THE IMPACT OF BIOGEOGRAPHY ON DIVERSIFICATION AND SURVIVORSHIP DURING 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. Despite the high rates of extinction, few large clades were lost and recovery taxa were distributed broadly across the evolutionary tree, suggesting that the extinction may have had minimal impact on ecological interactions. However, phylogenetic studies of brachiopods, trilobites, and crinoids suggest that the Late Ordovician had a long-term impact on the shape of the evolutionary tree. While clades containing species that were ecologically abundant during the Ordovician survived the extinction event, they diversified at a much lower rate and were replaced by new or sister clades that originated in the Ordovician but were not as diverse or abundant. One possible explanation for this pattern could be that surviving taxa became restricted to a few environments or biogeographic provinces, and were unable to move or adapt as their preferred environment shifted throughout the Silurian. We conducted a phylogenetic study of the biogeographic occupation of the Strophomenoidea, a diverse brachiopod group that was a major component of Ordovician ecosystems. We time-calibrated our phylogeny and used geographic occurrence data to investigate biogeographic shifts through time using the R package BiogeoBEARS. Our results suggest that there may be some correlation between tropical distribution and evolutionary responses to the Late Ordovician mass extinction. Several diverse clades during the Ordovician originated in tropical/subtropical environments and remained within those environments throughout their entire history. These tropical/subtropical clades preferentially went extinct during the Late Ordovician, though the clade that ultimately diversified after the event also originated in these tropical/subtropical environments. Cosmopolitan taxa (whose ranges extend from high latitude to the tropics) survived the event but were restricted to higher latitudes until later diversifying in the Devonian. Our results corroborate previous studies showing increased extinction in tropical faunas during the Late Ordovician, but they also illustrate the importance of clade independent processes in shaping how life responds to mass extinctions and recovery events.