Platystrophia ponderosa with
Geopetal structures are geologic features that indicate the original orientation of gravity-defined “up” and “down” at the time of rock formation. The specimen shown below is a broken orthid brachiopod shell (Platystrophia ponderosa Foerste, 1909; Animalia, Brachiopoda, Orthida, Platystrophiidae) containing syndepositional and post-depositional material that infilled the shell after decay of soft tissues. The light- to medium-gray material in the lower two-thirds of the shell is lime mud (sedimentary micrite). The whitish to pale yellowish-gray crystals in the upper third of the shell are calcite. Note that the calcite crystals have not completely filled up the available empty space.
So, which way was up? At the time of final deposition, the shell was oriented in the position you see it now. The calcite crystals are partially filling what was originally water-filled empty space in the shell. Not enough lime mud entered the shell interior to fill it up, so the mud obviously accumulated in the “down” portions of the shell. The “top” portions of the shell were left empty, until diagenetic or post-diagenetic fluids that were rich in dissolved calcium carbonate precipitated the calcite crystals.
Geopetal structure inside Platystrophia ponderosa brachiopod shell (2.8 cm across) from the Upper Ordovician of northeastern Kentucky.
Interestingly, the up-and-down orientation indicated by the geopetal structure is not the brachiopod’s biologic up-and-down orientation. The dorsal shell (biologic up) of this fossil sits below the lime mud accumulation. The ventral shell (biologic down) sits above the calcite crystals.
The brachiopod’s preferred life position, however, most likely had the anterior end facing upward and the hingeline area facing downward.
Stratigraphy: float from the Bellevue Limestone, Maysville Stage, middle Cincinnatian Series, Upper Ordovician.
Locality: loose piece from upper portions of the Maysville West outcrop (= large roadcut through Jersey Ridge along the Rt. 62/Rt. 68 bypass west of Maysville - the 1st hill south of Harsha Bridge over the Ohio River), northern Mason County, northeastern Kentucky, USA.