Found in the fossil record as early as the Cambrian Period more than 500 million years ago, the highly diversified brachiopods served as some of the most abundant reef-builders during the Paleozoic Era. The Permian extinction event around 251 million years ago that wiped out 96% of all marine species, including trilobites, diminished brachiopod diversity by two-thirds. While there are 5000 genera in the fossil record, just 100 remain. They live only in the ocean, avoiding strong currents and waves. They have an upper and lower shell which can be made of lime, phosphate, or other hard substances. Most attach to the floor of the ocean using a fleshy stalk that extends from the soft body inside the shell. In Illinois, brachiopod fossils are preserved in limestone and shale bluffs along the Ohio and Mississippi rivers.