Brittle stars from the British Oxford Clay: unexpected ophiuroid diversity on Jurassic sublittoral mud bottoms

Timothy A.M. Ewin; Ben Thuy

doi:10.1017/jpa.2016.162

Brittle stars from the British Oxford Clay: unexpected ophiuroid diversity on Jurassic sublittoral mud bottoms

Published online by Cambridge University Press: 03 April 2017

Timothy A.M. Ewin and

Ben Thuy

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Timothy A.M. Ewin: Affiliation:
Department for Earth Sciences, The Natural History Museum, Cromwell Road, South Kensington, London, UK SW7 5BD, UK 〈t.ewin@nhm.ac.uk〉
Ben Thuy: Affiliation:
Natural History Museum Luxembourg, Department of Palaeontology, 24 rue Münster, L-2160 Luxembourg City, Luxembourg 〈bthuy@mnhn.lu〉

Article contents

Abstract
Introduction
Materials and methods
Oxford Clay ophiuroid localities, stratigraphy, geology, and taphonomy
Systematic paleontology
Paleoecological considerations
References

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Abstract

Three new ophiuran species, Enakomusium whymanae n. sp., Aspidophiura? seren n. sp., and Ophiotitanos smithi n. sp., and an unnamed specimen assignable to the genus Dermocoma are described from the Callovian to Oxfordian Oxford Clay Formation of Great Britain. These determinations are based on new finds and a critical reassessment of historic specimens. The Oxford Clay ophiuroids represent two loose assemblages, one from the middle Callovian Peterborough Member and the other from the lower Oxfordian Weymouth Member. Both assemblages accord well with coeval midshelf mud bottom ophiuroid communities in terms of taxonomic composition and relative abundance of taxa. The British Oxford Clay ophiuroids are particularly significant as they are one of the rare instances where multiple species are represented, almost exclusively, by exceptionally preserved articulated skeletons. This provides an important window into the understanding of mid-Upper Jurassic ophiuroid paleobiology.

Information

Type: Articles
Information: Journal of Paleontology , Volume 91 , Special Issue 4: Progress in Echinoderm Paleobiology , July 2017, pp. 781 - 798
DOI: https://doi.org/10.1017/jpa.2016.162 [Opens in a new window]
Copyright: Copyright © 2017, The Paleontological Society

Introduction

Ophiuroid paleontology has undergone a remarkable revolution in the past few years. For many decades, most finds of fossil ophiuroids were dealt with as anecdotal oddities, worlds apart from the significance the group holds in modern marine environments (e.g., Stöhr et al., Reference Stöhr, O’Hara and Thuy2012). Until recently, comparatively few studies attempted a more systematic assessment of fossil ophiuroids, notable examples being Hess (Reference Hess1975) and references therein, and Kutscher and Jagt (Reference Kutscher and Jagt2000). Driven by recent advances in the morphological interpretation of the ophiuroid skeleton, particularly with respect to microscopical features pertaining to the spine-bearing lateral arm plates (Martynov, Reference Martynov2010; Thuy and Stöhr, Reference Thuy and Stöhr2011), efforts have been made to explore the ophiuroid fossil record from a much broader perspective. Now that fossil ophiuroids are better understood as taxonomically assessable testimonies of the evolutionary history of a diverse, widespread, and ecologically significant group of extant marine invertebrates, their systematic study has gained new momentum.

Dissociated lateral arm plates preserved as microfossils were recently identified as the most promising source of evidence to systematically assess the ophiuroid fossil record (Thuy and Stöhr, Reference Thuy and Stöhr2011; Thuy et al., Reference Thuy, Gale, Kroh, Kucera, Numberger-Thuy, Reich and Stöhr2012; O’Hara et al., Reference O’Hara, Hugall, Thuy and Moussalli2014). They can be very abundant and represent a time-averaged, taphonomically unbiased record of the ophiuroid assemblages at a given time and are therefore the most likely candidates to reflect the true ophiuroid paleobiodiversity as exhaustively as possible (Thuy, Reference Thuy2013, Reference Thuy2015). Fossils of articulated ophiuroid skeletons, by contrast, are rare finds bound to exceptional depositional conditions and therefore provide highly patchy glimpses into the fossil record of the group.

The fossil ophiuroids collected from the British Oxford Clay over the past 170 years are, however, a remarkable exception to this general pattern. In fact, the specimens known to date almost exclusively consist of articulated skeletons and, indeed, provide insights on the ophiuroid faunal spectrum otherwise only expected from dissociated lateral arm plates. The current state of knowledge on the Oxford Clay ophiuroids is limited to the original descriptions of Amphiura pratti Forbes, Reference Forbes1844 and Ophioderma weymouthiense Damon, Reference Damon1880, later revised by Hess (Reference Hess1964), of ophiuroid occurrences (Wright, Reference Wright1880; Martill and Hudson, Reference Martill and Hudson1991) and a preliminary report on a new species (Ewin and Thuy, Reference Ewin and Thuy2015).

