Ordovician Petraster Billings, 1858 (Asteroidea: Echinodermata) and early asteroid skeletal differentiation

A large and taxonomically diverse collection of Ordovician Asterozoa from France and Morocco enables new insights into the early evolution of the subphylum. Available specimens of both Asteroidea and Stenuroidea are comparatively few, the collection dominated by Ophiuroidea and to a lesser extent, Somasteroidea. Nearly all the asteroid specimens are assigned to Petraster Billings, 1858; P. caidrami-ensis n. sp. is differentiated based on the complexity of the extraxial skeleton. The genus Petraster is reviewed. Another asteroid fragment, Euaxosida sp. A


INTRODUCTION
Water-vascular tissues and their skeletal enclosure -the axial or ambulacral ossicles together with the adaxial or adambulacral ossicles-are fundamental to asterozoan organization and function, and once the basic ambulacral column configuration was established at the onset of subphylum history, evolutionary changes were limited.The remainder of the skeleton, the extraxial, is under the more immediate influence of local environmental conditions, and through time, homoplastic extraxial morphologies emerged among lineages.Axial and adaxial ambulacral construction of species of Petraster Billings, 1858, are comparatively uniform whereas the more varied extraxial expressions offer insights into complexity of early asteroid evolution.Extraxial variation cannot be simply linked to environmental specifics.Known Petraster occurrences, however, are too late in time to represent more than then-surviving indicators of earlier events.Euaxosida sp.A configuration also suggests early steps in asteroid evolution.Specificities of ossicular form -e.g."paxilliform", "platelike"are issues apart from skeletal series definition.
Following sections treating geologic setting and terminology, the taxonomy of Petraster is summarized, and succeeded by a discussion section interpreting morphological complexity.Species of Petraster are illustrated in figures one to six, specimens illustrating inferred phylogenetically significant variation in skeletal construction are illustrated in figures seven to ten.
In marked contrast with their Early Ordovician counterparts, later asterozoan faunas from the Middle and Upper Ordovician of France and Morocco have been little studied.The aim of this paper focusing on the asteroid genus Petraster is thus to contribute to a wider project devoted to the still poorly known diversification of asterozoans in high-latitude regions of the Mediterranean Province during the Ordovician.In recent years, several new occurrences of asterozoans were described in the Darriwilian and the Katian of the Armorican Massif (western France; Hunter et al. 2007;Blake et al. 2016).One of them, Petraster crozonensis Blake, Guensburg & Lefebvre, 2016, is redescribed here (see below), and its strati-
Contrary to the situation in the Armorican Massif, no asterozoan remains have been documented so far in the Middle Ordovician of Morocco.In contrast, the occurrence of diverse and locally abundant asterozoan assemblages in the Upper Ordovician of the Tafilalt (eastern Anti-Atlas) has been regularly mentioned (Gutiérrez-Marco et al. 2003;Lefebvre et al. 2007Lefebvre et al. , 2008Lefebvre et al. , 2010Lefebvre et al. , 2022;;Hunter et al. 2010).Although most of these assemblages are dominated by yet undescribed ophiuroids, some of them have also yielded well-preserved remains of asteroids, including the material of Petraster caidramiensis n. sp., which is described below.Assessing the precise stratigraphic position of each asteroid-bearing level in the Tafilalt area is challenging, because of the absence of key index taxa (e.g.conodonts, graptolites, palynomorphs) in most fossiliferous localities.Moreover, contrary to the situation in the Central Anti-Atlas, where the Upper Ordovician succession consists of lithologically well-contrasted units (with the sandstones of the Upper Tiouririne Formation intercalated between the shale-dominated Lower and Upper Ktaoua formations), the Ktaoua Group is predominantly composed of coarse siltstones and sandstones in the Tafilalt (Destombes et al. 1985;Destombes 2006a, b, c;Álvaro et al. 2022).In the eastern Anti-Atlas, the identification of the stratigraphic position of asteroid levels thus relies both on direct field observations made with J. Destombes in Oct. 2010, and on indirect estimations obtained by reporting the precise GPS coordinates of the sites on the two corresponding 1:200.000geological maps (Service géologique du Maroc 1986Maroc , 1988)).
