DO MASS EXTINCTIONS INFLUENCE THE MORPHOLOGICAL DIVERSIFICATION OF SURVIVING LINEAGES? MODELING THE TEMPO AND MODE OF TRAIT EVOLUTION IN ORDOVICIAN-SILURIAN FLEXIBLE CRINOIDS

The Late-Ordovician mass extinction (LOME) is estimated to have resulted in the extinction of >70% of marine species at a global scale. This extreme loss dramatically affected the diversification trajectory of major clades and led to significant discontinuities in the evolutionary history of their constituent higher taxa. Crinoids, a diverse and abundant clade of echinoderms, underwent a major transition between Early and Middle Paleozoic crinoid macroevolutionary fauna (CMF) as a consequence of the LOME. Although turnover between CMFs is well documented, questions regarding the timing of this turnover with shifts in the rate of morphological evolution, changes in disparity, and possible shifts and/or expansions in morphospace occupation remain largely unanswered. Here, we use the Flexibilia (Crinoidea, Echinodermata) as a model to explore macroevolutionary tempo and mode dynamics surrounding this major transition. Notably, flexibles originated during the Ordovician, but subsequently diversified to become an important constituent of the Middle Paleozoic CMF following the LOME.

We investigate patterns of phenotypic evolution in three continuous traits measured for Ordovician—Silurian species of Flexibilia, including possible ancestors within the paraphyletic lineage Cupulocrinus. These traits include body size, calyx complexity (i.e., number of plates), and filtration fan density (approximate number of feeding appendages). Using a Bayesian distribution of time-calibrated, informal phylogenies of flexible crinoids as a framework, we fit and compare multiple, process-based models of trait evolution to these three traits. In additional to fitting standard, canonical models (BM, OU, early bursts, etc.), we also fit paleontologically-informed models of trait evolution, including Slater’s (2013) “ecological release” and “release and radiate” models, to test whether the turnover in CEFs associated with the mass extinction resulted in temporal shifts in evolutionary rate, mode, or both. By quantifying variations in tempo and mode during this time period, we can begin to understand overarching questions relating to the impacts of mass extinction events on morphological evolution.