Belemnite guards and squid hooks have been found in huge numbers in the guts of ichthyosaurs and sharks. This is unclear. It is generally assumed that the fossil nautiloids were very much like the modern species, with a long lifespan and numerous reproductive periods, but the evidence is lacking. Hatching size of the Mesozoic and Tertiary nautiloids is broadly similar to the living species. Ammonites seem to be different, with a hatching size much more like that of the coleoids.
Sexual dimorphism is pronounced.
In some ammonites the shell eventually coils back on itself, almost sealing off the aperture. It has been suggested that like the modern octopus, the female ammonite brooded the eggs within the security of the shell, but didn't feed, and eventually died after the hatchlings swam away. On the other hand, some estimates of ammonite growth rates produce quite different results. Epiobionts, like serpulid worms, encrusting the shells of ammonites have been used to estimate lifespans of over 12 years. Fossil spirulids are known from the Pleistocene of the Canary Islands, the Pliocene of New Zealand, and the Miocene of Japan.