What Are Fossils?
A fossil is some evidence of a prehistoric animal or plant, preserved in rock, that gives a clue to the characteristics of the organism. The remains of animals or plants that lived during historic time are not considered fossils.
The oldest fossils in Illinois are found in rocks such as sandstone, limestone, shale. Some are only impressions of the outside of a shell; some are fillings of the inside. Parts of the original shell may be preserved, but in many fossils, the hard parts of the animal have been replaced by a material different from that of the original. Silica and calcium carbonate, which are readily preserved, commonly replace the original shell material.
Some fossils were made by marine worms that burrowed in the sand or mud of the sea floor. The worms themselves are rarely found as fossils, but their trails and burrows are common. The tracks left by prehistoric animals are fossils just as much as the animals themselves would be if they had been preserved.
In many places in Illinois, shells of clams, snails, and brachiopods are preserved with little change and are much as they were the day they were buried on the floor of the prehistoric sea.
The plant fossil materials that make up the coal beds of Illinois are the remains of primitive trees and plants that lived in swamps during the "Coal Age," or Pennsylvanian Period. When the plants died, they fell into the water and were preserved as peat, which later became coal. Many fine fossils found in the coal and overlying shales represent the roots, trunks, and leaves of the plants. A few of the insects that lived in the trees also are preserved.
Among the youngest fossils found in Illinois are the teeth and bones of bison, giant beaver, deer, and elephant-like animals called mammoths and mastodons, all of which lived during the "Ice Age," that is, the Pleistocene Epoch when glaciers moved in and out of Illinois. Complete skeletons of the animals are rare, but teeth and tusks are found in near-surface excavations.