All available specimens of P. caidramiensis n. sp. and Euaxosida sp.A were originally collected in the field by R. and V. Reboul, D. Vizcaïno, and F. Auvray, and subsequently donated to the paleontological collections of Lyon 1 University (Villeurbanne, France), where they are now deposited (UCBL-FSL).Asteroid specimens belonging to the Auvray and Reboul collections are from three distinct localities, all situated in a small geographic area, in the eastern Maïder, south of the N12 road between Msissi and Rissani: ECR-F12 (Tarafin Signit), ECR-F19 (El Kraouia), and Isthlou (ECR-F6) (for locality details and map, see Nohejlová & Lefebvre 2022).
Locality details and stratigraphic position of specimens of P. caidramiensis n. sp.belonging to the Vizcaïno collection (including the holotype, UCBL-FSL 712025) are not known precisely.Labels indicate they are from the El Caïd Rami area, which is named after a dried river (Oued El Caïd Rami) located a few kilometers away from the three above mentioned sites, north of the N12 road between Msissi and Rissani.The Lower Ktaoua and Upper Tiouririne formations are largely exposed in the classical El Caïd Rami area, where numerous fossiliferous sites (e.g.ECR-F2, ECR-F5) have been exploited in the early 2000s (see e.g.Lefebvre et al. 2007Lefebvre et al. , 2010Lefebvre et al. , 2022;;Hunter et al. 2010).By comparison with the other occurrences of P. caidramiensis n. sp., it can be reasonably postulated that the stratigraphic position of specimens from the El Caïd Rami is probably comprised between the middle part of the Lower Ktaoua Formation and the base of the overlying Upper Tiouririne Formation (early-middle Katian).Specimens from France and Morocco were compiled and latex casts prepared by BL, the casts forwarded to DBB, the latter author adding documentation available from other sources.TERMINOLOGY OF THE ASTEROIDEA Terminology begins with Spencer & Wright (1966) as reviewed by Dean Shackleton (2005) and Blake et al. (2020b).The aboral surface is directed toward the water column, the oral surface toward the substrate.The primary skeleton forms the body wall.The accessory skeleton includes the generally abundant spines, granules, and pedicellariae seated on all primary ossicles except axials.Axial (or "ambulacral") ossicles form a double series along the axis of the arm.The unpaired terminal is at the arm tip.Mouth-angle ossicles are the proximal-most ossicles of the axial series.Axial ossicles articulate with the adaxial ossicles (adambulacrals are adaxials).The remainder of the skeleton is extraxial.The body is edged by a single or double series of more or less clearly differentiated marginal ossicles.Because the term "marginals" has been broadly applied within echinoderms with unclear implications of homology, the genetically neutral term ambital framework has been proposed (Blake 2013).A single marginal series has been judged to be homologous throughout stem-group asteroids, it recognized as inferomarginal.Because focus herein is on the implications of the aboral surface of Petraster, all skeletal components "above" the inferomarginals for convenience are referred to as "aboral", including a second superomarginal series.The axillary (or "odontophore") typically is a more or less clearly differentiated unpaired ossicle aligned with the inferomarginal series at the interbrachial midline of asteroids.Abactinal ossicles are above the marginal series.Among many asterozoans, a medial disk centrale can be recognized, it enclosed by rings of more or less differentiated ossicles, a "primary circlet" or aboral ring.Midarm ossicles can be enlarged and/or otherwise differentiated to form a carinal series.In many asteroids, lateral abactinal series or adradialia are aligned with midarm carinals.A hydropore or madreporite provides opening to the water-vascular system.1966).The type species of Petraster, P. rigidus Billings (Fig. 1A1,  A2), is incompletely known leading to the use of P. kinahani (Baily, 1879) as the generic standard of comparison (Dean Shackleton 2005).These interpretations are followed here.

AbbreviAtions
Species inclusion in Petraster emphasizes irregularity of the skeleton aboral to the inferomarginals together with a fundamental uniformity of oral configuration.Five species are included, P. ramseyensis tentatively.Petraster caidramiensis n. sp., P. crozonensis, and P. kinahani are known from fairly complete specimens, in all three, aboral series (i.e., "above" the inferomarginals) of the arms are more clearly defined than those of the disk, in part because of limitations of preservation but also because disk ossicles appear less clearly differentiated.The subcylindrical proximal arm intervals of Petraster specimens were subject to compaction under sediment load exaggerating water-vascular channel breadth, the proportionately more robust distal arm intervals retaining a more natural water-vascular channel appearance.
