Compared to modern cephalopods, the fossil record reveals 20 times the number of species. The first cephalopods appeared during the late Cambrian (about 500 million years ago). They looked like squid, but with long, cone-shaped shells from which they extended their tentacles. Typically, the shells were divided into chambers (septa), added on as the animal grew. Some early cephalopods were enormous, as long as a school bus. For animals without backbones (invertebrates), they were sophisticated, with nervous systems and the ability to swim by jet propulsion. By the end of the Paleozoic (250 million years ago), cephalopods had diversified to include coiled shells like a modern nautilus. While the extinction event at the end of the Paleozoic impacted many species, cephalopods flourished again during the Triassic and Jurassic. The extinction event at the end of the Cretaceous (65 million years ago) left only the nautiloids and the coleoids (octopus and squid).

The earliest ammonoids on Earth, small organisms called Bacrites, had straight shells. Later ammonites had coiled shells made up of a spiraling series of chambers. They got their name from early Romans who mistook their fossils for ram?s horns (Ammon= a god with ram's horns). Ammonites became common in the seas of the Jurassic (200 million years ago). They were top predators, feeding on fishes, mollusks, arthropods, or other ocean creatures. Ammonites used good eyesight and tentacles to capture prey and feed it into a strong, crushing beak. Their closest living relatives are probably the modern nautiloids, but ancient ammonoids could be wider than a minivan. During the Cretaceous (about 150 million years ago), as ammonoids reached their heyday, some evolved to have shells that were coiled less or not at all. All ammonoids went extinct in the huge extinction event at the end of the Cretaceous (65 million years ago).