The post-Paleozoic ophiuroid fossil record has received increasing attention over the past few decades, beginning with the pioneering work of Hans Hess (summarized by Hess, Reference Hess1975). With few exceptions, however, most studies focused on taxonomically assessing post-Paleozoic ophiuroid paleobiodiversity, given the huge gaps still existing (e.g., Thuy, Reference Thuy2013) and hampering a comprehensive fossil-based exploration of the ophiuroid evolutionary history. Here, we provide an exhaustive systematic assessment of the ophiuroid material currently known from the British Oxford Clay Formation, including both historical finds and new discoveries, and attempt to put the studied material in a broader context in terms of paleobiology and systematics.

Materials and methods

The material studied comprises the holotype of Amphiura pratti and the additional conspecific specimen illustrated by Wright (Reference Wright1880), the neotype of Ophioderma weymouthiense and the associated specimens from the same locality (Hess, Reference Hess1964), a previously undescribed single specimen collected ex situ from the Fleet, near Weymouth, Dorset, an undetermined ophiuroid figured by Martill and Hudson (Reference Martill and Hudson1991), and the recently collected lot of specimens preliminarily described by Ewin and Thuy (Reference Ewin and Thuy2015).

Almost all material consists of articulated skeletons or parts of skeletons collected as macrofossils. The series of specimens described by Ewin and Thuy (Reference Ewin and Thuy2015) furthermore includes numerous articulated parts of disks and arms as well as dissociated skeletal plates collected by bulk sieving in water using a 150 µm mesh. Thanks to the mostly clayey matrix, most studied specimens are exquisitely preserved showing original stereom structures.

The material was examined using scanning electron microscopy (SEM), in case of the larger specimens using a Leo LV1455VP low-pressure environmental SEM without coating, and in case of the smaller ones using a JEOL Neoscope JMC-5000 with gold coating. Terminology followed Stöhr et al. (Reference Stöhr, O’Hara and Thuy2012) and Thuy and Stöhr (Reference Thuy and Stöhr2011). Lateral arm plates are abbreviated as LAPs in the following sections of the manuscript.

Repositories and institutional abbreviations

All material is housed in the Natural History Museum London (NHMUK) with the exception of the holotype of A. pratti, which is housed at the Geological Survey Museum (BGS), and the specimen figured by Martill and Hudson (Reference Martill and Hudson1991), which is housed in Leicester University Geology Collections (LEIUG).

Oxford Clay ophiuroid localities, stratigraphy, geology, and taphonomy

In England, the Oxford Clay Formation, middle Callovian–lower Oxfordian, crops out along a southwest/northeast trending band (Fig. 1) from the south coast of Dorset near Weymouth, through Wiltshire, Gloucestershire, and central England and into Lincolnshire and Yorkshire, where it again meets the coast at Scarborough (Cox and Sumbler, Reference Cox and Sumbler2002). Owing to the relatively unconsolidated nature of the formation, natural outcrops are only seen in areas of constant erosion, essentially coastal areas (Weymouth). However, the Oxford Clay Formation has been exposed in numerous inland areas at different times owing to various excavation activities. Several of these activities have produced the ophiuroid specimens that form the basis of this study (Fig. 1) and include railway line construction (Cheltenham and Christian Malford, Wiltshire in the 1840s; Wilby et al., Reference Wilby, Duff, Page and Martin2008), quarrying of overlying Pleistocene gravels (Coln Quarry, Gloucestershire; Dr. N. Hollingworth, personal communication, 2015), and clay extraction for brick manufacture (Westaway et al., Reference Westaway, Scotney, Younger and Boyce2015).

Figure 1

Map showing the outcrop surface of the Oxford Clay Formation in Great Britain and the positions of the ophiuroid localities (A: Ophiotitanos smithi, Ophioplax pratti; B: Enakomusium indet. C: Enakomusium whymanae, Aspidophiura seren; D: Enakomusium weymouthiense; E: Dermocoma sp. ).

The Callovian of southern and central England represents a time of steady sea level rise into the Oxfordian (Wright and Cox, Reference Wright and Cox2001). This is reflected in the widespread and laterally consistent deposition of the Oxford Clay, which has a silty/sandy base grading into fine and fissile clays and shales with a high organic content. The seas around Britain during the Callovian–Oxfordian were generally warm and clear. Land was located approximately 50–100 km away from any of the studied localities and lay mostly to the north, west, and east (Cox and Sumbler, Reference Cox and Sumbler2002). These lands were presumably the source of intermittent pulses of sediment that buried the ophiuroids.

The Oxford Clay Formation is subdivided into Taxon-range biozones and subzones using ammonites (Cox and Sumbler, Reference Cox and Sumbler2002) and comprises three members, the lowest Peterborough Member, the middle Stewartby Member (both Callovian), and the upper Weymouth Member (Oxfordian). The Oxford Clay has been the subject of many detailed studies owing to its economic importance as a source of self-firing brick clay and its abundant and exquisitely preserved fossils (Westaway et al., Reference Westaway, Scotney, Younger and Boyce2015; Martill and Hudson, Reference Martill and Hudson1991). The three members of the Oxford Clay Formation are remarkably consistent across their outcrop of over 200 km, which is indicative of its deposition during a wide transgression (Cox and Sumbler, Reference Cox and Sumbler2002). Figure 2 shows the stratigraphy of the Oxford Clay in central and southern England and the position of the ophiuroids studied herein.

