144-66 Million Years Ago
The Cretaceous witnessed the rise of flowering plants. Toward the end of the period, the northernmost point of the Mississippi Embayment renewed marine deposition in southernmost Illinois. The impact of a huge asteroid at the end of the Cretaceous Period decimated the dinosaurs and opened the way for mammals.
While dinosaur fossils have not yet been discovered in Illinois, fossils in the Hell Creek Formation of North Dakota record Cretaceous flora and fauna affected by the asteroid impact, including fresh water and marine animals, vegetation, and terrestrial vertebrates like Triceratops, a dinosaur common to North America.