The fossil record for octopus and their relatives is meager. They have no external shells, and soft parts do not fossilize well. However, the small, internal shells of squid, cuttlefish, and organisms called belemnoids have left a record that paleontologists interpret. Belemnoids were common during the Jurassic and Triassic (beginning about 200 million years ago), swimming with the ammonoids that flourished at that time. Belemnoids preyed on tiny marine organisms and served as prey for sharks and the large marine reptiles that cruised the warm seas. Their internal skeletons, found in abundance at some fossil sites, are small and bullet-shaped, and used to be mistaken for stones. Belemnoids disappear from the fossil record about 65 million years ago. While the massive extinction event at the end of the Cretaceous is famous for the dinosaur extinction, belemnites went extinct as well. Squid, octopus, and cuttlefish survived to diversify to the present.

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).