Carboniferous Period

Diorama of a Late Paleozoic forest floor
Cleveland Museum of Natural History diorama of a Late Paleozoic forest floor, photographed by James St. John

360-286 Million Years Ago

Known as the age of amphibians, the Carboniferous Period is also marked by the emergence of lizard-like amniotes, the ancestors of reptiles, dinosaurs, and mammals. In contrast with amphibians, amniotes lay eggs on land or retain the fertilized egg within the mother and lack a larval stage.

Mississippian Epoch: 360-320 Million Years Ago

Deposition patterns of shifting coastlines are preserved in limestone and Warsaw shale. Fossils include brachiopods, bryozoans, and crinoids.

Illustration of <em>Tullimonstrum gregarium</em> by Nobu Tamura
Illustration of Tullimonstrum gregarium by Nobu Tamura

Pennsylvanian Epoch: 320-286 Million Years Ago

The supercontinent Pangaea formed during a time of tropical swamp forests. An organism fossilized in Illinois and found nowhere else in the world is Tullimonstrum gregarium, affectionately known as the "Tully Monster". First discovered in 1955 by Francis Tully in Mazon Creek, a fossil bed in central Illinois, it became our state fossil in 1989. Coal and flourite also derive from this epoch.