Figure 2

Stratigraphic chart of the Oxford Clay Formation with ammonite zones, lithological units, and the positions of the ophiuroid localities (A: Ophiotitanos smithi, Ophioplax pratti; B: Enakomusium indet. C: Enakomusium whymanae, Aspidophiura seren; D: Enakomusium weymouthiense; E: Dermocoma sp.).

Ophiuroids are known from the Peterborough Member and probably (see the following) the Weymouth Member of the Oxford Clay Formation. None have conclusively been found in the Stewartby Member, which is regarded as generally poor in shelly fossils (Cox and Sumbler, Reference Cox and Sumbler2002). The Kosmoceras jason Taxon-range Biozone of the Peterborough Member has long been known as a source of numerous, exquisitely preserved fossils, particularly from sites near Christian Malford (mainly cephalopods and fish; Wilby et al., Reference Wilby, Duff, Page and Martin2008) and around the Peterborough brick pits where numerous marine reptiles were collected by the Leeds brothers but which is also known for its abundant invertebrate fauna (Martill and Hudson, Reference Martill and Hudson1991). The high diversity and organic content of the sedimentary rock of this member are attributed to high productivity (MacQuaker, Reference MacQuaker1994; Hudson et al., Reference Hudson, Taylor, Duff, Riding and Brown1994; Martill et al., Reference Martill, Taylor, Duff, Riding, Bown and Hudson1994). It is the Peterborough Member that four of the species (from three different localities) described herein come from. The other two species probably come from the Weymouth Member (“upper Oxford Clay”–lower Oxfordian) around the town of Weymouth, which is also well known for its fossil mollusk fauna (Cox and Sumbler, Reference Cox and Sumbler2002). The geologies of the five different localities are discussed individually in the following. Subdivision lettering matches that used in Figures 1, 2.

A: Christian Malford, NE Chippenham, Wiltshire. The specimens (Ophiotitanous smithi n. sp. and Ophioplax prattii n. comb.) studied herein are presumed to be from the Kosmoceras phaeinum Subzone, Peltoceras athleta Zone, upper part of the Peterborough Member, Oxford Clay Formation, upper Callovian, Middle Jurassic. While the precise localities of these specimens are unknown, they are both associated with the Christian Malford Lagerstätten. This famous site produced large numbers of fossils with spectacular soft tissue preservation, principally of cephalopods and fish, during the Victorian era (1840s–1850s). However, the precise site of the bed was kept secret by the Victorian collectors, and as such, locality names such as “Christian Malford” and “Chippenham” are all that is known. However, owing to the significance of these fossils, much work was recently undertaken to relocate the site, and it was indeed “rediscovered” in 2008 (Wilby et al., Reference Wilby, Duff, Page and Martin2008). Analysis of the ammonites returned from cores identified the site as belonging to the Kosmoceras phaeinum Subzone, from the Upper Peterborough Member (Wilby et al., Reference Wilby, Duff, Page and Martin2008). While no ophiuroids were collected during this recent work, it is assumed that these Victorian collections came from somewhere close to these localities and horizons.

The lack of precise locality detail and the extraction of specimens from the matrix leave little scope to assess the taphonomy. It is however assumed that burial must have been rapid (obrution?) as the specimens are preserved in articulation with minimal subsequent disturbance by burrowers (exclusion by sediment anoxia?). The environment outlined by Wilby et al. (Reference Wilby, Hudson, Clements and Hollingworth2004) for the preservation of soft-tissue coleoids (a very soft “soupy” anoxic sea floor) at Christian Malford, however, appears to be unfavorable for ophiuroids. As such, it is unlikely that the ophiuroids are from exactly the same horizon. That said, it is known that there was a high degree of variability in sea floor conditions at this time (Duff, Reference Duff1975), so it does not preclude these specimens from occurring close (geographically and stratigraphically) to the Christian Malford Lagerstätte.

B: Stewartby quarry, Stewartby Brickworks, Bedford, Bedfordshire, UK Bed 6a (of Duff, Reference Duff1974), Lower Oxford Clay, Kosmoceras medea Subzone, jason Zone (Callovian), Middle Jurassic. Collected by Keith Duff in 1970 from the base of the Peterborough Member, the ophiuroids (Enakomusium indet.) were found in a lens, which faded away in all directions and was 40 cm above the base bed 6A, in the medea Subzone of the jason Zone (Duff, Reference Duff1974). The sediment preserving the specimens is olive-green, rather well-laminated, shaley clay, belonging to the deposit-feeder bituminous shales biofacies of Duff (Reference Duff1975). It is in a facies Duff (Reference Duff1975) described as at the restricted end of this biofacies and was associated with a fauna dominated by Bositra and Procerithium, with subsidiary Pinna, Dicroloma, and Meleagrinella with rare nuculaceans (Palaeonucula and Mesosacella). While preservation of the ophiuroid skeleton is poor (stereome lost), the specimens consist of articulated individuals. Both oral and adoral surfaces are uppermost on the slab surface in different specimens that are located at slightly different levels throughout, with some overlapping one another. This is suggestive of short transportation and rapid deposition as occurs during an obrution event. There is no evidence of arm loss and regeneration.

