RELATIVE ABUNDANCE DISTRIBUTIONS OF BRACHIOPODS FROM THE LATE PALEZOIC OF NEBRASKA AND KANSAS

Stratigraphic boundaries typically are defined by faunal turnover, but not necessarily by any major changes in general ecological structures in communities. Here, we use relative abundance distributions (RADs) to summarize basic ecological structure among Pennsylannian and Early Permian brachiopod communities from Nebraska and adjacent regions. In particular, we assess whether differences in faunal composition correspond to degrees of difference in types of best-fit abundance distributions. We use brachiopod specimen counts from 100+ collections found in 14 formations, from the oldest date to the youngest date. For each collection, we find the best-fit models for five basic models: Geometric, Log-Series, Zero-Sum Multinomial, Lognormal and Zipf. The first three models assume that species compete for the same general resources (e.g., all are suspension feeders) and that factors such as immigration rates, population growth rates have much greater effects on relative abundance than do ecological interactions. The final two models assume that ecological interactions such as niche‑partition, niche construction, and direct interactions also have strong effects on community structure. We find the most likely distributions based on the probability of the observed numbers of species with 1, 2, etc., specimens given a hypothesized true number of species, a particular RAD and the sample size. We then contrast the distributions with differences in taxonomic composition and environment as indicated by rock lithologies.

Previous results and studies indicate that there is no clear temporal or environmental pattern in differences among RAD. There also is not a clear trend towards increasing “modernization” of RADs moving from the Pennsylvannian to the Permian, which is consistent with the notion that the major shift is concentrated in the Permian/Triassic transition. Our goal then is to see if any shift at all is occurring.