Petraster rigidus (Billings, 1857) (Fig. 1A) descriPtion Petraster in which arms are moderately elongate, R reaching at least c. 20 mm.Inferomarginals robust, rectangular, relatively wide on disk, more nearly equidimensional on arm; some ossicles appear to bear accessory pustules.Two available interbrachia of the holotype differ, one with a well-defined triangular axillary and a possible second small axillary enclosed on disk adjacent to MAO.Actinals on disk in well-defined single series.Second interbrachium, ossicular arrangement poorly preserved and ill-defined.Axillary, actinals not clearly defined.Adaxials rectangular, transversely elongate.

remArks
The poorly known type species is similar to other species assigned to the genus based on overall form and the limited data available on oral-surface expression, including inferomarginal shape, disk configuration, axillary, and actinal expressions.The diagnosis focuses on expression of the better-preserved interbrachium.The skeleton aboral to inferomarginals is almost unknown, the madreporite, axials, and MAO unknown.
Posited presence of two axillaries serves to separate P. rigidus from other Petraster species except from P. kinahani, with specificities of expression differing between these two.The more lateral axillary is similar to the single axillary of P. crozonensis, the adjacent inferomarginals differ in outline.A few small elongate ossicles lateral to the inferomarginals of one arm appear unlike expressions among other species, although these ossicles as preserved appear dissimilar from traces exposed on other arms.remArk Petraster caidrami is separated from P. crozonensis and P. kinahani by presence of only moderately differentiated enlarged aboral arm ossicles with numerous small intercalated granular accessary ossicles; presence of a single pentameral axillary similar in size to adjacent inferomarginals; presence of abutted, bilateral axials, the transverse ridge "T"-shaped.Blake, Guensburg & Lefebvre, 2016 (Fig. 4) Axials approximately equidimensional, sequential axials abutted, not overlapping; transverse ridge broadly J-shaped, transverse channel well-defined.MAO pair narrow, circumorals not thought to be dumbbell shaped.Adaxials with prominent "nose" articulating with axial.Adaxials possibly elliptical, longitudinally elongate prominent nose developed.remArks Petraster kinahani is separated from P. caidramiensis n. sp. and P. crozonensis by presence of distal abactinal arm ossicles that lack clear differentiation in form or into series; the absence of aboral intercalated granular accessories; the irregular arrangement of proximal superomarginals and intermarginals; the complex, multiossicular oral disk ossicular configuration with two axillaries; and equidimensional abutted axials with a "J"shaped transverse ridge.

Petraster crozonensis
A small ambulacral area (Fig. 6A1) is problematic.One area (horizontal arrow to upper left) shows a series of five axial transverse ridges offset from prominent noses of adaxials, the displacement accompanying sediment compaction, whereas another area (inclined arrow to lower left) suggests a series of five separate small ossicles between the axials and adaxials.Presence of an "extra" ossicle emplaced between the axial and adaxial is diagnostic of the Stenuroidea (Blake 2013).A similar expression was found in none of the other specimens of P. kinahani, the interval to lower left interpreted as preservational aberrancy while recognizing the occurrence might reflect paraphyly or polyphyly of the stenuroid condition.
Petraster?ramseyensis (Hicks, 1873) (Fig. 1B)  Branstrator (1982) based 'Petraster' wigleyi from the Ordovician Bromide Formation of Oklahoma on a fragment free of enclosing matrix and consisting of most of one arm as judged by arm taper, and the proximal interval of a second, the two linked by a small portion of the disk margin.The sharply angular, evenly tapered arms are marked by a clearly demarcated carinal series, the carinals differing little from ossicles of the well-defined longitudinally and transversely aligned adjacent abactinal series.The transverse series ossicles are aligned with those of the carinal series as to yield a uniform overall arm appearance.Abactinals are proportionately small, uniform, and in aboral aspect appear approximately diamond-shaped and separated by small and uniform apparent papular pores.
