Paleontology and geology
The shallow seas that had covered Michigan for so much of its early history withdrew for the last time during the early part of the Carboniferous (Mississippian). The sea left behind invertebrate marine fossils such as crinoids, blastoids, clams, and corals. By this time trilobites had become rare. In the Late Carboniferous (Pennsylvanian), nearshore coal-forming swamps were common. Plants dominate the fossil record of this time period. This map indicates that Carboniferous rocks cover most of the state south and east of Lake Michigan.