ASSESSING THE FIT OF ENVIRONMENTAL AFFINITIES TO PHYLOGENETIC TREES IN STROPHOMENOID BRACHIOPODS ACROSS THE ORDOVICIAN MASS EXTINCTION

The Late Ordovician mass extinction was the second largest mass extinction in the history of the Earth in terms of the percentage of species lost. Recent studies looking at bulk taxonomic data and individual phylogenetic patterns within clades have suggested strong selection across the event associated with taxonomic environmental affinities (e.g. depth) and selectivity against specific biogeographic provinces (tropical affinity). However, the long-term impacts and selectivity of the extinction are still a matter of significant debate, as species loss across the extinction appears to have been random with respect to taxonomic grouping and morphology, but the subsequent recovery in the Silurian may have been highly taxonomically selective. We propose that some of the conflicting signal between environmental and taxonomic selectivity may be explained by limited niche conservatism in these environmental preferences across the tree of life. To test this hypothesis, we utilized a phylogenetic framework from a previous study on the Strophomenoidea, a diverse brachiopod group that was a major component of Ordovician ecosystems. We time-calibrated our phylogeny and used lithological locality data and geographic occurrence data to investigate the impact of environmental affinity and biogeographic occupation on survivorship patterns for the clade. Environmental affinity was analyzed across three different parameters (carbonate vs siliciclastic preference, tropical affinity, and shallow vs deep water preferences). Affinities were estimated for each taxon using equations modified from Simpson and Harnik (2009) to generate a proportional variable that estimates the strength of environmental affinity for each taxon. These affinities were mapped onto the tree, phylogenetic signal was tested through a series of statistics, and affinity distribution was compared against a series of evolutionary models. Our results suggest that tropical affinity was likely heavily constrained within strophomenoid clades. These results corroborate our previous BioGeoBEARS work which suggested that predominately tropical taxa remain constrained to the tropics throughout the Ordovician and Silurian, whereas clades without a strong latitudinal preference retreated to high latitudes after the extinction.