The fossil record of the family Benthopectinidae (Echinodermata, Asteroidea), a reappraisal

Fossils assigned to the predominantly deep-sea asteroid family Benthopectinidae Verrill, 1894 are described and their affi nities reappraised. Detailed comparative morphology of ambulacrals, adambulacrals and marginal ossicles has revealed that only some extinct taxa fall within the morphological range of the modern representatives of the family. These include Jurapecten hessi Gale, 2011, J. infrajurensis sp. nov. (both Jurassic), J. dhondtae sp. nov. (Upper Cretaceous) and Nearchaster spinosus (Blake, 1973) comb. nov. (Lower Oligocene). A new Late Cretaceous genus, Punkaster gen. nov. (P. spinifera gen. et sp. nov. and P. ruegenensis gen. et sp. nov.), appears to be a highly derived benthopectinid. A possible benthopectinid is described from the Upper Triassic (Carnian) of China. Other described records are distantly related to, but convergent in gross morphology with, benthopectinids. Thus, Plesiastropecten hallovensis Peyer, 1944 is here referred to the Jurassic spinulosidan family Plumasteridae Gale, 2011 and Xandarosaster hessi Blake, 1984 is interpreted as Spinulosida Perrier, 1884 incertae sedis. The mid-Cretaceous Alkaidia sumralli Blake & Reid, 1998 is reassigned to the Forcipulatida (Zorocallina). The “fossil benthopectinid” of Spencer & Wright in Moore (1966) is shown to belong to the goniopectinid genus Chrispaulia Gale, 2005, of which two new Cretaceous species are described, C. wrightorum sp. nov. and C. spinosa sp. nov. Finally, we consider Henricia? venturana Durham & Roberts, 1948 to be an indeterminate asteroid.


Material and methods
The present study is based on fossil material contained in several North American and European museums (see list of institutional abbreviations below). Comparative Recent material of several taxa housed in a few institutional collections (Table 1) has also been used, as have partially macerated (bleach) specimens in the A.S. Gale Collection; the latter have not been formally registered. Specimens used for SEM examination were gold-palladium coated and imaged in a Jeol SEM.