Of the disk, only the marginal sequence and a few associated ossicles remain.The ambulacral furrow is closed, the outer surfaces of the adaxials weakly transversely elongate and nearly equidimensional.The remainder of the adaxials are obscured and axials are not exposed.Only a single marginal series is developed, the ossicles robust, rectangular, larger than the adaxials, and fewer in number.A somewhat weakly differentiated axillary appears to be present.As reconstructed by Branstrator (1982: fig. 76), a linear actinal series is developed on the disk, it as reconstructed similar to that of the type species P. rigidus.Small spinelets were reported (Branstrator 1982: 317).The concept of Petraster as developed here stresses irregularity of ossicular expression of the aboral skeleton, presence of two marginal series, and to a lesser extent, presence of cylindrical arms; 'P.' wigleyi Branstator, 1982 is not in accord with these delineations and it is not assigned to Petraster herein.
Withers & Keble (1934) assigned three new species from the Silurian of Australia to Petraster, two from the Yarravian Series.An aboral arm surface and its ossicular configuration of Petraster richi are essentially in accord with the present concept of Petraster; however, data adequate for interpretation are not available and therefore species assignment is left in abeyance.Little data are available for the other two species names, these not further considered.
Palaeasterina speciosa Miller & Dyer, 1878, was assigned to Petraster (Schuchert 1915) and later recognized as the type species of Jugiaster Blake, 2007.descriPtion Disk large, arms triangular, abruptly tapering; interbrachium thought abruptly rounded.Vaulting probably low.Aboral skeleton might consist of small, spicular ossicles.Any madreporite unknown.Specimen margin appearing marked by an ambital framework series of approximately equidimensional, triangular?ossicles.Actinals rectangular, elongate transverse to ambulacral ossicles.Axials known only from incomplete transverse ridges, axials appearing rectangular, podial basins relatively small.Adaxials appearing proportionately small, rectangular, and transversely elongate.remArks Ossicular preservation is poor.Arm taper and ossicular arrangement suggest arms are complete.The two remaining arm tips as preserved appear to have been somewhat rounded suggesting some specimen arching in life.Mouth angle ossicles are represented by incomplete ossicular swellings, these dilated as to indicate some specimen flattening with burial.Both surviving interbrachia exhibit now-disrupted irregular elongate ossicles, at least some directed away from the axials, these suggestive of somasteroid virgals; however, the ossicles do not correspond in number with the axialadaxials, they are not uniform, and orientation of distal ossicles appears irregular (Fig. 7A1).Although uncommon among asteroids, homoplastic ossicular alignment suggestive of virgal series is developed in crown-group Tremaster Verrill, 1879 (Fig. 7B1, B2), and Jurassic occurrences similar to that of extant Tremaster have been described

A B C1 C2
Early Asteroidea skeletal complexities (Hess 1981;Smith & Tranter 1985).Axial shape, including transverse ridge expression, is asteroid-like.Inferred euaxosidan affinities are based on the apparently weak linkage between the axial and adaxial based on ossicular positioning and presence of a pulled-apart and therefore weakly linked axial/adaxial series.A large disk with peripheral marginal series is unusual but occurs elsewhere (Fig. 9D) among Paleozoic asteroids.

DISCUSSION
interPreting the AsterozoAn skeleton Given the evidence provided by a sketchy fossil record, the taxonomic composition of the Asteroidea as generally recognized (e.g.Ubaghs 1953;Spencer & Wright 1966;Dean Shackleton 2005;Blake 2018) is readily treated as monophyletic while also recognizing potential for paraphyly or polyphyly.Beginning with designation of the Somasteroidea as stemward among asterozoans (Spencer 1951; Blake 2013), the history of the class Asteroidea was marked by relatively few fundamental changes in skeletal configuration, "fundamental" descriptively defined as differences likely to receive emphases at higher taxonomic levels.
Class-level asteroid apomorphies assuming a somasteroid ancestry consist of reduction of adaxial virgalia from the somasteroid series to a single ("adambulacral") ossicle; displacement of the axials toward the aboral surfaces of the adaxial; axial vaulting to form an ambulacral furrow; and loss of radial water vascular channel skeletal closure, closure among somasteroids likely serving to protect water-vascular tissues, protection in asteroids afforded by vaulting and furrow narrowing.Radial water vascular channel size was reduced among Asteroidea, although sediment load can produce potentially misleading channel spreading and flattening.Plesiomorphic expressions retained from the somasteroid evolutionary grade include lateral pairing of axials and adaxials, somewhat irregular alignment ("pairing") of axials across the ambulacral midline, and at least in some lineages, the capacity for coordinated lateral flexure of adaxials in the horizontal or body plane as reflected in configuration of the abradial margin of the axials (e.g.Eukrinaster Blake, Guensburg, Sprinkle & Sumrall, 2007;Cnemidiactis Spencer, 1918; Jugiaster Blake 2018: pl.2).Somasteroid-like abactinal expressions are thought to have been retained in the earliest Asteroidea.
