SILURIAN to PLEISTOCENE PERIOD: 435 million - 10,000 years ago
Squid belong to the family of invertebrates called CEPHALOPODS. Modern members include the nautilus, squid and octopus. They first appeared during the Silurian Period (435 million to 410 million years ago) and were abundant and widespread in the seas of the Jurassic and Cretaceous periods (175 million to 65 million years ago). Cephalopods are important index fossils — that is, they often link the rock layer in which they are found to specific geological time periods. Squid and cuttlefish fossils are rare. Impressions of their internal skeletons are all that are usually found preserved. In rare instances, soft body parts and tissue are found preserved, and even more rarely, the ink sac.
Modern squid and cuttlefish most likely can be equated with their prehistoric cousins. Extant species have some of the most highly developed nervous systems amongst all invertebrates. They are extremely adept at both evading predators and hunting prey on their own. Their stream-lined bodies are equipped with side fins that enable them to steer when moving. Some squid possess an ink sac which is used at the most strategic moment when pursued by a predator in order to confuse and escape imminent danger.