Morphological characteristics of extant benthopectinids
In basic construction, benthopectinids are similar to other Paxillosida Perrier, 1884 in possessing relatively elongated, tapering arms, with acutely angled to rounded interradii and conspicuous, spinebearing marginals that frame disc and arms and a fl at abactinal surface that is made of small ossicles set in a fl exible integument (Figs 1-2). Indeed, astropectinid taxa such as Dytaster Sladen, 1889, which bear single spines on each marginal ossicle and in which intermarginal fascioles are poorly developed, are superfi cially similar to benthopectinid genera like Pontaster. More detailed inspection of both the external and internal morphology, however, reveals that all extant benthopectinids share numerous distinctive characters; these are discussed below. Whether or not the presence of these justifi es the elevation of the family to an order (i.e., Notomyotida Ludwig, 1910) has been the source of controversy amongst taxonomists. In the phylogeny of Gale (2011a), benthopectinids belong to the Paxillosida, sister group to the other four families making up the group. Molecular studies suggest that benthopectinids are sister taxon to the Pseudarchasteridae, and belong to a clade which includes all Paxillosida (Mah & Foltz 2011: fi g. 1).
Distinctive features of benthopectinids include the following: • The presence of longitudinal muscles in the arms, which attach to specialised ridges on the ambulacrals (abr; see Fig. 3I), the dorsal body wall and the insides of the inferomarginals (Clark 1981).
• Papulae (specialised respiratory structures) are restricted to specifi c regions, called papularia, at the base of the arms on the abactinal surface. These are oval, or V-shaped, stand proud of the surface and are made up of distinctively thickened, triangular abactinal ossicles between which the papulae extrude (Fig. 1A).
• The presence of distinctive comb-like, multivalved pectinate (attached to more than one ossicle) and fasciculate (attached to a single ossicle) pedicellariae, in the form of a double comb of two opposing rows of valves (Fisher 1911;Jangoux & Lambert 1988). These bear a distinctive sculpture on the exterior of the valves. Pontaster has bivalved pedicellariae only (Clarke & Downey 1992). The precise homology with pedicellariae in other asteroid groups is uncertain.
• The articulation between successive inferomarginals is specialised. A fl ange is present on the proximal actinolateral surface of each inferomarginal, which carries a strip of smooth imperforate stereom for articulation with the distal part of the adjacent ossicle (Figs 3M, 4W).
• The ambulacrals are hourglass-shaped, with equally expanded triangular head and base ( Fig. 5I-M). The ambulacral heads of successive ambulacrals abut, and do not imbricate.
• The ambulacral-adambulacral contact is highly modifi ed (Blake 1973;Gale 2011a) in comparison to other paxillosidans ( Fig. 3A-B). Articulation ada3 is elongated and positioned on the abradial margins of ambulacrals and adambulacrals (Fig. 3B, F). Adada and ada2 are set on a raised ridge on the adambulacrals and the ada2 articulation surface is present on a ridge on the ambulacral base (Fig. 3F). The central part of the ambulacral base adjacent is concave and sites of attachment of the ambulacraladambulacral muscles (padam, dadam) are strongly asymmetrical, with a reduced padam (Fig. 3F). The muscle attachment site padam is carried on short, wing-like fl anges on the ambulacral bases. No other asteroids have this complex and highly derived ambulacral-adambulacral contact (Gale 2011a).
• The adambulacral construction and spination. The adradial margin of the ambulacrals projects into the groove, with an angled or rounded margin which carries a row of numerous adambulacral furrow spines. The central portion of the actinal adambulacral surface bears one or two large subadambulacral spines.
• The morphology and spination of the oral ossicles (Fig. 3C, J-K). The oral ossicles are trapezoidal in outline, with a convex actinal margin which carries a fringe of suboral spines. The inner face of the oral ossicles has dentition on the actinal border only.
• The presence of parapaxillae: distinctively modifi ed abactinal ossicles. Although these are diverse in form, they have a common structure -a broad fl at, rounded base, with a bevelled rim, and a raised central region which carries a centrally placed spine base, commonly surrounded by smaller spine bases ( Fig. 4X-Z). Parapaxillae are unique to benthopectinids.
• Enlarged abactinal and marginal spines, when present, have a distinctive construction, being cylindrical, tapering, with a concavity on the base for articulation with the underlying ossicle. They are made up of thorny stereom similar in form to the verticillate structure in echinoid spines.
• The odontophore is shield-shaped and fl attened, and the surfaces articulating with the oral ossicles extend along the length of the plate. This is unlike the odontophore of any other asteroids (Gale 2011a). The most recent revision of benthopectinids at the generic level was made by Clark (1981) and the Atlantic species of the family were thoroughly revised by Clark & Downey (1992; see also Mah 2020a). There has been a long debate about both the number of extant benthopectinid genera, and the assignation of species to these genera. Clark (1981) summarised the taxonomic history of the family and provided a tabular key to benthopectinid taxa, of which she recognised eight genera and four subgenera. In the present study, additional characters were found to be of taxonomic value, most especially the shape, sculpture and articulation of the marginal ossicles.

Morphological diversity of extant benthopectinids
The morphology of fi ve living benthopectinids is illustrated here with photographs of entire, dried specimens of Pontaster tenuispinus (Duben & Koren, 1846), Cheiraster gazellae Studer, 1883 ( Fig. 1), Pectinaster fi lholi Perrier, 1885, Benthopecten simplex (Perrier, 1881) and Nearchaster aciculosus (Fisher, 1910) (Fig. 2). The most conspicuous differences are the variable development of marginal spines; short and small in P. tenuispinus, larger and longer in the other taxa. Nearchaster aciculosus has elongated abactinal spines. The shape of the denuded marginal plates ( The skeletal morphology of fi ve species of extant benthopectinids (Table 1) has also been examined, using bleach preparation and SEM examination of ossicle types (Figs 3-5), in order to make comparisons with fossil taxa.

Species Collection Locality
Pectinaster fi

Phylogeny of the Benthopectinidae Verrill, 1894
The fossil material described in the present paper provides evidence of the timing and order of appearance of evolutionary novelties within the family. Firstly, the highly derived ambulacral-adambulacral articulation (see above) is ubiquitously present in all genera, and had evolved by the Lower Jurassic. Secondly, the abactinal ridges on the ambulacrals, and the presence of an articulation between the ambulacral base and the inferomarginals is a feature restricted to the crown group benthopectinids (all living taxa, plus their common ancestor) which did not appear until the Oligocene. The Mesozoic representatives of the family represent the stem group.

