DID ALTERNATING DISPERSAL AND VICARIANCE DRIVE BIODIVERSITY INCREASE DURING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT? A PHYLOGENETIC TEST USING BRACHIOPODS

During the Great Ordovician Biodiversification Event (GOBE), life diversified significantly at lower taxonomic levels for the first time in life’s history. This grand diversification has been correlated with large scale environmental changes such as global cooling, increased oxygenation, increased tectonism and volcanic activity. However, speciation occurs via reproductive and genetic isolation for a prolonged period of time, and large-scale directional changes alone would not facilitate the geographic isolation required to produce new species.

Stigall et al. (2017) proposed that the alternation of Biotic Immigration Events (BIMES), which disperse taxa from one geographic area to another, alternated with vicariance events, which isolated these basins and facilitate speciation, may work as a speciation pump to produce increased diversification during the GOBE. BIMEs would have been associated with a series of oceanic changes which potentially facilitated species migrations during the GOBE. Herein, we test this hypothesis through species-level phylogenetic biogeographic analysis of select brachiopod genera.

Ordovician brachiopods are extremely well preserved, abundant and have been extensively collected, providing an excellent opportunity to study them in a phylogenetic framework to understand speciation and paleobiogeographic history. Phylogenetic and biogeographic histories of three genera, Oepikina, Mimella, and Hesperorthis were estimated viaBayesian and parsimony methods using Mr. Bayes and PAUP*, and biogeographic history was reconstructed using BioGeoBEARS. These genera are ideal subjects due to their broad geographic distributions, abundance and well preserved physical internal and external features.

Phylogenetic analyses were based on a large suite of characters unique to each individual genus coded from museum specimens. Geographic distributions of taxa were determined through literature review and museum labels. Instances of dispersal and vicariance in each lineage were identified from biogeographic reconstructions.

Results showed that BIMES contributed to diversification throughout the GOBE. This diversification was associated with changes in sea-level which would have provided the geographic isolation required for speciation to occur via a series of BIMES.