The skeletal morphology of the Asterozoa, here focusing on the classes Somasteroidea and Asteroidea, is subdivided into a comparatively small number of skeletal categories or series, but interpretation of some series is difficult, here exemplified with reference to Petraster and Euaxosida sp. A. For evaluation, series are partitioned into categories in accord with two overlapping schemes.First, the three-fold subdivision of Spencer & Wright (1966: 9, as modified herein) consists of an "axial" series, including the ambulacral (or "axial") ossicles, the mouth angle ossicles, and the unpaired terminal ossicle at the distal tip of the arm, it here treated as axial.The terminal ossicle is difficult to recognize among fossil asterozoans because in many species it is little differentiated in size and form from immediately adjacent ossicles, and preservational events commonly further obscure its recognition.The terminal ossicle is important in that ossicular series (e.g.axials, adaxials, marginals) arise on the proximal side of the ossicle.However, it is the unknown evolutionary biology of the position on the proximal side of the terminal that is important.The terminal ossicle itself does not generate ossicular series.The problematic nature of somasteroid terminal ossicle recognition has been treated elsewhere (Blake & Guensburg 2015: 478, fig. 9)."Adaxial" series include virgals of somasteroids, the adambulacrals of asteroids, the laterals of ophiuroids, and the embedded virgals and outer virgals of stenuroids.The remainder of the skeleton is extraxial, including abactinals, the ambital framework, actinals, madreporite, and accessories.Because the extraxial skeleton partially encloses and obscures the axial and adaxial, it is the extraxial that is immediately obvious in most specimens and most readily provides characters for taxonomic evaluation, but it is the expression of the adaxial and axial that is crucial to interpretation at higher taxonomic levels, in part because the extraxial skeleton is under the more immediate influence of the local environment, and therefore more readily susceptible to evolutionary change.The axial/ adaxial series of asteroids, although crucial to interpretation, are incompletely understood because of typically limited exposure among fossils.
In a second scheme for partitioning, the extraxial ambital framework series of somasteroids and its asteroid homologous inferomarginal series serve to separate the extraxial aboral skeleton from the axial, adaxial, and extraxial oral skeleton.
Discussions here seek dissociation of expressions of relatively widespread uniformity that in a phylogenetic analysis would best provide apomorphies unifying clades assigned higher taxonomic rankings from the varied expressions more useful at lower taxonomic levels, the former largely made up of axial and adaxial characters whereas all three ossicular categories contribute usefully to the latter.Because more complete specimens are rare but discrete ossicles comparatively common at least in some European Jurassic and Cretaceous localities, specificities of ossicular variation have received much attention among paleontologists.Review and limitations of usage of ossicular morphology, and the need for comparative complete specimens, ancient and modern, has been reviewed (Villier et al. 2004: 811).Reviews of asteroid skeletal construction include Spencer & Wright (1966), Dean Shackleton (2005), and Blake (2018).evolution of the Asteroid AmbitAl frAmework And Petraster An ambital framework series is recognized in all known somasteroids (but see Hunter & Ortega-Hernández 2021; Blake & Hotchkiss 2022) and all or nearly all (but see Blake & Guensburg 1989) stem-group asteroids.Three ambital framework series are recognized, the "inferomarginal", which includes the "ambital necklace" subdivision (Blake & Guensburg 2015: 466), "superomarginals", and any "intermarginals" intercalated between the superomarginals and inferomarginals.
Descriptively, an ambital framework progression within Somasteroidea begins with the diffuse, delicate ambital network of Chinianaster Thoral, 1935 (Fig. 8A2), continues with the better defined but locally irregular series of Villebrunaster Spencer, 1951 (Fig. 8B) and the more clearly defined series of Ophioxenikos Blake & Guensburg, 1993 (Fig. 8C), and concludes with Archegonaster Jaekel, 1923.If only a single series is present it is considered "inferomarginal" presumably based on position and widespread occurrence of a so-called Early Asteroidea skeletal complexities "axillary", see below.Surveying the history of interpretations of "marginals" is beyond the scope of the present study.Of significance here, however, is that the extraxial inferomarginal series of Petraster, like the axial and adaxial series, are clearly recognized among species, their presence and definition providing the marker for separation and interpretation of the axial, adaxial, and extraxial oral skeleton from the entirely extraxial aboral skeleton.