Material examined
UCMP A-5018 (holotype no. 10675) is the type and only specimen; it exposes a partly disarticulated abactinal surface showing the disc and proximal portions of four arms. Large marginal spines and smaller abactinal ones are visible. Marginals, adambulacrals, ambulacrals and abactinal ossicles of the holotype were fi gured individually by Blake (1973).

Description
The abactinal surface of UCMP A-5018, embedded in matrix (Blake 1973: pl.16 fi g. 44), shows part of the disc and four proximal arms. Although the outline is retained, the ossicles are jumbled and largely dissociated, such that adambulacrals and ambulacrals are visible on the actinal surface. The marginal spines, largely in place, are elongated and tapering. The abactinal spines are much smaller, perhaps one-fi fth the size of those on the marginals. The adambulacrals are well preserved, subrectangular, with 2-3 large subadambulacral spine bases, and the concave inner (abactinal) surface and ridge bearing ada2 and ada3, characteristic of benthopectinids. The ambulacrals have the typical hourglass shape of benthopectinids, and asymmetry of the interambulacral muscles (P1 small, P2 large) is seen. The marginals are longer than broad with a convex, mound-like outer surface which carries 1-2 large spine bases and a number of sparsely scattered smaller ones. The inner surface of the marginals is fl at. The abactinal ossicles are parapaxillae, with centrally placed, single spine bases, surrounded by a ring of smaller spines.

Remarks
As recognised by Blake (1973), the distinctive characters of the ambulacral, adambulacral and marginal ossicles place this form fi rmly in the Benthopectinidae. Comparison with extant benthopectinid species studied here indicates that Mistia spinosa shares important characters with the Recent Pacifi c genus Nearchaster, including the following: 1. Adambulacrals are nearly identical in shape to those of Nearchaster aciculosus, and both carry 2-3 bases for subambulacral spines.
2. Marginals are closely similar to those of N. aciculosus in both shape and distribution of spine bases.
The overall form of the body, with large marginal spines, and shorter abactinal spines on the disc is broadly similar to the development in the genera Benthopecten, Nearchaster and Myonotus Fisher, 1911(see Fisher 1911. The proportionate sizes and distributions of spines in Mistia spinosa are closest to those in Benthopecten claviger Fisher, 1910, Myonotus intermedius (Fisher, 1910) and Nearchaster aciculosus (Fisher, 1910) (see Fig. 2E-F herein).
The abactinal parapaxillae of Mistia spinosa are very close in structure to those of N. aciculosus, with a central spine base surrounded by a ring of smaller ones.
In conclusion, Mistia spinosa is a benthopectinid which has remarkably detailed similarities of ossicle morphology to the present-day Pacifi c species Nearchaster aciculosus and it is therefore provisionally placed in that genus. The genus thus has a history in the Pacifi c Ocean of at least 33 million years.

Assigned species
In addition to the type species, J. infrajurensis sp. nov. and J. dhondtae sp. nov., both described below.

Remarks
Jurassic-Cretaceous benthopectinids are locally common among isolated ossicles in washed residues. All share the same distinctive sculpture type of prominent imperforate rugosities, conjoined by radiating strips of stereom (e.g., Figs 6N-O, 7G, I-J), absent on extant genera. In the extant genera Pontaster and Cheiraster, the rugosities on the superomarginals are smaller and more widely spaced (e.g., Jurapecten hessi -Gale 2011b: 76, fi g. 6a-h.

Diagnosis
Jurapecten in which the marginals bear a sculpture of discrete, rounded rugosities and 1-3 larger spine bases on inferomarginals. Ambulacral base short.

Material examined
The type specimen, NHMUK EE 13594, consists of a set of associated ossicles from the upper Oxfordian (Couches d'Effi ngen, bifurcatus ammonite Zone, stenocycloides ammonite Subzone) at Savigna, near Orgelet (Département du Jura, France; see Gale 2011aGale , 2011b. Additional material comprises several hundred ossicles and groups of associated ossicles from the type locality (NHMUK collections).

Remarks
There is little new information or material since the original description by Gale (2011aGale ( , 2011b; differences with J. infrajurensis sp. nov. are discussed under that species (see below).

Diagnosis
Jurapecten in which the ambulacral base broadens abradially; sculpture of superomarginals comprises a reticulum of conjoined rugosities.

Etymology
From the Latin 'infra', in allusion to the occurrence of the species in the Lower Jurassic.

