Rafinesquina ponderosa Ulrich in Hayes & Ulrich, 1903 - fossil brachiopod in limestone from the Ordovician of Ohio, USA. (view of ventral valve; hingeline at top)

This specimen is from the famous Cincinnatian Series of the tristate area of Ohio-Kentucky-Indiana. Rocks in the Cincinnatian were deposited in relatively shallow marine facies during the Late Ordovician. The Cincinnatian succession is mostly interbedded limestones and shales, all richly fossiliferous. Most of the limestones are event beds (= tempestites), deposited during ancient storms.

Seen here is a brachiopod, a group of 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).

Classification: Animalia, Brachiopoda, Articulata (also known as Rhynchonelliformea), Strophomenida, Rafinesquinidae

Stratigraphy: "Grant Lake Formation" (apparently from the Mount Auburn Member), Maysvillian Stage, middle Cincinnatian Series, Upper Ordovician