Illustration of <em>Amphibamus grandiceps</em>
Illustration by Nobu Tamura of Amphibamus grandiceps, from the Late Carboniferous of Illiniois, considered to have been close to the ancestry of modern amphibians

In Illinois, most of rocks of the Mesozoic and Cenozoic eras are eroded and not present in the geologic record, but there are vertebrate fossils from the Paleozoic Era, more than 245 million years old, and Quaternary Period, younger than 2 million years old. Paleozoic fossils include fish, lizards, and amphibians. Quaternary fossils include extinct animals like mammoths and mastodons, along with animals that still exist, such as horses.