Ammonites are an extinct group of marine cephalopod mollusks that lived between 240 million and 65 million years ago. They were closely related to the modern nautilus and had a similar spiral-shaped shell. They went extinct along with the dinosaurs at the end of the Cretaceous period. Ammonite fossils are found all over the world, and they are often used by paleontologists to date rocks and other fossils in the fossil record.