In Petraster, the inferomarginal series is well-defined and the superomarginal series difficult to interpret, the weak differentiation leading Spencer (1916: 106) to express uncertainty as to whether superomarginals of Uranaster kinahani (now Petraster kinahani) were as yet incompletely differentiated (i.e., emergent), or alternatively, in the process of being lost.Later, Spencer (1918: 126) suggested sup-eromarginals were "secondary" but he did not nominate a derivation whereas Dean Shackleton, (2005: 39) thought superomarginals of Hudsonaster "and similar taxa" were modified abactinals, and inferomarginals probaby so (Dean Shackleton 2005: 42).
In an alternative hypothesis for superomarginal origin, the irregular ambital framework series of Villebrunaster (Fig. 8B) and Ophioxenikos (Fig. 8C) suggest independent lineages might have led to both the single and double marginal series of asteroids.Additionally, the irregular marginal series of the early asteroid Eukrinaster (Fig. 9D) is suggestive of appearance of Villebrunaster thereby indicating that well-defined marginal series, either single or both single and double, could have originated after the origin of the asteroid condition, that is, after the origin of the defining asteroid ambulacral furrow.The more clearly developed series of enlarged ossicles aboral to the inferomarginal series of Petraster are recognized herein as superomarginal, and although superomarginal origin(s) appears apart from that of the inferomarginals, sourcing is considered uncertain.Regardless of series origin, specificities of asteroid marginal form are highly varied, in part correlated with overall arm shape.Ossicles commonly are enlarged relative to adjacent ossicles in species with flatter arms (e.g.Delicaster Blake & Elliott, 2003; Fig. 10A) whereas they are similar in size in more nearly cylindrical arms (e.g.Figs 1-6) (Spencer 1922: 202).
Inferomarginal series of Paleozoic asteroids with proportionately small disks and few actinal ossicles typically radiate from a so-called "axillary", it centered on the interbrachial midline and abutting or nearly abutting the mouth-angle ossicles.Spencer (1916: 62;details, 1919: 180) argued that the "odontophore" of the crown group is the stem-group axillary.Axillary recognition can be problematic in taxa such as Eukrinaster with an enlarged disk (Fig. 9D; Blake et al. 2007).The absence of a clearly defined axillary in P. kinahani (Fig. 6A) is reminiscent of expression of the early asteroid Aerliceaster Blake, Gahn & Guensburg, 2020b (Fig. 9A2) arguing potential emergence of the axillary within Asteroidea, and therefore the potential for homoplasy among axillary occurrences.
"Intermarginals" are by definition body wall ossicles bounded by the "inferomarginal" and "superomarginal" series.Occurrences in widely divergent stem-group (e.g.Xenaster Simonovitsch, 1871) and crown-group (e.g.Echinasteridae) asteroids demonstrate homoplasy.Given uncertainties of genesis of superomarginals, "intermarginals" in the first asteroids might be derivatives of either the complex ambital framework of a Villebrunaster -like ancestry or of abactinal origin and homoplastic.Present interpretation of the aboral skeleton differs from that of Dean Shackleton (2005: 93), who argued "true superomarginals are in direct abutting contact with inferomarginals over the entire arm length", in this interpretation apparently not recognizing intermarginal series.evolution of the Asteroid AbActinAl series And Petraster "Abactinals" are those ossicles other than the madreporite above all marginal series, a working usage independent of the hypothesis that so-called superomarginals and intermarginals might have had an "abactinal" source, and occurrences among taxa might be homoplastic.Three abactinal configurations are recognized within Somasteroidea: spicular and reticulated (e.g.Chinianaster, Fig. 8A1); plate-like and abutted (Ophioxenikos, Fig. 8C); and limited to an apparent granular layer lacking enlarged ossicles (Archegonaster).Skeletal reduction is a seemingly recurrent theme among Asteroidea and most readily visualized among the extant, including the Poraniidae and Acanthasteridae, the latter a derivative of the typically skeletally robust Oreasteridae (Blake 1979;Yasuda et al. 2006;Mah & Foltz 2011).Among somasteroids, differentiation into series (e.g.aboral ring, centrale, superomarginals, carinals, adradialia) has not been recognized.
