MORPHOMETRIC ANALYSIS OF THE TRILOBITE ELDREDGEOPS RANA TO ASSESS GEOGRAPHIC PATTERNS OF VARIATION IN THE MICHIGAN AND APPALACHIAN BASINS DURING THE MIDDLE DEVONIAN PERIOD

By investigating morphometric variation of the Middle Devonian trilobite Eldredgeops rana from the Great Lakes region, we can better understand the geographic distribution of genetic populations across the Appalachian and Michigan Basins. Following on a previous study, we have expanded our database to include over 700 specimens of E. rana from New York, Ohio, Michigan, Wisconsin, Indiana, Iowa, and Ontario. Measurements of cephalon length and width were used to determine growth trajectories of trilobite populations from the ancient basins. A subset of 350 specimens were used for geometric morphometrics, using 26 landmarks on the cephalon. Principal components analysis (PCA) and canonical variate analysis (CVA) were used to identify patterns of morphological variation among sampling locations.

Morphometric data suggest that there is a degree of divergence between Eldredgeops from the Michigan versus Appalachian Basins. Samples from New York, taken from a range of sampling localities and museum collections, show an exaggerated asymmetry to the right side of the cephalon that is not seen elsewhere. An unexpectedly strong north/south geographic divide was also observed in the CVA, with a weaker signal in the PCA. The CVA results suggest that the driver of this geographic variation is related to the size of the trilobite eye and eye stalks in proportion to the rest of the overall cephalon. This observation may be related to the overall water depth across and within the Michigan and Appalachian Basins during the Middle Devonian Period; trilobites with large, bulbous eyestalks are found in more nearshore areas (on either side of the Findlay Arch of northern Ohio), while trilobites with more narrow eyestalks are found more offshore (New York). Specimens from Ontario and northern Michigan also tend to have narrower cephalic margins and a wider glabella than specimens from Ohio or New York. Overall morphometric patterns suggest some separation of trilobite populations from the Michigan and Appalachian Basins, but with at least two shallow marine connections, one across northern Ohio and one across Ontario and northern Michigan, present during the Middle Devonian.