66 to 252 million years ago
The Mesozoic is the geologic era when dinosaurs lived. There have been no dinosaur fossils discovered in Wisconsin and Illinois because there are very few deposits from this time in these states. The area was exposed and subject to erosion during most of the Mesozoic. About a half dozen small non-marine gravel deposits in the west-central part of Wisconsin have been surveyed. These deposits resemble Cretaceous deposits in Minnesota, but no fossils have been recovered from them. At the very southern tip of Illinois, there are some sand and clay sediments that were deposited in the Cretaceous. Marine sediments contain the microscopic shells from foraminifera and deltaic river deposits include petrified wood from angiosperm trees that show similarities to beach, linden, and dogbane which indicates that a wet forest grew here.
Did dinosaurs roam about Illinois and Wisconsin? Probably, but no direct evidence has been discovered. We can infer that they did by examining fossils discovered in neighboring states. Non-marine deposits in eastern Missouri include clay deposits from coastal swamps. Dinosaur fossils, including hadrosaurid duck-billed dinosaurs and a small tyrannosaurid (possibly Albertosaurus), have been recovered from eastern Missouri, along with turtles, crocodiles, and lizards. In Iowa, fragmented pieces of bones, most likely from duck-billed and ankylosaurian dinosaurs, have been recovered. While to the northeast, in Minnesota, plant fossils of redwoods and ferns suggest a forest similar to the modern day temperate rainforests of the Pacific Northwest once covered part of Minnesota. One hadrosaur bone has been collected in Minnesota.