Guide to Fossils from Tropical Milwaukee

It may be hard to picture today’s prairie landscape as home to sea creatures, but there are reminders of that underwater life all around us. With patience and a keen eye, you can spot fossils of marine animals ranging from nearly microscopic bryozoans to foot-long cephalopods.

We hit the road from Chicago to explore the Milwaukee Formation, a geologic segment from the Devonian Period, about 420–360 million years ago. The formation stretches north from Milwaukee on both sides of the Milwaukee River and along the shore of Lake Michigan, extending up to Brown Deer, WI.

Here’s what to keep an eye out for when exploring the rocks and parkland around Milwaukee:

Strophomenid brachiopods

We spotted fossil seashells from a couple different kinds of brachiopods, a group of animals whose name means “arm foot.” While this particular fossil is only about three inches long, strophomenids included some of the largest brachiopods. These animals may look like bivalve mollusks, and while they do have two valves, they’re not related. You’ll recognize strophomenid fossils by their classic shell- or sometimes leaf-like appearance. For a bonus fossil, the round indentation to the left of this particular brachiopod is a crinoid (more on those soon).

Orthospirifer brachiopods

Another kind of brachiopod, an Orthospirifer, lay just inches away from the strophomenid. Orthospirifer is convex on both sides, sort of like pistachio shells, whereas strophomenids have one convex and one concave shell—making them flat like potato chips. Seeing these two encased in the same slab of rock really helps paint a picture of an undersea world populated by thousands of brachiopod species. Many have gone extinct, with around 300 species of brachiopod living today.

Bryozoans

Bryozoans often look like a thin layer of netting or lace embedded in the surface of a rock. As you start looking for fossils, keep an eye out for repeating patterns like this. Bryozoans are made up of many individuals called zooids, which form colonies and act as a single organism. Here we're looking at a fenestrate bryozoan, referring to the small “windows” in the netting where water once flowed.    

Crinoids

Fossil crinoids are often around the size of an eraser head, and you can spot them thanks to their perfectly circular shape. What looks like a little Cheerio-like ring is just one small section of a crinoid’s stalk—it’s much rarer to find a longer, preserved section of the stalk. Crinoids are related to starfish and almost appear to be a starfish attached to the end of a stalk. Another fun crinoid fact? Sometimes we find fossilized snails attached to the end of a crinoid’s anal tube—they reprocess the waste as food. Recycling!

Cephalopods

Cephalopod shells might be the hardest to spot, unless you’ve got a trained fossil-hunting eye. The one we saw in Milwaukee looks like a blobby, amorphous depression on the rock’s surface, about six inches long. Here, we’re only seeing remnants of part of the shell—based on its size, the full body might’ve been around a foot long. Those little dots are impressions of corals that encrusted the shell, either while the cephalopod was alive or after it died. Today, the only living cephalopod with a shell is the Nautilus.

Getting started

• What's a fossil versus just a rock? Keep an eye out for patterns, symmetry, or any marking that doesn’t seem to fit with the surrounding rock surface. The more time you spend looking, the more trained your eyes will become.
• Context is helpful in identifying a fossil, and a little advance research can go a long way. What’s the geologic formation? What animals and plants lived here, and when?
• Watch a fossil hunting tour with our Collections Manager of Fossil Invertebrates, Paul Mayer. He shows us around some of these exact Milwaukee Formation rocks and gives tips for spotting fossils.
• Looking for fossils in Michigan? Check out our Lake Michigan beachgoer’s guide.
• Wherever you go, make sure to follow all rules and regulations when collecting fossils and geologic specimens, and respect the rights of property owners.