PALEOBIOGEOGRAPHY OF THE LATE ORDOVICIAN RICHMONDIAN INVASION: INFERRING SOURCE REGIONS FROM PHYLOGENETIC PATTERNS

The fossiliferous rocks of the Upper Ordovician Cincinnatian Series record a major biotic immigration event known as the Richmondian Invasion. Previous studies have assessed paleoecological and paleobiogeographic dynamics associated with the Richmondian Invasion, but the biogeographic source of invaders remains unresolved. Competing hypotheses suggest that invaders were native to equatorial Laurentia or temperate basins surrounding the Taconic highlands. Phylogenetic hypothesis can provide an evolutionary framework for testing paleobiogeographic hypotheses surrounding the geographic source of the Richmondian invaders. Herein we assess the source of the invaders based on a phylogenetic biogeographic analysis of species-level phylogenies for Cincinnatian taxa.

In this study, species-level phylogenetic hypotheses for three genera of Late Ordovician articulate brachiopods were constructed: Glyptorthis Foerste, 1914; Hebertella Hall and Clarke, 1892; and Plaesiomys Hall and Clarke, 1892. Following phylogenetic analysis, taxon cladograms were converted to area cladograms. Fitch Parsimony was used to ascertain the biogeographic origin for two invasive species: Glyptorthis insculpta (Hall, 1847) and Plaesiomys subquadratus (Hall, 1847). A Lieberman-modified Brooks Parsimony Analysis (LBPA) was then used to assess the role of geologic processes driving evolutionary and paleobiogeographic patterns concomitant with the Richmondian Invasion.

Results of the phylogenetic biogeographic analysis suggest that species invasions occurred via multiple biogeographic pathways during the Richmondian Invasion. Basins oriented north and east of the Cincinnati region were the geographic source for the brachiopod species analyzed. Comparative biogeography of other taxa, including rugose corals, supports multiple geographic sources for the invader taxa. This support for disparate dispersal pathways in conjunction with the area relationships recovered from LBPA analysis suggest the Richmondian Invasion may have been mediated by tectonic events or shifts in paleooceanographic conditions.