540-252 Million Years Ago
During the Paleozoic Era, from the beginning of the Cambrian Period about 540 million years ago to the end of the Mississippian Epoch of the Carboniferous Period about 323 million years ago, Illinois was located south of the equator and parts were submerged beneath a tropical sea.
The Paleozoic Era marks life before the time of the dinosaurs. Remarkably diverse species of marine fauna such as trilobites, brachiopods, crinoids, blastoid echinoderms, and corals proliferated until devastated by the Permian extinction event 252 million years ago. Plants, invertebrates, and vertebrates such as reptiles and amphibians also developed, colonizing the land.