This is a large, 5.6" wide Harpoceras ammonite fossil from the from the Posidonia Shale of Germany. They have been compressed and are preserved on a black slate in a partially pyritized state. There are impressions of several other partial ammonites on the rock. It comes with a display stand, but there is also a hanger installed into the rock on the back of the plate allowing it to easily be hung on a wall.
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. 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.