Rafinesquina ponderosa Ulrich in Hayes & Ulrich, 1903 - fossil brachiopod in limestone in the Ordovician of Ohio, USA. (external surface of ventral valve)

This outcrop is in 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 fossiliferous limestones and shales. Most of the limestones are event beds (= tempestites), deposited during ancient storms. The exposure seen here consists of interbedded shales and nodular limestones of the upper Arnheim Formation.

The fossil at center is a weathered and partially eroded Rafinesquina ponderosa, a common strophomenid brachiopod in the Cincinnatian Series. 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).

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

Stratigraphy: upper Arnheim Formation, lower Richmondian Stage, upper Cincinnatian Series, upper Upper Ordovician