Cephalopod* phylogeny according to Fuchs et al. (2013). Contrary to conventional wisdom, belemnoids were not found to be extinct, rather, there appears to be a transitional taxa (Longibelus) between them and decabrachians (squids, cuttlefish, Spirula). True squid are thus a surprisingly young group and amazingly, lack a fossil record; fossils previously regarded as squid are now thought to be vampyropods (vampire squid and octopuses).
Fuchs, D. et al. (2013) Longibelus gen. nov., a new Cretaceous coleoid genus linking Belemnoidea and early Decabrachia. Palaeontology 56(5) 1081–1106.
* More specifically, covering crown-coleoids and one stem-coleoid, not all cephalopods.