144 - 65 million years ago
Wisconsin has no Cretaceous rocks, so it is impossible to say with certainty what Wisconsin was like during the Cretaceous Period. It is unknown whether sediments were deposited and later eroded away by wind, rain, snow, and glaciers, or if no sediments were deposited at all. However, Cretaceous marine deposits in Western Iowa and north-central Minnesota indicate that the western parts of these states had a shallow sea covering them. Fossils found in these states indicate that pterosaurs flew over this sea and plesiosaurs, mosasaurs, crocodiles, and sharks swam in this sea. Other animal which lived in this sea include fish, ammonites, gastropods, oysters, and other bivalves.
Near by non-marine deposits in western Illinois and eastern Missouri include clay deposits from costal swamps. Dinosaur fossils, including the duck-billed dinosaur hadrosaur and a small tyrannosaurid (possibly Albertasaurs), have been recovered from eastern Missouri, along with turtles, crocodiles, and lizards. No dinosaur fossils have been recovered from Illinois. Further 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 found in Minnesota and fragmented pieces of bones most likely from duckbilled and ankylosaurian dinosaurs have been recovered in Iowa.
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. This suggests that western Wisconsin may have had a similar environment as Minnesota during the Cretaceous, and it is reasonable to assume that dinosaurs might have roamed the state, but there is no direct evidence for this. Again, Wisconsin has no deposits positively identified as Cretaceous rocks, so it is impossible to say what Wisconsin was like during this period