1.55 Inch Subdichotomoceras Ammonite

Here's a classic Late Jurassic Jurassic fossil from Madagascar. It's just over 1.5 inches wide and still has some of the original matrix attached to it.

Ammonites were predatory mollusks that resembled squids with shells. These cephalopods had eyes, tentacles, and spiral shells. Though their shells resemble that of a nautilus, they are actually more closely related to living octopuses. Ammonites appeared in the fossil record about 240 million years ago, barely surviving several major extinction events. The last lineages disappeared 65 million years ago at the end of the Cretaceous.

Artist's reconstruction of an ammonite, by Nobu Tamura

More about ammonites