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Blast From the Past: Rocks Contain Fossils of Ancient Trilobites

If you dig deep enough into the land, eventually you will reach past the soil and hit layers of solid rock. These layers are called bedrock. In Illinois, our bedrock is mostly types of limestone and sandstone. These stones are both types of sedimentary rock. It is the only type of rock that can contain fossils.

As rocks are dug up in quarries to use for construction purposes, or as land erodes and rocks are exposed, all sorts of fossils can be discovered. Through these fossils we can learn about the living things and habitats that existed long ago. One of the most common fossils found in Illinois is from a creature called a trilobite.

What’s a trilobite?

Trilobites are ancient marine arthropods. Arthropods are animals with an exoskeleton, segmented body parts and jointed legs. There are many types of arthropods living today, including insects, spiders and crustaceans.

Illustrations of nine trilobite species.
Illustrations of trilobite species. (Illustration via Shutterstock)

There were many different types of trilobites. They ranged in size, shape, and number and length of appendages (legs, tails and spines). The smallest were less than a half-inch and the largest over 2 feet, but most were between 1 inch and 2 inches. Some had large compound eyes, while some didn’t even have eyes.

They also had different specialized mouth parts. From these differences, scientists have determined that trilobites had a variety of different diets. There were predators feasting on small fish and worms, scavengers consuming small pieces of waste and grazing on plant beds, and filter feeders eating microscopic plankton and algae.

What all species of trilobites had in common is the three lobes their body is divided into: left, center and right. It is these three lobes that give trilobites their name, tri-lob.

How long ago?

Trilobites swam in Earth’s oceans during the Paleozoic Era. The Paleozoic Era was a long, long, long time ago. Paleozoic means ancient life. This era began 541 million years ago and lasted until about 252 million years ago.

Trilobites were present for the entire 300 million years of this era. There were many different types of trilobites in the beginning. Over time, they became less and less common, until they finally disappeared with a mass extinction that ended the Paleozoic Era.

Where did they roam?

During the time of trilobites, Illinois was much closer to the equator and often under a shallow ocean. The climate in Illinois at the time was tropical, very similar to the Bahamas nowadays. While trilobite fossils are common, it is often just a part or piece that is preserved. Illinois stands out for having many well-preserved and complete trilobite fossils.


Words to know

Appendage: A projecting part of a living organism with a specific function.

Arthropod: An invertebrate animal, such as an insect, spider or crustacean.

Bedrock: Solid rock underlying loose soil deposits.

Exoskeleton: A rigid external covering for the body in some invertebrate animals.

Limestone: A hard, sedimentary rock used as a building material and to make cement.

Sandstone: A sedimentary rock consisting of sand or quartz grains cemented together.


Evidence shows trilobites were able to live in many habitats besides equatorial Illinois. Some crawled on the ground, some hung out in reefs and some swam in the deep seas. In fact, trilobite fossils have been found on every continent.

Many public buildings in our area have been constructed using limestone bricks. That means that just by accident or luck some fossils may be visible on the surface. Keep your eyes out for fossils as you pass by these buildings or walk over walkways made from large rock slabs. You might see evidence of shells, plants and maybe even trilobites!


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