Brachiopod shells are probably the most commonly collected fossils in Kentucky. Brachiopods are a type of marine invertebrate (lacking a backbone) animal. Their shells have two valves attached along a hinge, similar to clams. Although they had two shell valves protecting soft parts inside, as clams (bivalves, pelecypods) have, all similarity ends there. Brachiopods and mollusks are in different phyla (large divisions of life) because they have different body symmetries and internal structures. Brachiopods are a distinct phylum of organisms, containing many classes, orders, families, genera, and species. Clams are in the class Bivalvia, which is in the phylum Mollusca. Clam valves are mirror images of each other. The valves are symmetrical along a plane through the hinge. In contrast, brachiopod valves are dissimilar to each other. They are symmetrical along the midline of the valves perpendicular to the hinge.
In most fossil brachiopods, one of the valves has an opening through which a fleshy pedicle extended. This valve is termed the pedicle or ventral valve. The other valve is termed the brachial or dorsal valve. In life, the pedicle attached to the seafloor or other objects.
Brachiopods are marine filter-feeding organisms. They take in water and particles through the opening between their valves and filter food particles from the water through soft parts, called lophophores, within their shells. Brachiopods still exist today, but are much less common than clams (bivalves) and very rarely found as seashells on the beach. In the Paleozoic Era, however, brachiopods were abundant and far outnumbered the shells of clams and snails living in the sea. Brachiopods are common fossils in Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian rocks in Kentucky. They are the state fossil of Kentucky.
Some Kentucky brachiopods, at Kentucky Paleontological Society webpage