Ammonite

The Ammonite is a standalone fossil in the introduced in  that appears in all subsequent games to date.

In
When donating to Blathers in, he will provide the following information about the fossil:

"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."

In
When donating to Blathers in, he will provide the following information about the fossil:

"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!"

In
When donating to Blathers in, he will provide the following information about the fossil:

"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?"

In
After donating the fossil in, its plaque in the museum will read:

"Ammonites were sea creatures with shells ranging from a few inches to a few feet in diameter. However, theories speculate that giant varieties existed as well, with shells six feet in diameter. Oddly, the ammonite is a closer relative to the squid or octopus than to the chambered nautilus."

In
Halberd and Yū manage to dig up an ammonite fossil in during the summertime. Later, they, along with Ai, Sally, and Bouquet, go into a cave to hunt for more ammonite fossils. After going through a waterfall, they stumble upon a mountain of ammonite fossils. Later in the wintertime, when searching for the Johnny's spaceship part, Yu and the others see the same mountain of ammonite fossils.

Real-world information
Ammonites were a group of cephalopods which lived from the Devonian to the Cretaceous period. Despite their appearance, they are more closely related to the coleoids (shell-less cephalopods like squids, cuttlefish, and octopuses) than nautiluses (which have shells). They were extremely abundant during the Mesozoic - with some species used as index fossils to mark the beginnings of geologic stages - and remain the most common fossils on beaches. They went extinct at the same time as the non-avian dinosaurs.