Ewin and Thuy—Brittle stars from the British Oxford Clay Thus, the Callovian and Oxfordian ophiuroid occurrences


described herein are in line with previous records and seem to be part of the typical faunal spectrum of upper Middle to lower Upper Jurassic mud-bottom ophiuroid communities from mid- shelf depths. The material nevertheless stands out as it almost entirely consists of articulated skeletons and thus provides unpreceded insights into the morphology of the concerned taxa. They are among the very few cases in which exceptionally preserved ophiuroid fossils represent more than single-taxa occurrences (e.g., Thuy et al., 2011). Taphonomically, the finds described herein almost all


represent cases of articulated preservation. Dissociated LAPs, by contrast, were so far only recovered accidentally as bycatch within the set of articulated specimens of Enakomusium whymanae. The overrepresentation of articulated specimens in the studied material is without any doubt due to sampling bias. Systematic sieving of Oxford Clay sediments is very likely to produce more occurrences of dissociated LAPs. The ophiuroids that inhabited the Peterborough Member


were all found in fine, dark to medium gray or olive green, fissile shales. This suggests that the ophiuroids were probably


overwhelmed by distal obrution events while living in warm, relatively deep seas (i.e., below storm wave base) in open mar- ine conditions but with periodic seafloor euroxia. The remains of Aspidophiura seren are an exception here in consisting of completely dissociated arm plates found via screen washing among the articulated remains of Enakomusium whymanae. This hints that this species was allochthonous or that the remains were autochthonous and disarticulated before burial. There is also little (if any) indication of arm regeneration among the Peterborough Member ophiuroids. However, this is as likely due to the sample size, collection method (sieving of much of the material), and taphonomy as it is a true biological signal or low predation. Of the two species originating from the Weymouth


Member, there is no surviving lithological evidence with the Dermocoma specimen, and so further comment is impossible. However, the large number of E. weymouthensis specimens known are all preserved in a coarse, light gray-green, cross- bedded silt and appear to have been buried by a more proximal obrution event. Indeed, this locality was the most proximal (to land) of all the English Oxford Clay ophiuroid localities known (see the preceding). There are also several incidences of arm regeneration within this population (approximately 1:10 have regenerating arms). Many modern ophiuroids autotomize their arms as a defensive measure against predation; they sacrifice an arm but the rest of the animal is left unscathed. Thus, if a population has a large number of regenerating individuals, it can be an indication of predation pressure (e.g., Aronson, 1991). This appears to be infrequent in the Weymouth ophiuroids.


Acknowledgments


We thank J. Whyman for the kind donation of fossils; K. Duff, N. Hollingworth, and D. Martill for helpful comments, information, and advice; and P. Sheppard (British Geological Survey) and C. Watts (Leicester University) for access to col- lections. We are indebted to the reviewers D. B. Blake (Urbana- Champaign, Illinois, USA) and S. Stöhr (Stockholm, SE),


797


whose comments greatly improved an earlier version of this manuscript.


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