Ammonite

The Ammonite is a stand-alone fossil that can be donated to the museum in all 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."

Donating Ammonite in Wild World
"Hoo! Indeed, WOO hoo! Amazing! This fossil is an almost flawless specimen! The ammonite looks like it would be related to spiral-shelled creatures. But have I got a whopper of a surprise for you! They are actually part of the same family as squid and octopi! Just imagine! All that remains of these enigmatic creatures are their shells... Despite extensive research, we're still not sure what their bodies looked like. Oh! I'm terribly sorry. I went on rather long, didn't I? Forgive me!"

Donating Ammonite in City Folk
"Hoo my, simply wonderful! A truly rare fossil! An ammonite, you see, is not what it seems, eh wot? At first glance, you'd think it was related to the conch. But it isn't a member of that family at all. Hoo no! Actually, it's closer to the squid and octopus! Sadly, the only fossils we can find are mere shells, so we know not what sort of bodies they had. HOO! I... beg your pardon! I was so enthralled, I... I spouted hot air like a teapot, eh wot?"