Paleontology and geology
In the Ordovician, Wisconsin had a tropical to subtropical climate. A shallow sea covered the state, and sediments representing the nearshore environment contain fossils of colonial corals and bryozoans, as well as cephalopods. A brief ice age occurred at the end of the Ordovician. Although no glaciers reached Wisconsin, so much water was contained in glaciers elsewhere that sea level declined and drained the sea. These climate and sea level changes caused a mass extinction. Wisconsin provides one of the richest fossil records for the study of this worldwide extinction.