Fossil Ammonite Cluster - South Dakota

This is an ammonite (Hoploscaphities) cluster from the Fox Hills Formation of South Dakota. It's been wonderfully prepared on the hard concretion it was found in. It contains eight ammonites; four H. nicolletti, the largest being 2.1", ones H. spedeni, the largest 2.7" and three H. comprimus largest 1.4". The total height of this cluster is 4.2" In addition to the ammonites, there are a number of fragments and shells still partially concealed in the stone. The ammonites are iridescent and stand out nicely against the dark stone.

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