505-438 Million Years Ago
During the Ordovician Period, Illinois was entirely covered by a shallow sea, which contributed to the deposition of St. Peter sandstone and carbonate rock. The Appalachian Mountains began to form. Trilobites proliferated as horseshoe crabs and early corals developed. Cephalopods, the ancestors of octopus and squid, included Naedyceras, which had a spiral shell like a nautilus, and Orthoceras, which had a straight cone-shaped shell.