English: Pentamerid brachiopods in weathered fossiliferous dolostone from the Silurian of Illinois, USA.

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).

Pentamerids are usually large brachiopods that are moderately common in Silurian dolostones of America's Midwest. The calcite shell has usually dissolved away, leaving an internal mold.

Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Pentamerida, Pentameridae

Stratigraphy: Racine Formation, upper Niagaran Series (= upper Wenlockian), upper Middle Silurian

Locality: unrecorded/undisclosed site at or near the town of Grafton, southwestern Illinois, USA