Remarks
Jurapecten infrajurensis sp. nov. differs from J. hessi (see above) in the more elongated ambulacral base, and the more coarsely rugose sculpture of both infero-and superomarginals, which lack enlarged spine pits.

Diagnosis
Jurapecten in which the marginal ossicles possess a coarsely rugose sculpture and the inferomarginals bear a single, large, laterally directed spine base.

Etymology
Named after Annie V. Dhondt , specialist of Cretaceous bivalves and close friend.

Material examined
The inferomarginal illustrated here ( The partially preserved specimen (NHMM MD 4105), described and illustrated by Blake & Jagt (2005), may be conspecifi c, but recrystallisation of all ossicle types precludes detailed comparison of the sculpture of infero-and superomarginal ossicles and ambulacral ossicles are too poorly preserved. The type material of J. dhondtae sp. nov., from the middle Emael Member, is ca 200 000 years younger than NHMM MD 4105, from the basal Gronsveld Member (compare Keutgen 2018).

Diagnosis
Highly derived form in which the marginal ossicles are very elongated, proximal infero-and superomarginal pairs occasionally fused, and marginals may possess 1-2 large rounded bases for attachment of conical spines. Adambulacrals with deep rounded notch to allow extension of tube feet and very large furrow spines.

Etymology
From the similarity of the marginal spination to the 1980s punk hairdos.

Assigned species
In addition to the type species, P. ruegenensis gen. et sp. nov. (see below).

Remarks
The highly unusual marginal ossicles of this new genus (Fig. 9E, I-J, M, P) have been known for over 60 years, but they remained undescribed and it has hitherto not been possible to assign them to any family. The new material includes ambulacral and adambulacral ossicles, which demonstrate a likely affi nity with benthopectinids in the elongated, bar-like ambulacral heads and, especially, in the nature of the ambulacral/adambulacral articulation. This is typically benthopectinid, in the abradial position of ada3, the position of padam on a short wing-like process and the presence of ada2 on a steep ridge. The abactinal ossicles are closely similar to parapaxillae of modern benthopectinids. The presence of an abactinal ridge on the ambulacrals of P. spinifera gen. et sp. nov. may indicate that the species possessed longitudinal muscles in the arms. The genus perhaps represents a highly specialised offshoot from the mainline benthopectinids.

Diagnosis
Punkaster gen. nov. in which the marginal ossicles carry 1-2 cylindrical spine bases, which bore conical spines.

Description
Marginals are highly distinctive plates; supero-and inferomarginals elongated, broadly rectangular in lateral-abactinal aspect, with a length-parallel central rounded ridge, single oninferomarginals, bifurcating towards distal and proximal margins of superomarginals. Central, raised part of ridge carrying 2-3 spine bases, borne on cylindrical protrusions projecting from surface of ossicles. Superomarginals (Fig. 9E, I, P) with thin, narrow abactinal portion and opposing inferomarginals. Inferomarginals (Fig. 9J, M) with distinctive facet on internal surface for contact with adambulacrals. External surface of supero-and inferomarginals with complex sculpture comprising numerous small, rounded rugosities interconnected by network of stereom. Marginal spines (Fig. 10D-E) conical, thorny.
Abactinal ossicles fl attened parapaxillae, with irregular lobed outlines and bevelled rim ( Fig. 9F-G, S). Raised central area with fi nely rugose stereom which probably carried small spines. Some abactinals with single, outwardly directed spine base.

Remarks
Punkaster spinifer gen. et sp. nov. differs from P. ruegenensis gen. et sp. nov. (see below) in the unfused proximal supero-and inferomarginals and in the presence of large marginal spine bases. As seen in our reconstruction (Fig. 11), the marginals were paired, not alternating, and carried a transverse fan of 4-6 outwardly directed, large conical thorny spines (Fig. 11A). The species had very large tube feet, protected by large, lanceolate furrow spines (Fig. 11B).

Diagnosis
Punkaster gen. nov. in which the proximal supero-and inferomarginals form fused pairs; distally, they are separate. Large marginal spine bases absent.

Etymology
Named after the island of Rügen (northeast Germany) in the Baltic Sea.

