PALEOECOLOGY AND FAUNAL ANALYSIS OF THE PENNSYLVANIAN KEECHIE CREEK SHALE AT RAMSEY RANCH, TEXAS

Stratigraphic variation in fossil taxonomic abundance and composition—paleocommunities—can reveal large changes in biodiversity and taxonomic dominance that existed in the past. Fossil beds from the Pennsylvanian Midland Basin of Texas were once home to an abundance of invertebrate marine life, all inhabiting various ecological niches. Fauna within the offshore shale units of the cyclothems within the Midland Basin include a diverse fauna of not only brachiopods, crinoids, and bryozoans, but also echinoids and mollusks. Previous studies on the Pennsylvanian Finis Shale revealed variation through 1 million years in terms of the dominant groups and relative abundance of brachiopods and other mollusks. Here, we examine paleocommunity variation through a new locality (Ramsey Ranch) of the Keechie Creek Shale to evaluate similarities and differences to similar depositional environments from the Midland Basin, such as the Finis Shale.

We collected four 4L bulk-sediment samples from Ramsey Ranch. All fossil specimens >2mm were separated from the sediment, counted, and identified to genus. The abundance of the four samples from lowest to highest stratigraphic position were 44, 50, 204, and 55. These sample sizes were lower than the Finis Shale as well as the Mineral Wells Park outcrop of the Keechie Creek Shale. Richness for the Ramsey Ranch samples were lower as well, with richness from lowest to highest stratigraphic position, being 6, 10, 8, and 7. Evenness upsection was 0.68, 0.80, 0.69, and 0.71. Relative abundances within the samples from lowest to highest position were 34%, 26%, 50%, and 69% for brachiopods; 0%, 2%, 4%, and 2% for gastropods; 60%, 18%, 32%, and 23% for crinoids; 5%, 36%, 10%, and 2% for bryozoans;- and 0%, 16%, 4%, and 2% for other taxa. The relative abundances of the most common brachiopods were 4%, 0%, 10% and 0% for Rhipidomella;-and 50%, 46%, 15%, and 12% for Neochenetes. Throughout the Ramsey Ranch samples, there is a notable and distinct lack of bivalves, contrasting with other Keechie Creek and Finis Shale material. There is a generally low overall richness in Ramsey Ranch as well, in comparison to the other sites, but an unusually high abundance of crinoid and bryozoan material.