The Paleontology Portal (paleoportal.org) tells us this:
The youngest Paleozoic rocks in Wisconsin are the Devonian limestones and shales found north of Milwaukee. These rocks indicate the presence of a retreating shallow sea. North America was positioned over the equator during this time, and what is now Wisconsin was located in the tropics. The most common Devonian fossils reflect communities that inhabited warm, shallow seas and include brachiopods, bryozoans, trilobites, corals, bivalves, gastropods, cephalopods, sponges, crinoids, shells of single celled animals, conodonts, and the scales and teeth of early fishes and sharks.
But sources concentrate on the geology of land areas only. What about the geology of Lake Michigan? What lies on the bottom of our Great Lake? What animals bore the skulls found on Lake Michigan’s shores?