Remarks
The marginal ossicles are most unusual in that supero-and inferomarginal pairs are fused, perhaps uniquely amongst asteroids. The overall shape and sculpture of the ossicles suggest affi nity with the type species, Punkaster spinifera gen. et sp. nov. (see above), but the present form differs in the fused proximal marginals and in the absence of large spine bases. The inferomarginals share a number of similarities to those of benthopectinids ( Fig. 6A-E), notably the coarse, longitudinally arranged rows of rugosities, the presence of a large spine (or pedicellaria) base close to the abactinal margin of the plate, and the marked asymmetry of the distal inferomarginals, in which the distal height is less than the proximal one. The oral ossicles (Fig. 6F) are remarkably similar to those of benthopectinids in the evenly convex actinal margin, the distally angled apophyse and the shape of the adambulacral articulation ( Fig. 6G-I). However, the material available to date is too limited, and in the absence of ambulacrals and adambulacrals, this referral is very tentative.

Diagnosis
Arms long, narrow; disc small, with rounded interbrachial arcs; superomarginals meeting over radius along length of arm; distal marginals imbricate (emended from Gale 2005).

Remarks
The record in the Treatise of Invertebrate Paleontology (Part U; Spencer & Wright 1966) of a benthopectinid from the "Albian of England" is based on an arm fragment from the Albian Red Chalk of Yorkshire, United Kingdom (C.W. Wright, pers. comm. to ASG, 1978). We have examined this specimen (NHMUK PI EE 17997), which comprises fi ve marginal ossicles reconstructed on plasticene. The shape of the ossicles indicates that this specimen belongs to the genus Chrispaulia; it is here described as a new species, C. wrightorum sp. nov. (see below). In addition, we record another species from the Hauterivian (Lower Cretaceous) of northeast England and northern Germany.

Diagnosis
Chrispaulia with smooth marginal ossicles, in which the distal superomarginals possess a single large spine pit close to their distal, abactinolateral border.

Etymology
After the late brothers C.W. and E.V. Wright, who collected the specimen.

Material examined
The arm fragment (NHMUK PI EE 17997) from the Albian Red Chalk (Hunstanton Formation) at Speeton, Yorkshire (United Kingdom) is the holotype and single specimen known to date. The specimen was originally articulated; it was subsequently reconstructed on plasticene by the Wright brothers.

Remarks
Chrispaulia wrightorum sp. nov. differs from all congeners in the smooth marginal ossicles, lacking rugosities.

Diagnosis
Chrispaulia in which distal inferomarginals bear an oblique row of large, bifi d spine attachment sites, which carried long, fl attened spines.

Etymology
Latin for 'bearing spines', in reference to the row of spine attachment sites on distal inferomarginals.

Remarks
Chrispaulia spinosa sp. nov. differs from its congeners in its possession of 3-4 bifi d spine pits on distal inferomarginals.

Diagnosis
Five-to multi-armed asteroids possessing broad, short concavo-convex adambulacrals which possess a single row of large, transverse spine pits; adambulacrals articulate by means of transversely arranged specialised surfaces; abactinal ossicles stellate, large, with a central boss with which a long, glassy spine articulates.

Remarks
Although Plesiastropecten is 5-rayed, the adambulacrals, abactinal ossicles and marginals are closely similar to those of Plumaster (see Gale 2011b) and the genus is accommodated here.

Diagnosis
As for genus.

Material examined
The holotype (MZA L13a) is a complete asteroid in a claystone matrix, comprising part and counterpart, from the Lower Jurassic (Hettangian, liasicus ammonite Zone) of Hallau, Canton Schaffhausen, Switzerland (Peyer 1944). It is contained in the collections of the Museum zu Aller Heiligen, Schaffhausen, and has suffered from considerable over-preparation. Additional material includes a suite of specimens from the same locality and horizon, most notably, specimen MZA L 13b/32a-e, which is a partially dissociated individual of which one arm is well preserved (Blake 1984: fi g. 1a-g). This specimen is coated with a glue-like substance which obscures much detail and requires specialist preparation. The present description is based on these two individuals.

