USING PHYLOGENETIC BIOGEOGRAPHY TO LINK TECTONICS, SPECIATION, AND BIOTIC INVASION IN LATE ORDOVICIAN LAURENTIA

Geologic process such as tectonics and global climate change profoundly impact the evolution of life by facilitating episodes of biogeographic exchange or differentiation that influence patterns of speciation and diversification. Late Ordovician (middle Mohawkian) deposits of Laurentia record a dramatic faunal turnover across the M4/M5 sequence boundary. This faunal shift occurs coincident with the Taconian Orogeny and GICE related global cooling, both of have previously been hypothesized to have instigated the observed faunal turnover. In this study, we utilize a time-stratigraphic, species-level evolutionary framework for articulated brachiopods to assess the link between geologic and biologic change in North America via phylogenetic biogeographic analysis.

A Lieberman-modified Brooks Parsimony Analysis (LBPA) was performed on three clades of common and diverse orthid brachiopods: Glyptorthis Foerste, 1914, Plaesiomys Hall and Clarke, 1892, and Hebertella Hall and Clarke, 1892. Results of the phylogenetic biogeographic analysis indicate a fundamental shift in speciation mode across the M4/M5 boundary from a vicariance to dispersal dominated macroevolutionary regime. This shift in dominant speciation style corresponds to the onset of the Taconic tectophase and the development of an upwelling zone that introduced cool, nutrient-rich waters from the Sebree Trough into the epeiric seas of eastern Laurentia.

Together, orogenic activity along the margin of Laurentia and global cooling facilitated changes in habitat structure that reduced opportunities for isolation and vicariance. The subsequent increase in dispersal is associated with increased rates of biotic invasion and may have led to a regional suppression of speciation and increased faunal homogenization.