C: Coln Quarry, Lechlade, Gloucestershire, UK 51.6951º N, 1.7372º W. The specimens (Enakomusium whymanae n. sp. and Aspidophiura seren n. sp.) studied were collected by Ms. Janet Whyman from ex situ material that had been extracted from a drainage trench in the base of the quarry. The fossils originate from a fissile gray shaley clay from the jason Zone, lower Peterborough Member, lower Oxford Clay, Callovian, upper Middle Jurassic. Apart from unpublished observations and logs compiled by Dr. N. Hollingworth, the Oxford Clay at Coln Quarry is poorly known. Much about this locality is therefore here inferred by comparison with the better-known type locality for the Peterborough Member, near Peterborough, Bedfordshire, which was heavily studied in the early 1990s (e.g., Hudson and Martill, 1991, Reference Hudson and Martill1994; Hudson et al., Reference Hudson, Taylor, Duff, Riding and Brown1994; MacQuaker, Reference MacQuaker1994; Martill et al., Reference Martill, Taylor, Duff, Riding, Bown and Hudson1994). The Peterborough Member at Coln is composed of a richly fossiliferous series of shales and calcareous mudstones, dominated by the ammonite Kosmoceras and by bivalves and gastropods (N. Hollingworth, personal communication, 2015) although contextual material collected (by J. Whyman, 2008) along with the ophiuroids at Coln Quarry includes teleost and shark teeth, short sections of isocrinid pluricolumnals, crustaceans, brachiopods, and belemnites. The environment of the Oxford Clay surrounding Coln Quarry has been interpreted as a warm, shallow-shelf sea with normal salinity. Land probably lay close by, to the northeast, where marginal marine facies are seen in the East Midlands (Cox and Sumbler, Reference Cox and Sumbler2002). The bottom waters of the sea are suspected to have been euroxic, with the high carbon content attributed to high productivity (MacQuaker, Reference MacQuaker1994; Martill et al., Reference Martill, Taylor, Duff, Riding, Bown and Hudson1994) as at the type locality for the Peterborough member.

Specimens are randomly but densely distributed through a layer of rock approximately 2 cm thick, some overlapping each other. Both oral and adoral surfaces of different specimens are visible on the same bedding plain. Many individuals are articulated, missing only arm tips; however, most remains consist of long articulated sections of arm and isolated discs. There are also some isolated plates. All articulated material pertains to Enakomusium whymani n. sp. as does most of the disarticulated material; however, the latter also includes lateral arm plates of Aspidophiura seren n. sp. This suggests that the articulated E. whymani n. sp. were gregarious and buried after short transportation by an obrution event. The isolated material appears to be well preserved but suggests that the ophiuroids were disarticulated before deposition. It seems likely Aspidophiura seren n. sp. and E. whymani n. sp. coexisted but were not intimately associated (as the former has not been found articulated).

The arms of articulated E. whymani n. sp. specimens were not noted to be regenerating. This may be because predation pressure was low, but it is as likely due to the preservation and extraction of the majority of the specimens (most have disarticulated arms owing to being extracted through bulk sieving).

D: The locality of the type material (Enakomusium weymouthiense [Damon, Reference Damon1880]) is not specifically recorded and therefore may come from either Radipole Backwater or Ham Cliff, Weymouth, Dorset. Similarly preserved specimens, in similar matrix, from Weymouth, are recorded as coming from either ‘a Railway cutting near The Backwater’ or simply just ‘The Backwater.’ It is here assumed that all these specimens most likely come from the Radipole Backwater area. Unfortunately, the Oxford Clay at the Radipole Backwater is no longer accessible. The outcrop at Ham Cliff, however, was studied by Page (Reference Page2002), who, from ammonites, established that the succession there straddles the Callovian/Oxfordian boundary. It includes fossils from the Quenstedtoceras lamberti lamberti Subzone to the Cardioceras (Scarburgiceras) scarburgense Subzone and therefore includes both the top of the Stewartby Member and the base of the Weymouth Member. Unfortunately, there is little lithological distinction between the members at this site, and as the precise locality and horizon of the ophiuroid specimens is not known, it remains ambiguous as to where these specimens are from. They are, however, younger than the deposits at Coln, Christian Malford, and Peterborough.

The rocks that preserve the ophiuroids are coarse light gray/green cross-bedded silts that are burrowed. Numerous individuals (10 individuals on the surface of a slab measuring 110 mm x 150 mm) are found on single slabs of rock, displaying both dorsal and ventral surfaces with several overlapping and being separated by thin wedges of sediment. Most specimens are composed of disks and several arms although often the distal sections are missing. Long sections of isolated arms are also present. This suggests burial in the medial-distal part of an obrution event, where the brittle stars were transported a short distance and buried rapidly.

Several articulated specimens display arms in states of regeneration (smaller and shorter than other arms; approximately 1:10 specimens are regenerating). While this does not equate to a high degree of predation pressure, it is notable because it is unobserved in other British Oxford Clay ophiuroids. However, this is likely due to sample size and differing preservation in other locations rather than a meaningful biological signal.