Description
The overall form is well shown by the type specimen (Fig. 14A); the arms are moderately long and tapering, the interbrachial angles acute (R:r = approximately 6:1). The arms bear an even fringe of closely spaced marginal spines. The marginals are numerous, narrow and short with a three-tiered structure, resembling tiny 'cottage loaves' of bread (Fig. 14F). They are clearly visible in MZA L 13b/32a-e, and are robustly paxilliform, bearing a single large spine pit centrally. The abactinal ossicles are conspicuous and relatively large, convex to fl at and carry four to six, lobe-like lateral projections. Each abactinal has a single, large and centrally placed crater-like spine pit (Fig. 14B, D). The ossicles imbricate, and the lobes are notched on their inside (actinal) surfaces for contact with adjacent ossicles. These ossicles are more or less radially symmetrical in the disc, but elongated in the arm, where the abactinal ossicles are arranged in three rows comprising radials and two adradials (Fig. 14B). In the distal arm, the large radial and adradials are separated from adjacent ossicles of the same row by smaller inset ossicles. The adambulacrals were prepared in a small part of specimen MZA L 13b/32b. They are very broad and short (3:1), and carry 5-6 large, horseshoe-shaped spine bases arranged in a single transverse row (Fig. 14E). The adambulacrals of opposing rows are slightly angled (150°) in a proximal direction. The ambulacrals are only seen in abactinal view, and the ambh forms a conspicuous, elongated triangular proximal wing which overlaps the more proximal adjacent ambulacral (Fig. 14D). The ambb is oval and symmetrical. The marginal spines are conspicuous, forming an even comb-like fringe to the starfi sh. Each marginal plate carries a single tapering spine with a unique construction. The cross section is a shallow U-shape, and the abactinal surface bears a groove. The rounded actinal surface is made up of 4-6, length-parallel coalescing rods of trabecular stereom. The lateral margins of the spines carry outwardly directed short barbs, probably lost on the holotype through over-preparation. The base of the spine is swollen and rounded, and a simple socket on the base articulates with a boss on the marginal. The abactinal spines are shorter, and round in cross section; these are also composed of elongated trabeculae. The 5-6 adambulacral spines articulate with horseshoe-shaped bases, and are long and gently curved.

Remarks
The robust paxilliform construction of the marginals is not found in any benthopectinid asteroid, but is characteristic of modern solasterids and the Jurassic genus Plumaster, and each marginal carries only a single spine. We cannot agree with Blake (1984) that these resemble marginals of benthopectinids, other than in that they carry large spines. The large, oval or rounded, imbricating abactinal ossicles which are alternately large and spine-bearing and small and spineless in the arm are quite different from the parapaxillae of benthopectinids. The construction of the marginal and abactinal spines, with elongated trabeculae running along the length of the spines and a semicircular, concavo-convex cross section bearing two rows of lateral thorns, are dissimilar to benthopectinid spines, which are conical, cylindrical and carry numerous, irregular, distally directed thorns. The concavo-convex construction is otherwise seen only in the multi-armed Early Jurassic Plumaster ophiuroides (Wright, 1863). The transversely broad, short adambulacrals, carrying 5-6 large curved adambulacral spines, are quite unlike adambulacrals of benthopectinids which are narrow and rather elongated, but are similar to those of Plumaster (Gale 2011b: fi gs 13-14). The elongated, imbricating proximal fl anges of the ambulacral heads are not found in any benthopectinid, where the ambulacral heads are short, upright and do not imbricate, as in all paxillosidans (Gale 2011a), but are similar to those of Plumaster (Gale 2011b: fi g. 14b-c). Other than a superfi cial similarity in shape and the presence of elongated marginal and abactinal spines (also found in other asteroids), Plesiastropecten hallovensis does not show any of the characteristics of the Benthopectinidae, but bears a close similarity to the Pliensbachian-Aalenian multiarmed genus Plumaster, with which it shares broad, short adambulacrals with 5-6 large transversely arranged, hyaline spines and the fl anged, imbricating abactinal ossicles (Gale 2011b).

Material examined
The single specimen available (NMB M9683) comprises fragments of three arms and dissociated ossicles, intimately entwined with an isocrinid crinoid on a single slab, from the Bajocian of Reigoldswil (Canton Baselland, Switzerland). The specimen is very fragmentary and is not easy to interpret, because pressure solution has welded scraps of calcite onto most ossicles, making discrimination of features diffi cult.

