Cephalopods are the group of animals that includes the octopus, cuttlefish, squid and the nautilus. Cephalopods evolved from primitive molluscs during the Late Cambrian, approximately 500 million years ago. In primitive cephalopods like the one above its shell was external and superficially like that of a gastropod snail. Such a shell serves two functions: defence and buoyancy. The fossil above is possibly that of a Orthoceras Cephalopoda. Cephalopod from South Coast NSW Australia.

http://en.wikipedia.org/wiki/Cephalopods