Ammonite

The Ammonite is a stand-alone fossil that can be donated to the museum in all Animal Crossing games to date.

Donating Ammonite in Animal Crossing
''Well! Hoo and hoo again! An ammonite! My stars! A very impressive find! Ah yes, wonderful. This fossil is of exceedingly superior quality, wot wot. Now then, I know a thing or two about these creatures. Yes, indeed. Ammonites... Though ammonites lived in shells, they were not shellfish, but rather mollusks, like octopi and squid. The closest living relative to the ammonite is the chambered nautilus. Fascinating, no? Ammonites existed from the Devonian period, some 400 million years ago, to the end of the Cretaceous period. Their extinction coincided with that of the dinosaurs. Time and tide wait for neither man nor beast, wot! Oh, hoo. Blathers, you ninny! I've gone and done it again. Spewing out more information than necessary. So sorry.''