Two divergent emergent configurations from a posited somasteroid ancestry are recognized among early asteroids, one retaining proportionately small aboral ossicles (e.g.Eukrinaster; Fig. 9D) that are at least broadly suggestive of those of Ophioxenikos (Fig. 8C), and the second of enlarged plate-like abactinals exemplified by Aerliceaster (Fig. 9A1).In the latter, some linear alignment is recognized but not series readily correlated with those typical among more derived asteroids (Fig. 9C).The complexly varied abactinal series expressions among species of Petraster suggest variously emergent differentiation of aboral ossicular series.Unlike the stemward Somasteroidea, an unpaired aboral arm midline "carinal" series occurs widely among asteroids, in some genera bordered by generally less clearly differentiated "adradialia" (Figs 2A3, A6; 4A2, A3).Carinal series in taxa with rounded arms might arise as the support equivalent of a ridgepole of a gable roof (Blake & Rozhnov 2007: 526), and therefore carinal and any accompanying adradialia potentially are widely homoplastic.Petraster species exemplify varied carinal and adradial differentiation, and with that, potential homoplasy.
An "aboral ring" (synonym of "primary circlet") and variably developed "centrale" have been recognized among asteroids of all ages.Differentiation from the uniformity of the Somasteroidea could have been gradational, initially limited, and homoplastic.Because of poor preservation, recognition can be difficult, as potentially true of Petraster.The five ossicles of an "aboral ring" might represent the proximal ossicles of the carinal series and therefore a single ossicular group, but alternatively, in Carboniferous Delicaster (Fig. 10A1), the ring is well-defined and no carinal series is apparent, seemingly favoring independent series origins.Sediment compaction around the pentameral mouth frame can perhaps incorrectly argue "aboral ring" presence.
the AxiAl And AdAxiAl series Three fundamental changes can be recognized in the history of the axial/adaxial skeletal configuration of Asteroidea.The first the emergence of the ambulacral furrow.In somasteroids, axials and adaxial virgalia at rest lie in the oral plane, although delicately articulated ossicles suggest presence of a capacity that could yield temporary furrow-like flexure.Three ordinallevel lineages, the Euaxosida, Hadrosida, and Kermasida, are recognized based on extent of displacement of the axial series onto the aboral surfaces of the adaxials (Blake 2018).Axial displacement was accompanied by progressive deepening of the furrow, the latter expressed in the lessening of the at-rest angle between the axial pair across the arm midline.Although extent of these changes appears susceptible to convergence among lineages, a phylogenetic sequencing in an independent and unrelated study (Villier et al. 2018) emphasizing alternative evaluation criteria and directed toward other concerns reached many phylogenetic interpretations in common with the taxonomy of Blake (2018).
Next, among stemward asteroids, the water-vascular system is entirely external, the podia and any associated tissues (e.g.perhaps ampullae) roofed by solid skeletal surfaces, so-called "podial basins".In contrast, so-called "podial pores", passageways for podial tissues between sequential axials, emerged during the middle Paleozoic (Blake & Guensburg 1988) (Fig. 10A2), the generic diversity of occurrences favoring homoplasy.Podial pores are universal within the crown-group (e.g.Fig. 10B1, B2), the enclosure of podial tissues potentially favored for providing protection and freeing space in the furrow.Earlier studies argued presence of podial openings near the lateral margins of the ambulacral furrow, these interpreted as resulting from diagenetic displacement (Blake & Guensburg 1988).
Finally, is positioning of axials and adaxials.In all known stem-group Asteroidea -these as now known exclusively Paleozoic-, each axial directly abuts and is associated with a single adaxial, whereas among all known crown-group asteroids Asteroidea -these as now known exclusively post-Paleozoicaxials and adaxials are offset in a zipper-like arrangement (reviewed, Blake & Guensburg 1988).The configuration is the critical element in separation of stem and crown-group asteroids, a separation not recognized by earlier authors (e.g.Spencer & Wright 1966) but stressed more recently (e.g.Blake 1987;Gale 1987).
