Page "Cretaceous–Paleogene extinction event" Paragraph 22

Except for nautiloids ( represented by the modern order Nautilida ) and coleoids ( which had already diverged into modern octopodes, squids, and cuttlefish ) all other species of the molluscan class Cephalopoda became extinct at the K – T boundary.

These included the ecologically significant belemnoids, as well as the ammonoids, a group of highly diverse, numerous, and widely distributed shelled cephalopods.

Researchers have pointed out that the reproductive strategy of the surviving nautiloids, which rely upon few and larger eggs, played a role in outsurviving their ammonoid counterparts through the extinction event.

The ammonoids utilized a planktonic strategy of reproduction ( numerous eggs and planktonic larvae ), which would have been devastated by the K – Pg extinction event.

Additional research has shown that subsequent to this elimination of ammonoids from the global biota, nautiloids began an evolutionary radiation into shell shapes and complexities theretofore known only from ammonoids .< ref >