286-252 Million Years Ago
With the supercontinent Pangaea in place, the terrain in Illinois was above sea level, and there was no deposition. Since the Permian fossil record is missing from our geologic strata, we'll never know for sure if the mammal-like reptile Dimetrodon lived here, but its fossils have been found in the Southwestern United States. The period ended with a catastrophic extinction event of undetermined cause that concluded 252 million years ago. By that time, many invertebrates and over 95% of marine species and 70% of terrestrial vertebrate species had died out, leading to the rise of dinosaurs.