Prehistoric animals lived in water, on land, and in the air, and left
both direct and indirect evidence of their existence, evidence we now
call fossils.
Millions of ancient animals died without leaving a trace, but some,
especially those that had hard parts such as shells, bones, or teeth,
may be found preserved in rocks much as they were when buried beneath
sediment on the floor of an ancient sea. Sometimes only imprints of the
outside or fillings of the inside of the shells remain, the original
material having been completely dissolved. Footprints of land or
amphibious animals, burrows made by clams, or holes made by worms also
are fossils.
[Illustration: uncaptioned]
The animals whose remains are fossilized lived and died while the
sediments that contained them were being deposited, and they provide
clues to the types of life and climate then existing. Fossils of animals
characteristic of a certain time are an index to the age of formations
where they occur. For example, if a certain trilobite (an ancient
relative of the crayfish and lobster) is known to have lived only during
a definite time, then all rocks in which it is found are the same age.
Fossils of animals that lived in the sea are exposed in rocks in many
parts of Illinois, especially in quarries, river bluffs, and road cuts.
The oldest fossils found in Illinois are shells of marine
animals—snails, corals, crinoids, brachiopods, trilobites, pelecypods
(clams), cephalopods, bryozoa, arthropods, and others. The youngest
fossils are teeth and bones of prehistoric bison, giant beavers, deer,
mammoths and mastodons of the “Ice Age,” and snails found in glacial
loess.