784


Journal of Paleontology 91(4):781–798


productivity (MacQuaker, 1994; Martill et al., 1994) 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 trans- portation 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 articulatedE.whymani n. sp. specimenswere not


weymouthiense [Damon, 1880]) 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. Unfortu- nately, the Oxford Clay at the Radipole Backwater is no longer accessible. The outcrop at Ham Cliff, however, was studied by Page (2002), who, from ammonites, established that the suc- cession 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 WeymouthMember. 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


noted to be regenerating. Thismay 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


gray/green cross-bedded silts that are burrowed. Numerous individuals (10 individuals on the surface of a slab measuring 110mmx 150mm) 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 5kmlong) 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, 2002).


Systematic paleontology


Discussion.—Since the classification by Smith et al. (1995) was recently shown to be untenable (O’Hara et al., 2014; Thuy and Stöhr, 2016), we leave the species described herein in open nomenclature above genus level, suggesting an assignmentwith respect to the clades of O’Hara et al. (2014) whenever possible and providing descriptions enabling inclusion in subsequent phylogenetic studies (e.g., Thuy and Stöhr, 2016).


Class Ophiuroidea Gray, 1840


Clade A of O’Hara et al. (2014) Genus Enakomusium Thuy, 2015


Type species.—Ophioderma weymouthiense Damon, 1880, by original designation.


Diagnosis (emended from Thuy, 2015).—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 non- contiguous 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


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


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