A DIVERSE OPHIUROID FAUNA FROM THE UPPER PENNSYLVANIAN COLONY CREEK SHALE, TX

Ophiuroidea (brittle and basket stars) is significantly under-documented in the Upper Paleozoic, particularly the Pennsylvanian, due to rapid disarticulation of the skeleton shortly following death. Recently developed methods involving processing mudrock bulk samples and treating ophiuroid remains as microfossils have been successful in improving measures of ophiuroid diversity. Here we examine ophiuroid diversity through the Colony Creek Shale Mb., Caddo Creek Fm, Upper Pennsylvanian (Virgilian), north-central Texas. The Colony Creek is dominated by sparsely fossiliferous, friable shales with stringers of macrofossils, particularly mollusks and echinoderms. Microfossil residues are largely echinoderms, foraminifera, and sponge spicules consistent with a deep, open shelf interpretation.

Five bulk samples were collected from the lower 8 meters of the Colony Creek Shale along US 180, Palo Pinto Co., TX. Samples were processed and unbiased subsamples were picked for ophiuroid lateral arm plates (LAPs) to generate a species census for each sample. Ophiuroid abundance within Colony Creek is quite high with between 167 and 677 (mean 406) LAPs/g of sediment in the 2 phi fraction. Similarly, taxonomic diversity is high including 2 species of stenuroids, 2 species of cheiropterasterids, 3 species of furcasterids, 3 species of protasterids, and 5 species of stem modern clade ophiuroids, suggesting the Late Pennsylvanian may host an increased degree of species richness among modern types. Although archaic groups dominate taxonomically (10 / 15 total species), the modern clades are significantly more abundant comprising 80 - 95% of LAPs in the samples with only slight fluctuations in abundance up section. Modern taxa are also morphologically diverse showing an early diversification if this group. While Pennsylvanian species richness is as high as that seen in the middle Mississippian, the shift in species abundance is striking. Mississippian faunas are dominated by specimens belonging to archaic taxa whereas latest Mississippian through late Pennsylvanian faunas are dominated by modern clade taxa. This shows that the origination of the modern ophiuroid fauna was fully underway in the Pennsylvanian rather than a response the Permian Mass Extinction.