Date: Cretaceous Period (145-65 million years old)
Description: Ammonites were shelled marine invertebrates that were similar to modern cephalopods like octopi and squids. The first ammonites appeared in the Devonian period (400 million years ago), and they thrived in ancient oceans until the mass extinction event of the Cretaceous Period. They lived in chambered shells like the modern nautilus. These shells are what we find today, as only the hard parts of ancient creatures tend to fossilize. This particular fossil is a mold fossil of an ammonite shell, meaning an imprint of the shell that was left in the rock. It was found in an old Cretaceous seabed in Riverside Park.
Donor: Sioux City Academy of Science and Letters.