1.9" Cretaceous Fossil Ammonite (Sphenodiscus) - South Dakota

This is a 1.9" ammonite (Sphenodiscus lenticularis) from the Fox Hills Formation of South Dakota. The ammonite has been meticulously prepped to expose it from the hard concretion it was found in. There is a second, smaller ammonite that has been partially exposed, however the species is unidentified.

Comes with an acrylic display stand to assist with presentation.

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.

What an ammonite would have looked like while alive.
What an ammonite would have looked like while alive.

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.
Sphenodiscus lenticularis
North Central, South Dakota
Fox Hills Formation
Ammonite: 1.9", Rock: 3.3 x 2.2"
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