English: Platystrophia ponderosa Foerste, 1909 - geodized fossil brachiopod with geopetal structure in the Ordovician of Kentucky, USA.

This species is also known as Vinlandostrophia ponderosa.

Brachiopods are sessile, benthic, filter-feeding, marine invertebrates. They first appear in Cambrian rocks and were abundant in Earth's oceans throughout the Paleozoic. They were also common in Mesozoic oceans, but are scarce in modern oceanic biotas. Brachiopods have two shells, called valves, that are usually calcareous (made of calcite - CaCO3 - calcium carbonate). Each shell of a brachiopod is bilaterally symmetrical, unlike each shell of a bivalve (clam).

Seen here is a partially geodized brachiopod in fossiliferous limestone. The crystal-lined cavity at top represents a void in the original shell after burial. Fluids filled the void and precipitated calcite crystals (CaCO3 - calcium carbonate). The gray-colored material immediately below the crystal-lined cavity is micrite (= fine-grained calcite sediment; calcitic mud) that accumulated in the brachiopod shell.

The presence of a geodized void above and a micrite filling below makes this a geopetal structure - a geologic feature that indicates the original orientation of gravity-defined “up” and “down” at the time of rock formation. Which way was up? At the time of final deposition, the shell was oriented with the micrite at the bottom.

Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Orthida, Platystrophiidae

Stratigraphy: Corryville Member, McMillan Formation/Grant Lake Formation, Maysvillian Stage, middle Cincinnatian Series, Upper Ordovician

Locality: Flemingsburg South Outcrop - roadcut along Route 11, next to quarry entrance, south of the town of Flemingsburg, Fleming County, northeastern Kentucky, USA (28° 22’ 50.44” North latitude, 83° 44’ 54.93” West longitude)