Paleontology and geology
During the Devonian, shallow tropical seas continued to cover Michigan. Fossils are particularly abundant in the rocks of this time and include the trilobite, Phacops, many species of brachiopods, cephalopods, snails, crinoids, and Hexagonaria, the coral more commonly known as the Petoskey Stone (Michiganís state rock). The vertebrates are represented by plates of armored fish. This map indicates a broad exposure of Devonian rocks along the southern shore of the upper part of Lake Michigan, as well as in the southeastern and southwestern parts of the state.