CAPTURING BIOTICALLY-MEDIATED SPATIAL COMPLEXITY IN PENNSYLVANIAN NON-REEF COMMUNITIES: A BAFFLING CASE STUDY FROM TEXAS AND KANSAS

Spatial complexity (dimensionality) of the environment is an important influence on the biodiversity, trophic structure, and taxic composition of communities. Spatial complexity is partly a function of the topography of the physical substrate, but topography and microenvironment are often biologically mediated through baffling species of great affect (Vermeij, 1987) such as colonial organisms and algae. Reefs are obvious examples of this phenomenon in the marine realm. But does biotically-influenced spatial complexity play a major role in non-reef ecosystems? To address this, we analyzed 75 limestone and shale samples from the Pennsylvanian (Missourian) Winchell Fm. (TX) and Plattsburg Ls (KS). With the exceptions of phylloid algae, echinoids, and crinoids, all taxa were identified to genus.

Polar (Bray-Curtis) ordination of samples using relative Sorenson distance measure, demonstrates a strong gradient on the first two axes (71% of variation explained) in the percentage of baffling organisms, including crinoids, bryozoa, and phylloid algae (multiple regression on the first two axes, adj. r-square=0.63, p < 0.0005). The result is notable because (1) the pattern demonstrates that the relative abundance of species of great affect determines community type and composition, even in a non-reefal setting; (2) the gradient is largely independent of individual baffling taxa; (3) no evidence exists for a gradient based on stratigraphy, locality, or physical variables such as depth, lithology, or grainsize; and (4) the pattern is lacking when the analysis is performed on the limited dataset of brachiopods only. The more comprehensive dataset used in this study captures biologically-mediated gradients, suggesting that evidence for biotically-influenced structure in fossil communities may require more complete taxonomic sampling. Although paleocommunity sampling has tended to focus on limited taxa for logistic reasons, failure to include all available taxa in the analysis may make recognition of important biotic processes impossible.