Original: The ancient ( cohort Belemnoidea ) and modern ( cohort Neocoleoidea ) coleoids , as well as the ammonoids , all diverged from the external shelled nautiloid during the middle Paleozoic Era , between 450 and 300 million years ago , although the coleoids may be polyphyletic .
Simplified: The ancient ( cohort Belemnoidea ) and modern ( cohort Neocoleoidea ) coleoids , as well as the ammonoids , all diverged from the external shelled nautiloid during the middle Paleozoic Era , between 450 and 300 million years ago .