EVOLUTION ON A TEMPORAL SCALE: A CASE STUDY OF A DEVONIAN BRACHIOPOD SPECIES IN THE TRAVERSE GROUP OF NORTH EASTERN MICHIGAN, USA GIVETIAN, ALPENA, PRESQUE ISLE, ATRYPID, STASIS, MORPHOMETRICS

Geometric morphometrics were used to examine the evolutionary and ecological processes affecting the shell morphology of an atrypid brachiopod species lineage. Seven landmark measurements were taken on dorsal, ventral, anterior and posterior regions of over 1100 specimens of an atrypid species from the Middle Devonian Traverse Group of Michigan to quantify shell shape. Specimens were partitioned by their occurrence in four stratigraphic horizons (Bell Shale, Ferron Point, Genshaw and Norway Point). Multivariate statistical analyses were performed to test patterns of morphological shape change of species over 5 m.y. time. Maximum-likelihood method was used to determine the evolutionary rate and mode in morphological divergence in this species.

MANOVA showed significant differences in mean shape between stratigraphic units (p≤0.01), but with considerable morphological overlap. There was little change in the lower section, but a large jump in morphology between Genshaw and overlying Norway Point Formation in the upper section. Maximum likelihood estimation suggests that morphological evolution was lightly constrained, but was not subject to strong stasis. Rates of evolutionary change were slow to moderate. Euclidean based cluster analysis demonstrated that samples from successive units were more similar than widely separated ones. Changes in water depth do not show any statistical correlation with changes in shell shape with shallow depth samples being significantly different from medium ones.

The change in Norway Point samples could be interpreted as the origin of a new species, either from environmental selection pressure or by an immigration event. Comparison of Michigan Basin sections with the contemporary Appalachian Basin sections suggests that morphologies from the uppermost units in the Traverse Group show abrupt deviation from the lowermost units unlike Hamilton Group where morphological overlap was prominent between the lowermost and uppermost units. Thus, morphological trend in this atrypid species in the Michigan Basin appears to be local in scope.