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.
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.