BOOM AND BUST; ANALYZING SHIFTS IN EVOLUTIONARY RATES OF STROPHOMENOID BRACHIOPODS DURING THE LATE ORDOVICIAN MASS EXTINCTION USING A PHYLOGENETIC FRAMEWORK

The influence of mass extinction events on evolution has been an important field of study for both macroecologists and evolutionary biologists. Specifically, do large scale extinctions affect long-term diversification dynamics of clades after the event? Modern phylogenetic methods allow for statistical tests of packaged shifts in evolutionary rates on a phylogenetic tree through time as well as across clades. We present a preliminary study of evolutionary rates of brachiopods across the Ordovician extinction using fossil phylogenies.

We analyzed rates of diversification and extinction within the diverse and abundant brachiopod order Strophomenida (strophomenides) through time as well as across clades. We first analyzed rates on a previously published family-level phylogeny (Congreve et al 2015) that included 61 exemplar species from over a quarter of the total genera within the order. Although taxonomic resolution was coarse, statistical analyses of evolutionary rates using RevBayes suggested speciation rates were initially higher in the early Ordovician and decreased before the extinction event. Similarly, extinction rates were lower during the early Ordovician with a pulse at the late Ordovician. The post-extinction recovery suggests a subtle shift in diversification dynamics, with a steady increase in speciation rates after the extinction, and a drop off in extinction rates to slightly below Ordovician levels. These results may provide some additional support to previous taxonomic-based analyses suggesting the Late Ordovician mass extinction had long term effects on the diversification dynamics of Paleozoic brachiopods (Krug and Patzkowsky 2015).

These results were then compared to evolutionary rates calculated from a new detailed phylogeny focused on a predominately Ordovician strophomenide clade, consisting of species originally assigned to the families Rafinesquinidae, Glyptomenidae, and Strophomenidae. This group radiated extensively during the Ordovician radiation and was strongly affected by the extinction event, but did not experience a substantial recovery after the extinction. Our results demonstrate the importance of scale and clade specific responses when studying the evolutionary impacts of mass extinctions.