E: The Fleet, Weymouth, Dorset. The single specimen (Dermocoma sp.) recovered from this locality has been completely removed from the matrix, and as such, all lithological context has been lost. The Fleet is a large lagoon (over 5 km long) that stretches westward from Weymouth where both the Weymouth and Stewartby members of the Oxford Clay Formation are known to occur. However, the specimen studied herein is coated in pyrite, which hints that it belongs to the more fossiliferous Weymouth Member as ammonites between Tidmore Point and Weymouth are similarly preserved (Cox and Page, Reference Cox and Page2002).

Systematic paleontology

Discussion

Since the classification by Smith et al. (Reference Smith, Paterson and Lafay1995) was recently shown to be untenable (O’Hara et al., Reference O’Hara, Hugall, Thuy and Moussalli2014; Thuy and Stöhr, Reference Thuy and Stöhr2016), we leave the species described herein in open nomenclature above genus level, suggesting an assignment with respect to the clades of O’Hara et al. (Reference O’Hara, Hugall, Thuy and Moussalli2014) whenever possible and providing descriptions enabling inclusion in subsequent phylogenetic studies (e.g., Thuy and Stöhr, Reference Thuy and Stöhr2016).

Class Ophiuroidea Gray, Reference Gray1840

Clade A of O’Hara et al. (Reference O’Hara, Hugall, Thuy and Moussalli2014)

Genus Enakomusium Thuy, Reference Thuy2015

Type species

Ophioderma weymouthiense Damon, Reference Damon1880, by original designation.

Diagnosis (emended from Thuy, Reference Thuy2015)

Ophiuroid genus with robust overall skeletal plating; thick disk scales devoid of removable granules; leaf-like oral papillae, completely separated; genital slit devoid of papillae; arms with small and mostly noncontiguous dorsal and ventral arm plates at least in proximal to median arm segments; lateral arm plates thick, robust, rounded rectangular; outer proximal edge commonly with spurs and paralleled by a conspicuous, deep furrow; spine articulations integrated into outer surface stereom, moderately large and prominent, and composed of a round muscle opening proximally tightly encompassed by a small, thin, commonly denticulate ridge and distally separated from a slightly smaller nerve opening by a large, commonly curved vertical ridge; arm spines commonly longer than half an arm segment; tentacle openings developed as deep, ventralward, pointing notches in proximal to median arm segments and as distalward-pointing within-plate perforations in median to distal arm segments.

Enakomusium whymanae new species

Figures 3, 4

Figure 3

Enakomusium whymanae n. sp., NHMUK EE 16205 (holotype), Coln Quarry, Lechlade, Gloucestershire, UK 51.6951N, 1.7372W; ex situ layer of fissile gray shaley clay from the jason Zone, Peterborough Member, lower Oxford Clay, Callovian, late Middle Jurassic. (1) Ventral side of disk; (2) dorsal side of disk; (3) detail of mouth plating; (4) detail of ventral interradius; (5) ventral side of proximalmost arm segments; (6) dorsal side of proximalmost arm segments; (7) detail of spine articulation. (1, 2) Scale bar=1 mm; (3–6) scale bars=500 µm; (7) scale bar=50 µm. AbGP=abradial genital plate; AOP=apical oral papilla; AOS=adoral shield; AS=arm spine; DAP=dorsal arm plate; LAP=lateral arm plate; LOP=lateral oral papilla; MO=muscle opening; OP=oral plate; OS=oral shield; RS=radial shield; TS=tentacle scale; VAP=ventral arm plate; 1VAP=first ventral arm plate.

Figure 4

Enakomusium whymanae n. sp., Coln Quarry, Lechlade, Gloucestershire, UK 51.6951N, 1.7372W; ex situ layer of fissile gray shaly clay from the jason Zone, Peterborough Member, lower Oxford Clay, Callovian, late Middle Jurassic. (1) NHMUK EE 16206 (paratype), dorsal side of disk; (2) NHMUK EE 16207 (paratype), ventral side of arm fragment; (3) NHMUK EE 16208 (paratype), dorsal side of arm fragment; (4) NHMUK EE 16209 (paratype), proximal lateral arm plate in (4.1) external and (4.2) internal views; (5) NHMUK EE 16210 (paratype), median lateral arm plate in (5.1) external and (5.2) internal views and (5.3) with detail of spine articulation; (6) NHMUK EE 16211 (paratype), distal lateral arm plate in (6.1) external and (6.2) internal views. (1–3) Scale bars=1 mm; (4, 5.1, 5.2, 6) scale bar=200 µm; (5.3) scale bar=50 µm. CPP=central primary plate; IP=interradial plate; MO=muscle opening; NO=nerve opening; RS=radial shield.

Holotype

NHMUK EE 16205.

Diagnosis

Relatively small species of Enakomusium with pairs of radial shields separated interradially by single plates similar in shape and size to radial shields; LAPs comparatively thin, with a weak constriction but devoid of spurs on the outer proximal edge; large, prominent spine articulations not sunken into depressions and bearing conical, pointed arm spines exceeding the length of an arm segment.

Occurrence

Coln Quarry, Lechlade, Gloucestershire, UK 51.6951ºN, 1.7372ºW; ex situ layer of fissile gray shaley clay from the jason Zone, Peterborough Member, lower Oxford Clay, Callovian, late Middle Jurassic.

