Morphological Evolution in an Atrypid Brachiopod Lineage from the Middle Devonian Traverse Group of Michigan, USA: A Geometric Morphometric Approach

Geometric morphometrics were used to assess evolutionary mode and correlation with environmental factors in the shell morphology of an atrypid brachiopod species lineage. Seven landmark measurements were taken on the dorsal valve, ventral valve, anterior and posterior regions of over 1,100 specimens of Pseudoatrypa cf. lineata taxon from the Middle Devonian Traverse Group of Michigan State to quantify shell shape. Specimens were partitioned by their occurrence in four stratigraphic horizons (Bell Shale, Ferron Point, Genshaw and Norway Point) from the Traverse Group of northeastern Michigan outcrop. Geometric morphometric and multivariate statistical analyses were performed to test patterns and processes of morphological shape change of species over 5 m.y. interval of time. Maximum-likelihood method was used to determine the evolutionary rate and mode in morphological divergence in this species over time. Three hypotheses were tested regarding patterns of evolutionary change: (1) if the species conforms to a punctuated equilibrium model, there should be no significant differences between successive stratigraphic samples; (2) if the species evolved in a gradual, directional manner, then samples from successive stratigraphic units should be more similar than ones more separated in time; (3) if morphological shape was affected by change in environmental factors like water depth, etc. then we would expect a strong correlation between changes in such factors and changes in shell shape. MANOVA showed significant shape differences in mean shape between stratigraphic units (p ≤ 0.01), but with considerable overlap in morphology. There was little change in the lower part of the section, but a large jump in morphology between the Genshaw Formation and the overlying Norway Point Formation at the top of the 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. However, shallow water depth samples were significantly different than medium depth samples. Collectively, these results suggest that shell morphology did not change through the lower 61 m, but made a sharp jump between the Genshaw and Norway Point formations. Erosional unconformities below Norway Point Formation coupled with environmental heterogeneity during this time interval, may have lead to provinciality in the Michigan Basin sections, thus resulting in greater morphological change. Thus, the change in the 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 intervals in the Traverse Group show abrupt deviation from the lowermost intervals unlike the Hamilton Group where morphological overlap was prominent between lowermost and uppermost units. Thus, the morphological trend observed in the P. cf. lineata lineage in the Michigan Basin appears to be local in scope.