Fossils provide a unique window into ancient life, giving scientists and enthusiasts alike crucial clues about the Earth's biological and geological past. Illinois has a rich fossil record, from ancient shallow seas to lush, swampy forests and the legendary Tully Monster. The prehistoric environments in this state created ideal conditions for fossilization, delivering fossil-rich sites like the Mazon Creek beds, Thornton Quarry and more.

Table of Contents

• How Are Fossils Formed?
• What Fossils Are in Illinois?
• Were Dinosaurs in Illinois?
• What Are Popular Fossil Sites in Illinois?
• Unearth Dinosaur Fossils at Jurassic Gardens

How Are Fossils Formed?

Fossils are the preserved remnants of ancient life, typically found in rock formations built up over millions of years. The process of fossilization is rare, requiring particular conditions to prevent decay and preserve an organism's structure over time.

Plant and animal decomposition is usually quick, but if the remains are buried in sediment like sand, ash or rock, they may be shielded from decay and scavengers. Over time, more sediment layers accumulate, compressing and protecting the remains from these elements. The layers eventually harden into sedimentary rock, which encases the remains.

The underground conditions will determine the next step in fossilization. In some cases, the organism's hard parts, like shells, bones or teeth, are replaced by minerals seeping through groundwater, turning them into stone. This process is known as permineralization. Other fossil types, like plants or soft-bodied organisms, can form as impressions or molds in the surrounding rock.

Fossilization is a time-consuming process that requires just the right environmental factors, which is why these finds are so extraordinary. 

What Fossils Are in Illinois?

Fossils in Illinois are mostly from the Carboniferous period, around 300 million years ago when much of the state was covered by warm, shallow seas and lush, swampy forests. This unique landscape created ideal fossilization conditions, preserving a diverse range of species, from early land plants to marine invertebrates and even primitive vertebrates. Popular fossils in Illinois include:

• Sea lilies: Crinoids, or sea lilies, are common in limestone deposits. They are marine animals related to stea stars that collectors can find in rock formations and riverbeds.
• Various ferns: The state's Carboniferous swamps preserved ferns and seed ferns like Alethopteris and Neuropteris, which give us intricate details of leaves and fronds from prehistoric vegetation.
• Fish teeth: Fossilized shark and fish teeth occasionally surface in Illinois, especially in deposits around rivers and coal mines. These small but significant finds show just how diverse the marine life in the area was.
• Ammonoid fossils: Fossils of brachiopods — marine animals with shells, and ammonoids, which are related to modern octopuses and squid, are widespread in the state. These are typically found in shale deposits and limestone from the ancient seafloor.
• The Amphibamus: The amphibamus, meaning equal legs, forms part of the amphibian family tree. It shows a small creature that may mark the evolutionary turning point where frogs and salamanders split from mainstream amphibian evolution.
• Lepospondyl amphibian: The lysorophus is an eel-like amphibian in the lepospondyl family. It has vestigial limbs and is mostly found in the state's southwest corner.
• Mammoths and Mastodons: Although Illinois was mostly geologically unproductive, herds of American Mastodons and Wooly Mammoths passed across the state during the Pleistocene epoch.

The Prairie State has several sites which are world-famous for their exceptionally well-preserved fossils, including rare soft-bodied creatures. These fossils attract scientists and enthusiasts as they provide valuable insight into the ancient ecosystems that once thrived in this region.

What Is the Tully Monster?

The Tully Monster, or Tullimonstrum gregarium, is the state's most iconic fossil. It is an off-soft-body marine creature with a long, segmented body, claw-like appendage and stalked eyes. It was first found in Mazon Creek, and as this creature still lacks an exact classification, it is a fascinating subject for paleontologists. The Tully Monster was recognized as the State Fossil in 1989.

Were Dinosaurs in Illinois?

While dinosaurs may have existed in the state in theory, no fossils support this idea. It is possible that the rocks containing these fossils have eroded away, but there is only proof of small nocturnal mammals.

What Are Popular Fossil Sites in Illinois?

There are several great fossil sites that showcase Illinois' rich prehistoric past. Here are some must-visit fossil sites:

Mazon Creek Fossil Beds

Near Morris, Illinois, Mazon Creek is one of the most famous fossil sites in the world. The Mazon Creek fauna is world-renowned, and the site boasts incredible fossils found in nodules — from horsetails to rare sea creatures and ferns. The site is open to collectors with permission and during scheduled fossil-hunting events.

Pit 11 is the most well-known location in Mazon Creek. Originally a strip mine, this site is rich in ancient ecosystem fossils. These include worms, jellyfish and unusual specimens that illustrate the diversity of life in ancient Illinois waters.

Thornton Quarry Reef Outcrop

In the Chicago area, Thornton Quarry is one of the largest limestone quarries in the world. It dates back about 400 million years to the Silurian period and contains fossils like crinoids, brachiopods, trilobites and corals. Although it is not open for public collecting, the fossil specimens found here are often studied and displayed in museums.

Shelbyville Reservoir

Lake Shelbyville in central Illinois is a notable site for fossil enthusiasts. It offers a glimpse into the marine life of the Mississippian period, around 300 million years ago. The surrounding rock formations in exposed areas along the lake's shoreline and nearby creeks contain a variety of marine fossils, including branchiopods, crinoids, coral and trilobites. The best time to explore is during periods of lower water levels. Be sure to check local regulations about fossil collecting, as there may be some restrictions to protect natural resources.

Unearth Dinosaur Fossils at Jurassic Gardens

Whether you are a seasoned paleontologist or an enthusiastic explorer, Illinois has several sites with fascinating fossils from ancient swamps, seas and forests. As we uncover more fossils and expand our understanding, Illinois' rich geological records remind us of the ever-changing nature of our planet and our role in preserving these natural treasures for future generations.

For those looking to learn more about fossils with their kids, look into the Volo Museum's family-friendly attractions, Jurassic Gardens. Kids can pan for their own dinosaur fossils, play games in the Raptor arcade and learn more about prehistoric life from Illinois and beyond. Step back in time and explore our 15,000-square-foot premises with life-like animatronic dinosaurs and a massive outdoor play area. Visit our Frequently Asked Questions page for more information on how to plan your visit.

Get your tickets today and spend a day in the Jurassic Gardens at the Volo Museum in Illinois.