Description of holotype

EE 16205 (Fig. 3) is an articulated disk (4.58 mm disk diameter) exposing both dorsal and ventral sides and with proximal segments of one arm preserved; ventral side covered by an arm fragment comprising eight median to distal segments and belonging to the holotype or to another specimen of the same species; dorsal side of the disk covered in its central part by few small, round, imbricate and moderately thick scales and a slightly larger circular central primary plate; remaining dorsal side of the disk covered by large, rounded isosceles-triangular radial shields (Fig. 3.2) accounting for approximately two-thirds of the disk radius joined in pairs over their entire length and separated interradially by a single plate similar in shape to the radial shields but slightly narrower; no removable granules or spines or conspicuous stereom ornamentation discernible on the dorsal disk plates and scales.

Ventral side of disk with relatively few interradial scales similar in shape and size to the dorsal disk scales, devoid of removable granules or spines. Genital slit extending over at least half of the disk radius; no genital papillae discernible; abradial genital plate (Fig. 3.4) length approximately four times width, with longitudinal ridge but devoid of perforation on exposed side. Oral shield (Fig. 3.3) arrow-shaped, with pointed, acute proximal angle, concave lateral edges, and rounded rectangular distal portion; adoral shields (Fig. 3.3) comparatively broad, with wide distal portion encompassing almost half of the second oral tentacle opening; proximal tips of adoral shields meeting in front of oral shield; second oral tentacle pore (Fig. 3.3) opening inside mouth slit; oral plates stout and wide; first ventral arm plate radially compressed pentagonal, wider than long, with obtuse distal angle, pointed lateral angles, and rounded rectangular proximal edge; lateral oral papillae (Fig. 3.3) fully separated but contiguous, rounded rectangular except for pointed distal-most papilla, second distal-most papilla largest, remaining papillae continuously decreasing in size toward the dental plate; single, short, conical apical oral papilla, slightly smaller than neighboring lateral oral papillae; teeth not exposed.

Only proximal arm segments preserved (note that the median to distal arm portion covering the ventral side of the holotype is ignored here as it might well belong to another individual), segments nearly as long as wide; LAPs with weak constriction and coarsely meshed outer stereom with slightly enlarged trabecular intersections; spine articulations on elevated part of distal LAP portion, at some distance from the distal edge of the plate; spine articulations (Fig. 3.7) relatively large, evenly distributed over the entire distal portion of the LAP, with a dorsalward increase in size of the spine articulations and in the distance separating them; dorsal-most spine articulation visible in dorsal view; spine articulation composed of a large round muscle opening proximally bordered by a thin, irregularly denticulate vertical ridge and distally encompassed by a wider, more strongly protruding, smooth, semicircular and oblique vertical ridge; nerve opening slightly smaller than muscle opening, distally bordering the semicircular ridge; arm spines (Fig. 3.6) conical, pointed, with rough longitudinally reticulate outer surface, slightly longer than one arm segment; ventral edge of lateral arm plates with deep tentacle notch, distally encompassed by a thinned ventro-distal edge of the lateral arm plate. Ventral arm plates (Fig. 3.5) widest distally, with straight distal edge, deeply concave latero-proximal edges, and pointed proximal tip, separated by lateral arm plates on all exposed segments; tentacle openings large, covered by two oval, nearly equal-sized tentacle scales, developed as between-plate openings in all preserved arm segments. Dorsal arm plates (Fig. 3.6) small, fan-shaped, with right proximal angle and convex distal edge, widely separated by lateral arm plates.

Paratype supplements

EE 16206 (Fig. 4.1) is an articulated disk (4.63 mm disk diameter) exposing both ventral and dorsal sides; ventral disk morphology well in agreement with that of holotype; dorsal disk plating better preserved than in holotype, with clearly radial shield pairs abutting over their entire length interradially separated by a single plate similar in size and shape to radial shields.

EE 16207 (Fig. 4.2) is an articulated arm fragment composed of seven proximal to median segments; proximal segments similar to those of holotype; median arm segments becoming progressively more elongate; median LAPs without constriction and with smaller and less prominent spine articulations; arm spines of median segments as long as half an arm segment; tentacle pores large and developed as between-plate openings in two proximal-most segments; tentacle pores of the following segments developed as within-plate perforations, strongly decreasing in size and progressively moving from a ventral to a ventro-distal position toward the distal-most preserved segment; proximal-most within-plate tentacle perforation with two oval scales; following tentacle openings with no discernible scales; ventral arm plates strongly decreasing in size from proximal-most to distal-most preserved segment; the two proximal-most ones similar to those observed on holotype, the following ventral arm plates fan-shaped, wider than long.

EE 16208 (Fig. 4.3) is an articulated arm fragment composed of nine proximal to median arm segments; proximal ones similar to those of holotype, median ones more elongate; median LAPs devoid of constriction but with similar outer surface ornamentation as proximal lateral arm plates; dorsal-most spine articulations decreasing in size and moving from a nearly dorsal to a lateral position from the proximal-most to the distal-most preserved segments; arm spines of median segments equaling half the length of an arm segment; dorsal arm plates discernible on all preserved arm segments.

