Ordovician Sea and Fossils

Written by Jan Okeson

Did you know that, in addition to stalactites, stalagmites, and other cave formations, Cave of the Mounds contains a wide variety of fossils?  That’s right, ancient creatures are preserved in the rock right beneath our feet!  Looking at these fossils can take our imaginations back to a time when Wisconsin was a totally different—and amazing—place.

Why would fossils be found in a cave? It’s because of the rock called limestone, which has a strange and peculiar origin: it’s seafloor that hardened into stone. That means Wisconsin was once covered by, perhaps surprisingly, a shallow, salty ocean inhabited by shelled animals.  Coincidentally, this fossil-rich rock is also a great place for caves to form, because it’s easy for water to sculpt and dissolve.

That means that, as we walk through the cave’s water-formed passageways, we can also find fossils of prehistoric ocean life. For instance, there are ancient snails called gastropods, clam-like animals called brachiopods, sea-lilies known as crinoids, and corals shaped like cows’ horns (known as “rugose” corals, so named for their wrinkly or ridged surfaces). There are also a variety of cephalopods—ancestors of the squid, octopus, and nautilus—that carried elaborate, hat-like shells. These shells could be straight, or wound into a variety of coils. We’re lucky to have a six-foot-long, straight-shelled example that’s easy to see from the cave trail.

All of these sea animals have something in common: they’re soft-bodied creatures, without bones.  That’s because the sea that made the cave’s limestone existed a really, really long time in the past—over 400 million years ago.  That’s hundreds of millions of years before woolly mammoths, dinosaurs, and all but the most primitive fish!  A cave tour only takes an hour, but it’s like taking a long trip back in time!

Fossil fanatics take note: with the exception of the six-foot cephalopod fossil, the fossils in Cave of the Mounds require some guidance to locate. We recommend one of our Focus on Fossils tours in February if you want to see the harder-to-find fossils underground here.