Description
The ossicles of the ambulacral groove are the best preserved and most distinctive part of this specimen (Fig. 15A). The ambh imbricate strongly proximally, and are elongated and triangular. The ambulacrals are waisted, and the ambb carry short asymmetrical fl anges for padam and dadam, and articulation surfaces ada1 and ada2. The adambulacrals are rectangular and very elongated, being approximately three times longer than broad; half to two-thirds of the actinal face is occupied by a V-shaped depression for the adadm (Fig. 15B-C). An oblique ridge runs from the proximal part of the V to the distal abradial margin. Proximal adambs carry two large swollen spine bases set obliquely on the proximal face of the ossicle, and more distal adamb have a single spine base.
Several oral ossicles are visible. These are very broad and gently convex on the external surface, and an adambulacral articulation ridge and deep V-shaped concavity for the oradm muscle are present. The abactinal ossicles are of even, small size, have a lobed base and carry a large centrally placed, rounded spine boss. Possible marginals are elongated-rectangular and imbricate distally, with a very large round spine base on the proximal part of the external face. The abactinal and marginal spines are conical and moderately long, and have swollen bases. They are made up of elongated trabeculae of stereom.

Remarks
The ambulacral/adambulacral articulation of X. hessi is utterly dissimilar to that of benthopectinids. The ambh is broad and short in X. hessi, with a strong, short transverse actinal ridge. In benthopectinids, the ambb is triangular and elongated. Xandarosaster hessi has short, oval and nearly symmetrical facets for padam and dadam, which are triangular and strongly asymmetrical in benthopectinids. In X. hessi ada2 and adada are fused and ada3 is absent. In benthopectinids, all three facets are discrete, and ada2 and adada are placed on a ridge adjacent to a concavity on both ambulacrals and adambulacrals.
The construction of the spines, with elongated hyaline trabeculae, is unlike that of benthopectinids in which the spines are made of thorny stereom with distally directed barbs. Moreover, the elongated, strongly imbricating, asymmetrical ambh are not present in benthopectinids, where the heads are short and do not imbricate.
In summary, X. hessi does not possess a single character of the Benthopectinidae, but has features unique to the Spinulosida (sensu Gale 2011a), including spines constructed of elongated hyaline trabeculae and elongated, triangular proximal ambh which strongly imbricate proximally. Its affi nity with other spinulosidans is uncertain, although some aspects of the ambulacrals and adambulacrals are broadly comparable with those of solasterids. The very elongated rectangular adambulacrals are unique to X. hessi. Adambulacrals of this type, currently indeterminate (Fig. 15D), are also found uncommonly in Jurassic sedimentary rocks such as the lower Oxfordian of Andelot-Morval, France.

Diagnosis
Zorocallinids with extra-axial arm constructed of seven rows of ossicles organised with one row of radials, two rows of adradials, two rows of superomarginals and two rows of infromarginals; both marginal rows extend to arm tip.

Remarks
The family Terminasteridae is sister group to the Eocene-Recent Zoroasteridae Sladen, 1889, which is widespread in bathyal to abyssal depths of the present-day oceans.

Diagnosis
Terminasteridae with elongated rhombic superomarginals directed obliquely towards radials; inferomarginals with tall, distally swollen central spine bases and prominent groove for spine attachment; radials (except primary radial) rhombic in outline.

Assigned species
In addition to the type species, A. megaungula Ewin & Gale, 2020.

Diagnosis
Alkaidia in which the primary radial ossicles are elongated and the terminal ossicle is not deeply notched on its proximal margin.

Material examined
The holotype (Texas Memorial Museum, number 1786 TX1) is from the Grayson Formation (lower Cenomanian) at the Waco shale pit (Waco, Mclennan County, Texas, USA). Additional material comprises a magnifi cent individual (NHMUK PI EE 15225), collected by Frank Holterhoff from the Grayson Formation (lower Cenomanian) of Dottie Lynn, Fort Worth, Texas and illustrated here ( Fig. 16A-C), as well as numerous dissociated ossicles from the same locality (NHMUK PI EE 18005-18007, 18009).

Remarks
The affi nities of A. sumralli have recently been discussed in some detail by Ewin & Gale (2020) and the evidence for its inclusion in the Forcipulatida (Zorocallina) and the family Terminasteridae can be summarised briefl y as follows: the presence of abundant, straight 'duck-billed' forcipulate pedicellariae is a characteristic of the Zorocallina (Fig. 16D); the construction of the abactinal surface is closely   similar to that of zoroasterids and terminasterids, which also have large, Y-shaped fi rst superomarginals and a row of robust, lobed, quadrangular radial ossicles which imbricate proximally and each carry a centrally placed conical spine (Ewin & Gale 2020).
Additionally, the morphology of the adambulacrals and ambulacrals, and the nature of their articulation is similar in zoroasterids, Terminaster and Alkaidia (Fig. 17). Ada1a and ada2 are concave on the adamulacrals (Fig. 17A-C) and positioned on a process on the ambulacrals (Fig. 17G-H, J-K).
The dadam and padam facets are subequal in size, broad and short ( Fig. 17A-C).
The abactinal construction is never seen in extant benthopectinids, in which the abactinal ossicles in the arms are small and parapaxilliform or very small, and never imbricate. Additionally, in benthopectinids the abactinal surface is invariably fl at, and the arm section is not subcylindrical.

