English: Composita sp. - fossil brachiopod from the Pennsylvanian 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).

The brachiopod seen here is an athyrid from the Pennsylvanian-aged Greenup Limestone in Illinois. This limestone is a thin interval in a cyclothem, a cyclical, unconformity-bounded package of nonmarine and marine sedimentary rocks (sandstone, coal, shale, limestone). Cyclothems are common in the Pennsylvanian - they formed in response to numerous sea level rises and falls (transgressions and regressions), which in turn were caused by waxing and waning of glacial ice sheets on the ancient small supercontinent of Gondwana.

Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Athyridida, Athyrididae

Stratigraphy: Greenup Limestone Member, lower Greenup Cyclothem, upper Mattoon Formation, Virgillian Series, upper Upper Pennsylvanian

Locality: eastern banks of the Embarras River, 0.2 miles north of Ryan Bridge, north of Lick Hollow, along the Cottonwood Township-Union Township line, northeastern Cumberland County, southeastern Illinois, USA

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See info. at: en.wikipedia.org/wiki/Composita and

en.wikipedia.org/wiki/Cyclothems