UNRAVELLING THE PALEOZOIC OPHIUROID MYSTERY: IDENTIFICATION OF MISSISSIPPIAN AGE BRITTLE STAR (OPHIUROIDEA, ECHINODERMATA) VERTEBRAE AND LATERAL ARM PLATES FROM DISARTICULATED OSSICLES

Paleozoic ophiuroid biodiversity is poorly documented globally. Here, we describe the ophiuroid fauna from the Upper Mississippian (Chesterian) Sulphur, IN locality of the Indian Springs Shale Mb. of the Big Clifty Fm. based on disarticulated ossicles picked from fine-grained shale washings. In the past, Paleozoic ophiuroid taxa were described using articulated skeletons that provided little information on ossicle morphology. Our disarticulated material shows unexpectedly high generic diversity while using techniques common in studies of Mesozoic and Cenozoic taxa.

Collected sediment was boiled overnight in a hydrogen peroxide and water bath to separate matrix from skeletal debris. Cleaned sediment was sieved and picked for ophiuroid ossicles, primarily the information rich vertebrae (ossicles coring the arms) and lateral arm plates (LAP, bear spine articulations). We recovered at least five ophiuroid taxa from washed residues including at least two different species of furcasterid and one eospondylid species from both vertebrae and LAPs, an Onychaster-type species currently known from a LAP, and a modern type species currently known from only vertebrae. Furcasterid type 1 vertebrae are rectangular, slightly longer than wide with a distinct median groove and are correlated with J-shaped LAPs with large identifiable spine articulations. Type 2 furcasterid vertebrae are rectangular in lateral view and triangular in cross-section with large conspicuous podial basins. They are associated with more robust LAPs with a rhombus shape in lateral view and possess fewer spine articulations. Rounded “ear-like” LAPs with less-noticeable spine articulations and a concave inner surface are attributed to an Onychaster-type taxon. Eospondylid vertebrae are wider than tall in both lateral and cross-sectional view with a large central nerve opening and clearly defined podial basins. Their associated LAPs are fan shaped with small spines lining the distal edge. Modern type vertebrae are rectangular, slightly longer than wide, have a distinct median groove, and large dorsal and ventral processes. While their vertebrae are relatively abundant, their non-descript associated LAPs have yet to be recognized. Future work will identify these species and infer relationships among Paleozoic and Recent ophiuroids.