If you think 200 years of statehood is a significant milestone, consider this: trilobites, in one form or another, lived in the seas that once covered Illinois for a span of almost 300 million years. The oldest specimens date to about 500 million years ago, while the last trilobites died out about 200 million years ago. This specimen dates back to the Silurian Period (408-438 million years ago) and was found near Grafton, Illinois. Trilobites were a successful group, and they are very common in the fossil record. They are often used as index fossils, allowing geologists to date the layers where trilobites and other fossils are found. Trilobites are also popular with fossil collectors due to their diversity of shapes and sizes.
Trilobites get their name from the three lobes that make up their bodies (tri-lob(e)-ite). Download this PDF to learn more about Illinois trilobites and to see a timeline showing when they lived and how they changed and evolved through time.