PHYLOGENETIC PALEOBIOGEOGRAPHY OF LATE ORDOVICIAN LAURENTIAN BRACHIOPODS

Species immigration and dispersal events are common in the fossil record. Reconstructing species migration pathways within a phylogenetic context can provide insight into Earth system events that facilitated species dispersal. Analyses that constrain both timing and directionality of large scale species migration events are important for identifying feedbacks between biodiversity, tectonics, and oceanographic events. In this study, we reconstruct dispersal pathways and mechanisms that operated during a specific episode of interbasinal species invasion, the Richmondian Invasion, which is recorded in the Katian age strata of the Cincinnati Series in the Cincinnati, USA region.

We used Lieberman-modified Brooks Parsimony Analysis (LBPA), a phylogenetic biogeographic method, to identify biogeographic patterns among Richmondian invaders. LBPA provides two outputs: (1) a reconstruction of the biogeographic evolution associated with the speciation events within individual lineages and (2) a reconstruction of the relative order the biogeographic areas underwent vicariance and geodispersal. Previously generated phylogenetic hypotheses of four clades of rhynchonelliform brachiopods were first converted into area cladograms. Then ancestral geographic ranges were optimized and speciation events were characterized as occurring via dispersal or vicariance, when possible. Geodispersal and vicariance matrices were then coded and analyzed via parsimony to reconstruct area relationships.

Of the focal clades examined, three species migrated to the Cincinnati Basin during the Richmondian Invasion from three different ancestral areas: the paleoequatorial region north of the Transcontinental Arch, the midcontinent region, and a peripheral basin. The resulting area cladograms indicate tectonic and oceanographic changes were the primary geologic drivers of biogeographic patterns within the focal taxa. The Taconic tectophase contributed to the separation of the Appalachian and Central basins as well as the two midcontinent basins; whereas sea level rise following the Boda Event promoted interbasinal dispersal. Three migration pathways into the Cincinnati Basin were recognized, which supports the multiple pathway hypothesis for the Richmondian Invasion.