BRITTLE STAR (OPHIUROIDEA, ECHINODERMATA) SKELETAL ELEMENTS FROM A NEW LOCALITY (FLORAVILLE, IN): COMPARING THE FAUNAL DIVERSITY OF TWO MISSISSIPPIAN-AGE LOCALITIES

Preliminary studies of disarticulated brittle star (Ophiuroidea) lateral arm plates (LAPs) show an unexpectedly high diversity in the late Mississippian. Over the past three years, this study focused on ophiuroid LAPs from two Mississippian localities and numerous new taxa have been identified as a result of our investigation. Here we test whether a pattern of high diversity among LAPs previously reported is typical for two similarly aged faunas, the Ridenhower Formation of the Paint Creek Group (Upper Mississippian, Chesterian) near Floraville, IN and Big Clifty Fm. (Upper Mississippian, Chesterian), near Sulphur, IN. Faunas were compared while utilizing similar methodologies and collecting techniques.

Fresh, fossil-laden shale was collected from in the field from each locality. The highly friable shale was disaggregated, washed of clays and sieved to appropriate mesh size (>250 microns). Sediment residues were examined, and brittle star fossils were identified and removed by hand using a wet paintbrush. Approximately 90 brittle star plates were recovered from each gram of sediment at grain sizes between 250 and 500µm attesting to the abundance of brittle stars in the fauna.

Preliminary results show a pattern of high diversity from the Ridenhower Fm. that is similar to that of the Big Clifty Fm., each having between 14 and 15 species. Furthermore, both faunas contain similar genera but, in some cases, different species. Both faunas have the large ophiuroid Cholaster but lack Onychaster. Both contain the small stenuroid Umerophiura and three similar species of protsterid/encrinasterids. Furcasterids are common at both localities with three species in the Ridenhower Fm. and four species in the Big Clifty Fm. The cheiropterasterid Vandelooaster is present at both localities as well as a large, smooth form of unknown affinities with large spine bases. Modern type ophiuroids are represented at both localities by Aganaster and a very small, new taxon. Unique to the Ridenhower Fm. are small, distinctive, stenuroid-like ossicles with large spine bases. The similarities of these faunas show that the high diversity ophiuroid faunas previously reported for the Upper Mississippian are not ususual, suggesting that additional research is needed to fully understand Mississippian ophiuroid biodiversity.