GEOGRAPHIC VARIATION AND GROWTH OF THE MIDDLE DEVONIAN TRILOBITE ELDREDGEOPS RANA AS A MEANS OF TESTING THE GEOGRAPHIC ISOLATION OF THE MICHIGAN AND APPALACHIAN BASINS

The extent of mixing of marine faunas of the Michigan and Appalachian Basins during the Middle Devonian period is unclear. Using growth trajectories and morphological variation of the trilobite Eldredgeops rana, we attempt to discover the degree of divergence between populations in these basins. Morphological divergence may be used as a proxy for genetic and geographic isolation; a lack of divergence implies the two ocean basins were not fully separated during the Middle Devonian.

Nearly 300 specimens of E. rana were examined from university and museum collections; they represent a large geographic range encompassing both the Michigan and Appalachian Basins. Included localities are: the Silica Shale near Sylvania, Ohio, and Milan, Michigan; the Plum Brook Shale in Erie County, Ohio; the Moscow Shale in western New York; and the Traverse Group of Alpena County, Michigan. Measurements of the glabella length and width were taken and plotted against each other to create growth trajectories of each geographic population. Trajectories were fitted to a power function. Specimens with adequate preservation were then photographed and subjected to morphometric analysis, using 26 landmarks across the trilobite cephalon. Morphological variation within and between populations was analyzed to infer the degree of genetic variation and geographic divergence.

Growth trajectories for each basin were nearly identical across the sampled size range. Morphometric data suggest a lack of divergence, indicating that populations of E. rana were likely mixing across different geographic areas, leading to genetic stasis. The Michigan Basin and Appalachian Basins therefore likely maintained a connection throughout the Middle Devonian period.