Comparative Trends in Evolution Across Correlated Geological Strata

While many Devonian correlated stratigraphic units are yet to be tested for species evolution, the middle Devonian Hamilton Group and Traverse Group have been studied to investigate the same. Fossil brachiopods were used as tools that allowed close investigation of the evolutionary response of these species across correlated strata in Eastern America. Results show that their evolutionary patterns are uncoordinated in nature. Thus, this study reveals that evolutionary patterns in brachiopod species lineages do not seem to match in correlated strata at the regional scale in the United States. Abrupt change was noted in the upper strata of the Traverse Group of Michigan samples from the lower ones. This 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 evolutionary patterns are not linked in nature. Morphologies from the uppermost units in the Traverse Group show sudden deviation from the lowermost units, unlike Hamilton Group where morphological overlap was prominent between the lowermost and uppermost units. Thus, morphological trend in a fossil brachiopod species lineage in the Michigan Basin appears to be local in scope. Other correlated Devonian strata like the Silica Shale of Ohio and the Arkona Shale of Ontario are yet to be tested for species evolution.