FIRST LOOK AT LOWER PERMIAN OPHIUROID DIVERSITY USING DISARTICULATED OSSICLES

Ophiuroidea (brittle stars) are poorly documented in the Late Paleozoic because their multi-element skeletons rapidly disarticulate into sand-sized ossicles upon death. Even though ophiuroid elements can be abundant (up to 650 plates/g in 2 phi fraction), they have typically been ignored because these ossicles were considered taxonomically non-diagnostic. New methodologies treating ophiuroid lateral arm plates (LAPs) as microfossils show them to be species diagnostic, opening the brittle star fossil record to rigorous analysis. Recent studies investigating Mississippian and Pennsylvanian brittle star faunas found LAPs to be abundant and diverse with up to 15 co-occurring species. Here we expand this work into the Early Permian (Cisuralian) to investigate abundance and diversity levels.

Shale samples collected from the lower Wolfcampian (Asselian) Camp Colorado and Watts Creek Mbs. of the Moran Fm. and the uppermost Wolfcampian (Artinskian) Elm Creek Fm. of TX, and lower Wolfcampian (Asselian) Bader Fm. of KS were processed for microfossils and ophiuroid material was collected from unbiased sieved subsamples, and censused. These techniques confine our samples to muddy environments as limestones cannot be processed at present.

The Permian samples show consistently high LAP abundance and comparatively low diversity. As with Mississippian and Pennsylvanian ophiuroid samples all major clades are present including archaic taxa (protasterids, furcasterids, stenuroids, cheiropterasterids), and modern clade ophiuroids. Archaic brittle stars dominate Mississippian-aged faunas whereas modern clades dominate Pennsylvanian age faunas. Similarly, Permian faunas are dominated by modern types, representing up to 95 percent of LAPs, particularly a thick-plated smooth species. The Elm Creek Fauna, unusually, is dominated by an ophioscolecid (modern clade) while the smooth modern form is absent, reflecting changes in benthic communities between Asselian icehouse (Bader and Moran Fms.) to late Artinskian / Kungurian non-glacial conditions (Elm Creek Fm.). Archaic taxa are present in all samples but only in lower abundance. This study shows a continuation of the Pennsylvanian transition from an archaic to modern ophiuroid faunas and that this transition took place much earlier than previously thought.