QUANTIFYING THE ECOLOGICAL COMPONENTS OF PHENOTYPIC DIVERSIFICATION DURING THE “AGE OF CRINOIDS” WITH PHYLOGENETIC COMPARATIVE METHODS

Understanding how ecological differences among species affect patterns of diversification is crucial for elucidating general patterns and processes that regulate the origin, maintenance, and modification of Earth’s biodiversity. Phylogenetic Paleobiology provides a useful template for documenting patterns of ecological trait evolution in a tree-based statistical framework and is a powerful approach for testing ecological theories of evolutionary radiation using fossil data.

Historical geology textbooks commonly characterize the Carboniferous as the “Age of Crinoids” because it marks the maximal Phanerozoic diversity and abundance of these geologically significant echinoderms. However, few studies have examined patterns of morphological evolution during this interval and only rarely within a phylogenetic context. Moreover, most workers have investigated this radiation in terms of total morphological disparity rather than the evolution of ecologically pertinent traits, which is arguably more important when investigating links between ecology and diversification.

To better characterize the ecological components of this major radiation, I first collected measurement data for morphological characters related to niche differentiation and life history using a global sample of middle to upper Paleozoic eucladid species. Next, I constructed an informal backbone supertree for these species and inferred a Bayesian posterior distribution of time-calibrated phylogenies using fossil tip-dating methods. Specifically, I used the time-varying (piecewise constant) implementation of the sampled-ancestor fossilized birth-death model, which accounts for temporal variation in diversification, fossil preservation, and sampling ancestor-descendant relationships. Finally, I applied phylogenetic comparative methods over a sample of trees from the Bayesian posterior distribution to evaluate the fit of alternative process-based models of phenotypic diversification (e.g., Brownian motion, Trend, Late Burst, OU, etc.). Preliminary results find multiple clades rapidly evolved similar morphologic designs during the late Carboniferous, which suggests a shared response to similar ecologic or selective pressures and is associated with a major transition in global crinoid faunas.