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.
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. Ammonites appeared in the fossil record about 240 million years ago and they barely survived several major extinction events. The last lineages disappeared 65 million years ago at the end of the Cretaceous.