To discuss what creatures were present millions of years ago in what is now Illinois, we must first talk about what this area and the rest of the earth was like in that time.
First, the climate during this time was much warmer than it is today. Most of the area that is Illinois today was actually part of a swampy tropical forest.
North America did not look like it does today. A vast gulf of water separated most of eastern North America from the western mountains. North and South America were also separated by water.
Because of the warmer temperatures and ocean currents at that time, ice caps at the poles were not present. This time period is thought to be near the end of the dinosaurs.