EE 16209 (Fig. 4.4), EE 16210 (Fig. 4.5), and EE 16211 (Fig 4.6) are dissociated proximal, median, and distal LAPs, respectively; rounded rectangular outline; proximal LAP nearly as long as high, distal one approximately 1.5 times wider than high; dorsal edge weakly concave, distal edge straight to very weakly convex; proximal edge deeply concave, devoid of discernible spurs but paralleled by deep, poorly defined furrow; weak horizontal striation near central part of furrow; outer surface ornamentation as in LAPs of holotype; four spine articulations in proximal and median LAPs, three in distal one; spine articulations relatively large, prominent, on slightly elevated part of distal LAP portion, tightly encompassed by outer surface stereom; strong dorsalward increase in size of spine articulations and in distance separating them; spine articulation morphology as in holotype but better preserved. Inner side of LAPs with large, rounded triangular dorsal contact surface with opposite LAP; ventral contact surface much smaller; short, well-defined, prominent ridge in central part of inner side, with tongue-shaped, dorso-proximalward-pointing dorsal tip; inner side of distal LAP edge with slightly more densely meshed stereom but devoid of discernible spurs; proximal and median LAPs with large, deep, sharply defined ventralward-pointing tentacle notch; distal LAP with large tentacle perforation distally bordering the central ridge; no other perforations discernible.

Etymology

Species named in honor of Janet Whyman (Coventry, United Kingdom), who discovered the type material and generously donated it to the Natural History Museum London.

Materials

NHMUK EE 16206, EE 16207, EE 16208, EE 16209, EE 16210, EE 16211 (paratypes); NHMUK EE 15466 (28 complete specimens and a collection of several hundred arm and disk fragments).

Remarks

The Ophiomusium-like form, with stout overall plating and at least part of the tentacle pores developed as within-plate openings, is one of the most widespread in Jurassic ophiuroids. The taxonomic status and systematic position of these forms was subject to considerable controversy, amendments of genus diagnoses, and the introduction of the regrettably misrouted genus Mesophiomusium Kutscher and Jagt, Reference Kutscher and Jagt2000 (Thuy and Meyer, Reference Thuy and Meyer2013; Thuy, Reference Thuy2015). To put at least some of the Jurassic Ophiomusium-like forms on solid taxonomic ground, in accordance with the latest insights on ophiuroid morphology, Thuy (Reference Thuy2015) introduced the genus Enakomusium to accommodate Ophiomusium-like forms with rounded rectangular LAPs, relatively large spine articulations bearing relatively long spines, and tentacle pores developed as between-plate openings in proximal and often median arm segments.

The present material fully accords with the diagnosis of Enakomusium. Among the currently known species of the genus, it most closely resembles the type species E. weymouthiense (see below), particularly in respect to the radial shields, the single interradial plates, and the mouth plating. The present specimens differ, however, in their smaller size, a less robust plating, and particularly, possession of lateral arm plates with a constriction (Figs. 4.4 to 4.6) and larger, more prominent spine articulations that are not sunken into depressions and which bear spines that exceed the length of an arm segment. The LAPs of the here described material furthermore lack any discernible spurs on the outer distal edge, have a less well-defined furrow paralleling the distal edge, and are generally less robust. At least part of these differences cannot be explained by ontogenetic variation and thus preclude assignment to E. weymouthiense, despite their stratigraphical and geographical proximity.

Other species assigned to Enakomusium generally have stouter LAPs, smaller and less prominent spine articulations, and discernible spurs on the outer distal edge and therefore cannot be confused with the present material. It thus seems warranted to erect a new species. Within the morphological spectrum of Enakomusium, the new species is the least Ophiomusium-like, with the generally more fragile plating, the conspicuous and prominent spine articulations, and the unusually long arm spines. Modern representatives of the Ophiomusium-Ophiosphalma-Ophiolipus complex tend to be much more morphologically conservative. The Jurassic forms generally contrast with their modern relatives in showing a much larger range of morphologies, with the present one even superficially reminiscent of ophiacanthid morphologies with the large, prominent spine articulations, the constricted LAPs, and the long spines (e.g., Thuy, Reference Thuy2013).

Enakomusium sp. indet.

Materials

LEIUG 73036. One specimen containing three poorly preserved articulated individuals and three sections of disarticulated arm.

Occurrence

Stewartby quarry, Stewartby Brickworks, Bedford, Bedfordshire, UK Bed 6a (Duff), lower Oxford Clay, medea Subzone, jason Zone (Callovian), Middle Jurassic.

Remarks

This poorly preserved specimen was figured by Martil and Hudson (Reference Martill and Hudson1991). The articulated ophiuroids from the Peterborough Member of Stewartby, Great Britain, are too poorly preserved to enable a confident species-level identification. With respect to the few morphological features discernible on the specimens, assignment to Enakomusium seems warranted, and there are indeed superficial similarities to E. whymanae n. sp. from Coln Quarry, Gloucestershire, but we nevertheless prefer treating the specimens as an unidentified record of Enakomusium.

