Ammonite Fossil Shells Crystalized

Ammonite fossils are the shells of a group of predators called cephalopods that lived in the seas between 240 and 65 million years ago, and became extinct alongside the dinosaurs. They are known for their spiral-shaped shells with ribs, and are often considered to be one of the most well-known fossils. The name "ammonite" comes from the Greek god Ammon, who had ram horns. 

Ammonites are useful as guide fossils in stratigraphy because they are common, easy to identify, and have a worldwide distribution. They also evolved quickly, so each species had a relatively short lifespan.