Cephalopod
The old (cohort Belemnoidea) and modern (cohort Neocoleoidea) coleoids, as well as the ammonoids, all diverged (evolved away) from the external shelled nautiloids during the middle Paleozoic Era, between 450 and 300 million years ago. Most ancient varieties had protective shells. These shells at first were conical but later developed into the curved shapes seen in modern nautilus species. Shells inside the body still exist in many living cephalopod groups, such as cuttlefish. The most famous group with external shells, the ammonites, became extinct at the end of the Cretaceous period.