An ammonite fossil cut in half to reveal inner structure.
Ammonites were predatory mollusks that resembled squid. These cephalopods had eyes, tentacles, and spiral shells. They are more closely related to a living octopus, though the shells resemble that of a nautilus. Ammonites first appearing in the fossil record 240 million years ago, descending from straight shelled cephalopods. The last lineages disappeared 65 million years ago at the end of the Cretaceous.