These ancient mollusks had coiled shells ranging in size from less than a quarter inch to more than six feet in diameter. During the age of the last dinosaurs, the Western Interior of North America was covered by the Cretaceous Seaway, a shallow sea that advanced and retreated several times. Many species of ammonites lived in the Cretaceous Seaway, and their fossils can be found throughout western North America today. Dr. Kirk Johnson is the DMNS curator of fossil plants and invertebrates. These fossils became the centerpiece of the Museum's Cruisin' the Fossil Freeway exhibition as well as the subject of an advanced invertebrate paleontology course in 1999.
Paleontologist unearths a fossil concretion.
Invertebrate fossils are sometimes preserved in concretions, or round, hard rocks. The chemical processes that occur when the animal dies precipitate calcium carbonate in the surrounding mud, causing it to harden quickly. As with vertebrates, prospecting for invertebrates starts with knowing where and how to look. Badlands don't support much vegetation, and this means they expose plenty of bare rock that might be rich with fossils. Riverbanks, beaches, and road cuts are also good places to look since erosion is active on fresh bedrock in those spots.