6.4" Fossil Ammonite (Sphenodiscus) in Rock - South Dakota

This is a large Sphenodiscus sp. ammonite that was collected from the Fox Hills Formation of South Dakota. Most of it has been prepped free of the hard concretion it was found in. Portions of the shell exhibit gorgeous iridescence.

There are repaired cracks through the ammonite where the nodule split during collections. The ammonite also broke within the rock during fossilization, explaining the shift in the ammonites orientation. It comes with an acrylic display stand.

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.

What an ammonite would have looked like while alive.
What an ammonite would have looked like while alive.
Sphenodiscus sp.
South Dakota
Fox Hills Formation - Trail City Member
6.4" Wide ammonite, entire specimen is 6.6 x 6.4"
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