BIODIVERSITY AND TAXONOMIC TURNOVER OF LATE PENNSYLVANIAN – EARLY PERMIAN OPHIUROIDS OF TEXAS, OKLAHOMA AND KANSAS, USA

Characterization of fossil brittle star (Ophiuroidea) faunas has been revolutionized by investigating assemblages of lateral arm plates (LAPs) as microfossils. These abundant and distinctive skeletal elements can be identified to species. Because each ophiuroid has hundreds to thousands of these elements, small (approximately 3 liters / one gallon) sediment samples generally yield all members of the fauna. Specimens collected in bulk samples of poorly lithified mudstones and shales are often exquisitely preserved and offer an easy method to characterize these otherwise taphonomically fragile species in faunas. Pilot studies in the Upper Mississippian (Chesterian / Uppermost Visean) Big Clifty Fm. and Glen Dean Fm. of the Illinois Basin recovered 12 and 14 ophiuroid species respectively. While not studied quantitatively, estimated relative rank abundance suggest that these faunas were dominated by stem lineage ophiuroids including Furcasterids, Stenuroids, Cheiropterasterids and Protasterids whereas two species of modern type taxamade up significantly less than half of the specimens. Initial samples of Upper Pennsylvanian – Lower Permian faunas from Texas, Oklahoma and Kansas generally have fewer ophiuroid taxa (mostly between 6-10 species) but often have extremely high specimen abundance of up to 240 LAPs per gram retained on the 2 phi sieve. Nearly all genera of Late Mississippian faunas are present and at least three additional genera occur only in these younger faunas. Where Late Mississippian and Late Pennsylvanian/Early Permian faunas differ most is in the abundance of modern type brittle stars. For example, the Lower Permian Watts Creek Fm. contains a few specimens of Furcasterid, Cheiropterasterid, and Protasterid species, but two modern type species comprised 97.4 percent of LAPs collected. In fact, all of the Late Pennsylvanian – Early Permian faunas so far sampled are dominated by a modern type species. Future work will sample these faunas across paleoenvironments to determine if the different ophiuroid taxa display habitat partitioning, providing insight into the ecological dynamics controlling biodiversity and clade replacement in Paleozoic ophiuroids.