Palaeontological Institute of the Russian Academy of Sciences, Moscow) for providing numerous, well-preserved specimens of Bolboporites sp. for this study, and for insightful and constructive discussions on their systematic affinities. Hans-Arne Nakrem and Franz-Josef Lindemann (Natural History Museum, Oslo) are thanked for access to the Norwegian material of Bolboporites and Coelosphaeridium, and Michel Creuzé des Châtelliers and Blandine Bärtschi (Lyon 1 University, Villeurbanne) for access and dissection of specimens of the Recent spiny starfish Pentaceraster mammilatus. Jih-Pai Lin (National Taïwan University) is also acknowledged for having made available specimens of Timorocidaris for comparison. Frédéric Herbst (UMR6303 ICB, Burgundy University, Dijon) acquired the EBSD data. This study also benefited from useful comments from and/or discussions with Daniel B. Blake (University of Illinois, Champaign), Gilles Cuny and Vincent Perrier (Lyon 1 University, Villeurbanne), Georgy Mirantsev (Palaeontological Institute of the Russian Academy of Sciences, Moscow), Elise Nardin (Observatoire Midi-Pyrénées, Toulouse), Colin D. Sumrall (University of Tennessee, Knoxville), and Gary D. Webster (Washington State University, Pullman). Finally, Thomas E. Guensburg (Field Museum, Chicago) and Stephen K. Donovan (Naturalis, Leiden) are greatly thanked for their constructive and helpful reviews.