Platystrophia ponderosa with
GEOPETAL STRUCTURE
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.