Enakomusium weymouthiense (Damon, Reference Damon1880)

Figure 5.1–5.3

Figure 5

Enakomusium weymouthiense (Damon, Reference Damon1880), Callovian of Liesberg, Switzerland, specimens from the original material of Hess (Reference Hess1963). (1) Proximal lateral arm plate in (1.1) external and (1.2) internal views; (2) median lateral arm plate in (2.1) external and (2.2) internal views; (3) distal lateral arm plate in (3.1) external and (3.2) internal views. Aspidophiura? seren new species, Coln Quarry, Lechlade, Gloucestershire, UK 51.6951N, 1.7372W; ex situ layer of fissile gray shaley clay from the jason Zone, Peterborough Member, lower Oxford Clay, Callovian, late Middle Jurassic. (4) NHMUK EE 16212 (holotype), proximal lateral arm plate in (4.1) external and (4.2) internal views and (4.3) with detail of spine articulations; (5) NHMUK EE 16213 (paratype), median lateral arm plate in (5.1) external and (5.2) internal views; (6) NHMUK EE 16214 (paratype), proximal lateral arm plate in (6.1) external and (6.2) internal views; (7) NHMUK EE 16215 (paratype), distal lateral arm plate in (7.1) external and (7.2) internal views and (7.3) with detail of spine articulations. (1–4.2, 4.4–7.2) Scale bars=200 µm; (4.3, 7.3) Scale bars=100 µm. MO=muscle opening; NO=nerve opening; TN=tentacle notch.

1880 Ophioderma weymouthiense Reference DamonDamon, plate 17, fig. 10, 10A.

1963 Ophiomusium calloviense Reference HessHess, p. 1143.

1964 Ophiomusium weymouthiense Reference HessHess, p. 780.

2015 Enakomusium weymouthiense Reference ThuyThuy, p. 95.

Materials

NHMUK E 1607b (neotype), specimens figured by Hess (Reference Hess1964) from the same locality include E 1607a, E 3500. Eighteen other specimens from the Weymouth area containing over 50 individuals are also known in the NHM collections but did not provide any new information.

Description

Redescription of the original LAPs of Hess (Reference Hess1963): LAPs moderately large, thick, proximal ones slightly higher than wide, distal ones slightly wider than high, rounded rectangular outline with very weakly convex dorsal and distal edges; proximal edge strongly concave, with two poorly defined, vertically elongate, thin spurs dorsally and a much shorter, equally poorly defined spur ventrally; proximal edge paralleled by a conspicuous, deep furrow; distal half of ventral edge in proximal and median LAPs weakly concave; distal LAPs with evenly convex ventral edge. Outer surface stereom of LAPs moderately finely meshed and with weakly enlarged, vertically elongate, and densely packed trabecular intersections; faint horizontal striation lining the distal edge of the furrow paralleling the proximal LAP edge. Three to four relatively large spine articulations sunken in shallow depressions of the distal portion of the LAP at some distance from the distal edge and tightly encompassed by the outer surface stereom; strong dorsalward increase in the size of the spine articulations and the distance separating them; spine articulations composed of a large, round muscle opening proximally tightly encompassed by a relatively low, semicircular, irregularly denticulate ridge and distally separated from a slightly smaller nerve opening by a thicker, more prominent but shorter vertical oblique ridge composed of slightly more densely meshed stereom.

Inner side of LAPs with large ventral and dorsal contact surfaces with opposite LAP; short, wide, well-defined, and prominent central ridge on elevated part connecting the ventral and dorsal contact surfaces with the opposite LAP, widest ventrally and with a tongue-shaped, slightly proximalward-pointing dorsal tip; inner side of distal edge lined with poorly defined, very weakly prominent vertical surface composed of more densely meshed stereom; large, deep, sharply defined ventralward- (proximal LAPs) to ventro-distalward-pointing (median LAPs) tentacle notch; distal LAPs with large ventro-distalward-pointing tentacle perforation. No further perforations discernible on inner side.

Remarks

The neotype and associated specimens were duly described and illustrated by Hess (Reference Hess1964). There is nothing to add from our observations of the type material. To facilitate comparison with other species, in particular those known exclusively from dissociated LAPs, we redescribe and illustrate the LAPs from the Callovian of Switzerland originally described by Hess (Reference Hess1963) as Ophiomusium calloviense and shortly after synonymized with Ophioderma (now Enakomusium) weymouthiense (Hess, Reference Hess1964) using the terminology by Thuy and Stöhr (Reference Thuy and Stöhr2011).

The LAPs of Enakomusium weymouthiense bear strong similarities with those described by Thuy (Reference Thuy2015) as Enakomusium leckenbyi (Wright, Reference Wright1880) and Enakomusium ferrugineum (Boehm, Reference Boehm1889), corroborating their assignment to the same genus (Thuy and Stöhr, Reference Thuy and Stöhr2011). The main difference put forward by Thuy (Reference Thuy2015) was the grouped ventral spine articulations separated from the dorsal-most one by a gap. Our redescription suggests a strong but continuous dorsalward increase in the distance separating the spine articulations, rather than a ventral grouping and a dorsal gap. Another noteworthy difference with the LAPs of E. leckenbyi and E. ferrugineum is that the spine articulations of E. weymouthiense are sunken in shallow depressions. Differences with the new species Enakomusium whymanae are discussed in the preceding.