Community paleoecology of the Pennsylvanian Winchell Formation, north-central Texas

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Schneider, Christie Lynn

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Many previous fossil community studies have focused on biodiversity issues without examining community structure and dynamics. The research herein investigates how much community information beyond biodiversity can be recovered. An echinoid Lagerstätten in the Lake Brownwood Spillway locality of the Pennsylvanian (Missourian) Winchell Formation contains four new species of echinoids. Archaeocidarids also host Crurithyris and bryozoan epibionts on their spines. The relationship between the echinoids and epibionts was commensal, benefitting the epibionts. Epibionts received associational defense, transport, water currents, settlement sites, and decreased competition. Four recurring community types occurred in Winchell outcrops from the Lake Brownwood Spillway, Perrin, and RP1, and in Kansas outcrops from the Kansas City, Lansing, and Shawnee Groups. Large productids, Neospirifer, echinoids, and Aviculopinna are characteristic fauna of communities in argillaceous limestones. In shales, highly abundant bryozoans, small attaching brachiopods, and crinoids characterize one community type, whereas small attaching brachiopods and tubuliporate bryozoans plus Neospirifer, productids, and Myalina typify a different community type. In fine grain limestones, abundant Composita and echinoids, along with Antiquatonia and bryozoans characterize another community type. Other distinct, non-recurring communities included those containing abundant echinoids in the Brownwood black shale Lagerstätte; Composita and Acanthopecten in packstone layers in the Brownwood black shale, and diverse bivalves plus Parajuresania and Minilya in concretions at Brownwood. Insights into community structure, not just changes in community diversity through time, are possible with detailed sampling and analysis. Ordination analyses revealed limestone/shale patterns in scatterplots and a ternary gradient arrangement of unbaffled, fenestellid-baffled, and phylloid algae-baffled communities. Competition is inferred to occur between large productid brachiopods. Spatial competition is directly seen in fenestellid-baffled shale communities as interspecific overgrowths and non-interference by conspecifics. Epibiosis is very common in all shale communities and provides evidence for unpreserved substrates, but the biotic or abiotic nature of these host surfaces is unknown. Cluster analyses were performed on Brownwood samples using Bambach’s (1983) guild classification, guilds based on the potential for strong competitive interactions, and individual taxa. Guild and taxon analyses agree on large-scale community types, but differ in sample arrangement within units and lithologies. Researcher bias in sampling appears to be minimal.