The Carboniferous derived (order Kermasida) stem-group species Delicaster enigmaticus (Kesling, 1967) (Fig. 10A2, A3) in comparison with the earlier asterozoans usefully exemplifies important evolutionary changes.Delicaster does not represent an aberrant potentially misleading lineage because it is similar to Permian asteroids from Australia (Kesling 1969) and dissimilar from other later Paleozoic asteroids (e.g.Kesling & Strimple 1966;Blake et al. 2014).Axial ossicles of Delicaster are fully aboral to the adaxials, and, as in crowngroup species, well-developed medial podial pores are present, but the ossicles are "T"-shaped, lacking the wing-like tissue flanges found for example in the crown group Anthenoides Perrier, 1881 (Fig. 10B1, B2), these correlated with the zipper-like axial-adaxial arrangement.Marginal ossicles of most crown-group asteroids are developed in a double series (although only a single series occurs in the Zoroasteridae; Blake 1987;Blake & Elliott 2003), but in a single series in Delicaster (Fig. 10A1-A3).The triangular, fully external axillary (Fig. 10A3) is unlike the nearly universal internal occurrence of the homologous crown group "odontophore".Also unlike the typical crown group expression, no actinals are apparent in Delicaster, the axillary closely fitted against the adjacent MAO, proximal inferomarginals, and adaxials (Fig. 10A3).Arm abactinals are uniform, small and granular, the ridgepole carinal configuration arguably not useful to the flat, strap-like arms of Delicaster.Ossicles of the robust aboral ring (Fig. 10A1) are wholly and clearly differentiated from the small arm abactinals suggesting series differentiation, unlike the ambiguous differentiation between any aboral ring and adjacent abactinals of Petraster.interPreting euAxosidA sP.A Known only from the incomplete fragment of a single specimen exposed in oral aspect, Euaxosida sp.A (Fig. 7A1, A2) is problematic, but like Petraster, it suggests intermediacy in the emergence of morphology typifying derived Asteroidea.Euaxosida sp.A is assigned to the Asteroidea at the ordinal level based on what can be seen of axial form and relationship of the axials to the adaxials.Overall form and abactinal and ambital framework series, insofar as they are known, are broadly somasteroid-like and tentatively interpreted as plesiomorphic.
Early Asteroidea skeletal complexities More problematic is interpretation of the ossicles between the marginals and the adaxials, these of an elongate form and positioning suggestive of somasteroid virgals, but interpreted as asteroid actinals because they are not aligned with the axials, size differences, and irregularity of shape.Crown-group asteroid actinal series alignment among a few species, including Tremaster mirabilis Verrill, 1879 (Fig. 7B1, B2; also see Hess 1981;Smith & Tranter 1985), are reminiscent of somasteroid virgalia, but no other evidence is available that might argue in favor of a more direct linkage between somasteroids and later asteroids.

SUMMARY STATEMENT
Fundamental axial and adaxial skeletal expressions of the Asterozoa originated within the Somasteroidea, and following reduction of adaxial virgal series among the derived lineages, were largely retained through time.This stability is argued as reflecting intimate linkages with the inferred functionally and evolutionarily comparatively stable soft tissues of the water vascular system.Early configurations of extraxial series in contrast are varied and interpreted as emergent and widely homoplastic among later derived asterozoans, the relative plasticity reflecting evolutionary effects of more localized environmental conditions.These interpretations are argued as supported by extraxial complexities of Petraster and Euaxosida sp.A.; available specimens of both, however, are Late Ordovician in age and therefore can only provide inferred guidelines to earlier events.
Because of relative accessibility among both ancient and extant representatives, the extraxial skeleton has received emphasis in phylogenetic reconstruction and taxonomy, although it is the axial and adaxial series that are critical to the evaluation of basal asterozoan events and relationships.

Family
, genus, and species unknown Euaxosida sp.A (Fig. 7A1, A2) mAteriAl exAmined.-UCBL-FSL 712909 (coll.Auvray), the oral surface of about one-half of a single specimen consisting of Early Asteroidea skeletal complexities COMPTES RENDUS PALEVOL • 2024 • 23 (17) two interbrachia, two arms, and a portion of the disk.Only overall ossicular form survives, the surface texture of remaining ossicles altered and pitted after removal of the secondary rust-colored deposits.Radii of two remaining arms approximately 19 mm and 23 mm; two remaining interradii 11 mm and 13 mm, the distended mouth frame indicating flattening with concomitant increases in radii values.locAlity.-Tarafin Signit, Morocco; middle part of the Lower Ktaoua Formation, early Katian.