8.5" Fossil Ammonites (Sphenodiscus) - South Dakota

Fossil Ammonites (Sphenodiscus) - South Dakota #144027

This is a large (8.5" wide) Sphenodiscus sp. ammonite and a much smaller (1" wide) ammonite of the species Hoploscaphites nicoletti, collected from the Fox Hills Formation of South Dakota. These ammonites have been prepped mostly free of the hard concretion they were found in. The Sphenodiscus ammonite required crack repair and restoration in the form of gap filling.

This specimen is quite heavy, weighing just over 41 lbs. Due to the weight, it will be shipped in a crate.

These 70 million year old ammonites lived when South Dakota was a shallow inland sea. They were found preserved in concretions when split open. They then had to be hand-prepared to remove the hard rock surrounding them from their shells, a very time consuming task.

Ammonites were predatory cephalopod mollusks that resembled squids with spiral shells. They are more closely related to living octopuses, though their shells resemble that of nautilus species. True ammonites appeared in the fossil record about 240 million years ago during the Triassic Period. The last lineages disappeared 65 million years ago at the end of the Cretaceous.

What an ammonite would have looked like while alive.

More about ammonites