APPLYING BAYESIAN AND MAXIMUM-LIKELIHOOD METHODS TO CENTURY-OLD HYPOTHESES: THE LATE ORDOVICIAN RICHMONDIAN INVASION

The Late Ordovician Richmondian Invasion involved the immigration of over 60 genera of benthic invertebrates into the Cincinnati Basin, OH region. Multiple competing hypotheses about the geographic origin of the invaders and the processes that facilitated the faunal migration event have been debated over the past century. Parsimony methods, along with stratigraphic analyses at the basin scale, have been applied to biogeographic areas occupied by the invaders to determine their source location(s). However, new advances in probabilistic models provide new quantitative tools for characterizing vicariance and dispersal events using maximum likelihood and Bayesian approaches.

Ten published species-level phylogenetic hypotheses of brachiopods and trilobites spanning the Middle to Late Ordovician were used to test the performance of new maximum-likelihood and Bayesian methods on Paleozoic taxa to interpret dispersal pathways on a global setting during the Late Ordovician Richmondian Invasion, with special attention to Laurentia. Phylogenies were time stratified across four time slices (T0-T3) and speciation events were characterized within each phylogeny using the R package BioGeoBEARS.

Results of the analysis indicate that speciation via dispersal was more prevalent than vicariance within the clades across all timeslices. In addition, long-distance founder-event speciation, or jump dispersal, was found to occur in all ten phylogenies. This is the first time jump dispersal, which is typical among modern island clades, has been detected in deep-time phylogenies using rigorous statistical methods. These results identify jump dispersal as an important speciation type among benthic Paleozoic taxa. In addition, several multidirectional dispersal paths were operational before and during the Richmondian Invasion interval. A suite of tectonic and paleoclimate factors, along with larval type of invasive taxa, influenced dispersal direction and pathways used by the invaders.