4.8" Wide Ammonite Cluster - South Dakota

This is a 1.9" 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 "friends" still buried in the rock. The entire piece measures 4.8 x 3.7"

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.

South Dakota
Fox Hills Formation
Largest Ammonite 1.9" Wide, Rock 4.8 x 3.7"
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