The Paleozoic rocks of Illinois have long been known for their abundant and well-preserved trilobite fossils. Cambrian age trilobites have been found in a few small outcrops in north-central Illinois and in several cores drilled from deeply buried rocks at various localities throughout the state. Some of the better trilobite collections have been made from the more accessible Ordovician and Silurian rocks that are exposed in the northern and southwestern parts of the state. Trilobites have been found in some post-Silurian Paleozoic rocks in Illinois, but they are much less abundant and diverse than those in the older rocks. Trilobite dating The study of trilobites is not just an academic exercise.
Trilobites are useful in determining the relative age of some sedimentary rocks.
Knowing the relative age is important for economic reasons, particularly where it is necessary to locate and identify strata containing oil, natural gas, coal, and ore deposits. The study of such index fossils and their relationship to the strata in which they are found is called biostratigraphy. Geobits may also be ordered by mail, telephone, fax or email.