From Ohio History Central
Brachiopods (Phylum Brachiopoda) are bottom-dwelling marine organisms with two shells, or valves, made of calcium carbonate. But, unlike clams, the shells differ in shape, and sometimes size. Although brachiopods range from Cambrian to Recent, they were dominant marine invertebrates in the Paleozoic. Many Paleozoic limestones and shales, particularly those of Ordovician age in Ohio, are literally a fossil hash made up mostly of brachiopod shells. Most species were small, less than an inch, but some were up to two inches in longest dimension. Although most species have ribbed shells of various forms, some brachiopods of the Mississippian and Pennsylvanian developed long, thin, sharp spines on their shells, probably as a defense against many species of shell-crushing sharks that became abundant at that time.
Ordovician and Devonian marine rocks, particularly limestones and limy shales, have a great diversity and abundance of brachiopod remains. Silurian carbonates have locally abundant brachiopods but in many areas dolomitization of Silurian limestones has destroyed the calcium carbonate shells. Sandy marine shales and some limestone beds locally have abundant brachiopods, although in some case preservation is not ideal. Marine limestones and shales of Pennsylvanian age have yielded brachiopod fossils. Brachiopods are easy to find and collect and are a good starting point for the beginning collector. Their diversity makes them of interest to the advanced collector as well.