THE BIOSTRATIGRAPHIC AND BIOGEOGRAPHIC DISTRIBUTION OF THE MIDDLE DEVONIAN BRACHIOPOD ATRYPA SP. EHLERS AND KESLING, 1970

The Middle Devonian brachiopod Atrypa sp. Ehlers and Kesling, 1970 (to be formally named by Jensen and Wicander, in press) is a very abundant and distinctive species present in Devonian-age (early middle-late Givetian) limestones and calcareous shales in Michigan and possibly elsewhere on the North American craton. Originally known as Atrypa sp. by Ehlers and Kesling (1970), this species is currently known only from Middle Devonian strata within the Michigan Basin, and thus has the potential to be a good guide fossil for that time interval. Despite its great abundance and wide distribution within the Michigan Basin, little is known about its biostratigraphy or paleogeographic distribution outside the Michigan Basin.

Currently, Atrypa sp. has only been reported from within the Michigan Basin, where it has been collected from four formations: the Bell Shale, Ferron Point Formation, Genshaw Member of the Long Lake Limestone, and the Gravel Point Formation. The Bell Shale, Ferron Point Formation, and the Genshaw Member constitute its distribution in eastern Michigan, whereas its western Michigan range is restricted to the Gravel Point Formation.

Biometrical analysis performed on 387 specimens indicates a significant difference between specimens from the eastern and western regions respectively of Michigan. The specimens from the western region of Michigan were generally larger than those from the eastern region. The western specimens averaged at 28.9 mm and 29.3 mm, in terms of length and width respectively, as opposed to the eastern specimens average of 23.4 mm and 24.2 mm. However, most specimens fall within the maximum variance limit set by Jensen and Wicander (in press) for the distinctive species, with only a few specimens exceeding those limits. It is highly probable that the differences in size are due to environmental conditions of the Michigan Basin during the Middle Devonian or could represent evolutionary trends to a new species.