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An illustration of what the Chicago area would have looked like about 309 million years ago.

An illustration of what the Chicago area would have looked like about 309 million years ago.

Courtesy of K. Carr and the Field Museum of Natural History

What prehistoric creatures roamed Chicago?

If you want to see dinosaur bones in Chicago, the obvious choice is to hit up the Field Museum. There, you’ll find large displays like Máximo the titanosaur from Argentina or Sue, the tyrannosaurus from South Dakota.

But what about dinosaur bones found in Illinois?

A group of third and fourth graders from Urban Prairie Waldorf School in the city’s Pilsen neighborhood asked Curious City if dinosaurs ever walked in Chicago.

The unsatisfying answer: we don’t know for sure. Dinosaur fossils are almost exclusively found in sedimentary rock. But rocks from the Mesozoic Era, about 252 to 66 million years ago haven’t been discovered in the region. However, several experts we spoke to said Chicago still has a lot of stunning prehistoric discoveries dating back to before dinosaurs walked the Earth. These fossils give scientists an idea of how the region looked when sea creatures ruled the place.

An illustration of a Silurian reef

Illinois in the Silurian, about 425 million years ago. Nautiloids swim above a Silurian reef near what one day will be Chicago.

Source: Diorama at the Milwaukee Public Museum, from the Collection of the Milwaukee Public Museum, #1E013.

A city of ocean and ice

We know Chicago today for its snowy winters and flat landscape. But 425 million years ago, during the Silurian period, the city was under an ocean and set 15 degrees south of the equator. Scientists are able to study rocks from that time period, but there’s a big gap in the region’s geological history.

“We have this big, tremendous record of rocks from 420 million years ago, [and] some a little bit further from the area from about 310 million years ago,” said Roy Plotnick, professor emeritus of Earth and Environmental Sciences at the University of Illinois Chicago. “Then nothing in the rock record until 14,000 years ago.”

Through fossil findings, scientists have discovered sea creatures from hundreds of millions of years ago, like relatives of modern clams, octopi and sea anemones.

Coal age forest reconstruction at the Field Museum of Natural History.

Illinois in the Pennsylvanian, about 310 million years ago. Coal age forest reconstruction at the Field Museum of Natural History.

Diorama photograph by John Weinstein for the Field Museum of Natural History

Then, about 21,000 years ago, Chicago was covered in ice, about a kilometer of it. That ice started to melt, revealing hills around the region. The water retreated even more to form the Great Lakes and eventually, the current shoreline. Bone discoveries from around 14,000 years ago show that animals like mastodons and mammoths, related to modern elephants; and castoroides, bear-sized beavers, were roaming around.

A fossil mother lode

The first fossil discoveries were made in the 1800s, as stone quarries were being built for construction.

The Thornton Quarry just south of Chicago is a place where scientists have been able to see how those prehistoric reefs changed over time. The quarry opened in 1836 as a limestone mining operation, but it’s been a wealth of fossils of ancient sea creatures.

Another major area for fossil discoveries is Mazon Creek in Grundy County, southwest of Chicago. The place is known as a lagerstatte, a German term used in paleontology to describe a place that has extraordinary fossils with exceptional preservation.

“It’s a place where things that are soft bodied – worms and shrimp and sea anemones, which usually don't preserve are preserved,” Plotnick said. “These were first described in the 1850s and have been collected by both professionals and amateurs.”

But major finds have even happened in people’s yards. Federal Judge Joseph Sam Perry was expanding a pond at his home in suburban Glen Ellyn in 1963. He uncovered the bones of a mastodon.

It may be harder to uncover these larger bones today because of the urban environment. But Plotnick said if you look carefully, you can still find fossils. Next time you’re at the beach, take a look at the rocks and pebbles. You might find evidence of the prehistoric reef.

Susie An is Curious City’s editor.

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