Remarks
The unique holotype, from the Upper Cretaceous Chico Formation of the North Fork of Matilija Creek, Ventura County (California, USA), is contained in the palaeontological collections of the University of California. Part (number 4866B) and counterpart (4866A) show the abactinal and actinal surfaces, respectively.
The type specimen is an external mould of a near-complete asteroid with moderately long, narrow arms and a small disc, which has lost the abactinal ossicles and exposes the ambulacrals on the abactinal surface. Unfortunately, parts of the moulds have been worn by erosion and few details are well preserved. The ambulacral heads are elongated, and the adambulacral ossicles are clearly visible, as are probable small marginal ossicles bearing spines. The groove is very wide. Although Blake (1984) assigned the taxon to the Benthopectinidae, there does not seem to be any compelling evidence for its taxonomic affi nity, and it is considered here to be an indeterminate asteroid.

Discussion
The hypothesis that Mesozoic asteroids (Table 2) which bear conspicuous, elongated marginal and abactinal spines are members of, or closely related to, the extant deep-sea family Benthopectinidae is tested, using comparative morphology of diverse ossicle types. It is concluded that all three members of the subfamily Paleobenthopectininae Blake, 1984 are convergent in gross morphology with certain extant benthopectinids and should be correctly assigned to distantly related groups (i.e., Spinulosida, Plumasteridae; Forcipulatida, Terminasteridae). The other fossil benthopectinid cited in the literature (Spencer & Wright, 1966) is here assigned to a new species of the goniopectinid genus Chrispaulia.
In the present study, we demonstrate the taxonomic value of characters ubiquitously present in extant benthopectinids. In particular, the ambulacral/adambulacral morphology is of value, specifi cally the nature of the articulation between the ambulacrals and adambulacrals; this provides an excellent criterion for correct recognition of fossil benthopectinids (see also Gale 2011a). The distinctive benthopectinid ambulacral/adambulacral articulation appeared early in the Jurassic (Toarcian) and is retained in all members of the family to the present day. Possible benthopectinid ossicles are already present in the Triassic (see above), but in the absence of ossicles of the ambulacral groove and mouth frame this identifi cation is uncertain.
The Benthopectinidae is represented in the Jurassic and Cretaceous (Toarcian-Maastrichtian) by the genus Jurapecten, which resembles the extant Pontaster in the morphology of the marginal plates and spines attached to these (squat, block-like; spines short), but in which the ambulacrals lack a raised transverse abactinal ridge; they did not articulate with the inferomarginals, as is the case in all extant genera. The absence of the abactinal ridge can be taken as evidence that Jurapecten lacked longitudinal muscles in the arm. The new genus Punkaster gen. nov., from the Upper Cretaceous, appears to be a highly derived benthopectinid, in which the marginals are elongated, sometimes in fused interradial pairs, and carried large conical spines. The main evidence of assignation of Punkaster gen. nov. to the family is the nature of the ambulacral/adambulacral articulation. The oldest representative of extant benthopectinid genera is Nearchaster (formerly Mistia) spinosus comb. nov. from the Lower Oligocene of Oregon, USA (Blake 1973).
The transition between the plesiomorphic Mesozoic-type benthopectinid morphologies (lack of abactinal ridges on ambulacrals, ambulacrals not articulating with the inferomarginals) and derived Cenozoic types in which the abactinal ridges supported longitudinal muscles, is poorly known, but the oldest Cenozoic fossil benthopectinid (Nearchaster spinosus comb. nov.) is present in the Early Oligocene. It is therefore likely that the radiation of the family took place during the early Paleogene. The presence of a number of derived characters in all living and Cenozoic benthopectinid taxa supports their monophyly. However, the very fragmentary nature of the benthopectinid fossil record (mostly isolated ossicles) does not lend itself to cladistic analysis of the group.