This Specimen has been sold.
Fossil Ammonites (Sphenodiscus) in Rock - South Dakota
These are two Sphenodiscus lenticularis ammonite specimens collected from the Fox Hills Formation of South Dakota. It's been wonderfully prepared in the hard concretion it was found in. The largest Sphenodiscus ammonite is 1.4" wide.
These 70 million year old ammonites lived when South Dakota was a shallow inland sea. It was found preserved in a concretion that was split open. It then had to be hand prepared to remove the hard rock surrounding it from the shell, a very time consuming task.
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.