3.5" Sphenodiscus Ammonite - South Dakota
This is a beautiful 3.5" Sphenodiscus ammonite specimen from the Fox Hills Formation of South Dakota. It's been nicely prepared from the hard concretion in which it was found and the base is flat enough that it will display nicely without the use of a stand. This ammonite has a number of small "friends" still buried in the rock.
Ammonites were predatory mollusks that resembled a squid with a shell. 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. True ammonites appeared in the fossil record about 240 million years ago. The last lineages disappeared 65 million years ago at the end of the Cretaceous.