PALEOCOLOGY OF FOSSIL COMMUNITIES IN PENNSYLVANIAN SHALES OF TEXAS

The Pennsylvanian of Texas is characterized by cyclical deposition of sandstone, coal, black shale, limestone, and gray shale, in repeated successions known as cyclothems. The gray shale lithofacies within these cyclothems often contain a well-preserved and abundant benthic fauna. Here, we examine the paleocommunity of the gray shale within and through seven stratigraphic sections. Our goal is to test if ecological, evolutionary, or environmental variables are the most likely cause for community variation.

We collected 2-3 bulk sediment samples at different stratigraphic intervals from seven localities in north Texas. Samples were disaggregated, sorted, and counted to calculate relative abundance, richness, and evenness both throughout all seven sites and within the individual sections. Within the stratigraphic sections we examined variation by conducting nonmetric multidimensional scaling (NMDS) ordination.

Combining all samples from each locality, the Finis Shale (895) and Keechie Creek at Mineral Wells Fossil Park (834) had the largest sample size, followed by Jasper Creek Shale (475), Colony Creek Shale (154), and Keechie Creek at Ramsey Ranch (84). The Lake Bridgeport Shale (15) and Salesville Shale (1) were fossil-poor and were omitted from further analysis. There was no stratigraphic trend in richness, which ranges from 14 (Jasper Creek) to 51 (Finis Shale). Evenness among all sites varied from 0.23 (Keechie) to 0.88 (Colony Creek). Within-unit variation in richness and evenness differed among the units. The relative abundance of brachiopods, the dominant taxon in the Keechie Creek and Finis Shale, varied from 93% to 70%, respectively, with the Finis Shale containing a larger proportion of mollusks. NMDS samples clustered by formation, not by stratigraphic position. Based on previously collected data, paleoproductivity and redox proxies did not correspond with community variation. This suggests that, over this stratigraphic interval, environmental conditions of shale deposition were similar during each depositional cycle. There were no consistent faunal or evolutionary trends through or within the seven units. Thus, community composition was likely driven by yet-to-be-determined ecological factors.