ENVIRONMENTAL AFFINITIES AND THE DIVERSIFICATION OF EARLY PALEOZOIC BRACHIOPODS

The Cambrian and Ordovician diversification of skeletonized marine benthic invertebrates took place in an environmental framework where classes and orders expanded and contracted their distributions along two main environmental axes, onshore-offshore position, and preference for carbonate or siliciclastic substrates. The patterns of expansion are well known at higher taxonomic levels, but how these shifts occurred at lower taxonomic levels is less well understood. Here we address the question whether genera within brachiopod orders expanded their environmental distributions over time, or if the environmental shift occurred primarily through origination and then remained constant within genera over time? We use environmental affinities for lithology, latitude, water depth, and grain size to test for niche conservatism during the Ordovician radiation to address this question. The current study uses over 29,000 occurrences from the Paleobiology Database to evaluate environmental affinities in seven well-sampled brachiopod orders. We account for variation in environmental sampling with an affinity metric based on the log-odds ratio of occurrence counts. Multivariate analysis of the resulting affinities indicates that lithology and latitude are most important in explaining the environmental distribution of these groups during the Ordovician. The Atrypida, Billingsellida, Pentamerida, and Rhynchonellida show net shifts in affinities from carbonates to siliciclastics over time, and from tropical to extratropical latitudes. The Orthida and Strophomenida show no strong affinities for lithology or latitude, and the Lingulida maintained an affinity for siliciclastics and extratropical latitudes. Results for the Atrypida show that most genera shift their lithologic affinity to include a greater proportion of siliciclastic occurrences over time. We will present results to evaluate whether affinity shift of genera is common across all brachiopod orders during the Ordovician radiation.