Trilobite

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

In Animal Crossing
"Well, what do we have hoo? A trilobite, if I'm not terribly mistaken. Yes, yes. Very nice indeed. This is quite a beautiful specimen, as well. Quite! You're to be congratulated! Well then, where to begin? Trilobites...Ah, yes...Trilobites were hard-shelled segmented arthropods, which populated the Paleozoic seas long before dinosaurs existed. There were over 15,000 species of known trilobites, and more are discovered each year, wot wot! This makes them the single most diverse group of extinct organisms ever! Amazing! Truly remarkable, you know! They are, however, extinct: no living descendants whatsoever. The constant perils of nature and quite formidable indeed."

In Wild World
"Hoo my, simply marvelous! This fossil is an outstanding specimen. Trilobites existed long before the dinosaurs, I'll have you know. At the time, there were very few enemies, so they absolutely thrived, wot! For protection, much like a...blech...pill bug, they would roll up into a ball. Oh, forgive my babbling! The very thought of these beasts gets me atwitter!"

In City Folk
"Hoo, this fossil is a truly outstanding specimen! Trilobites were around long before the age of the dinosaurs, eh wot? They were quite numerous, and as a result, much research has been done on these fellows. In point of fact, scientists date geological strata based on them. It's almost like they're your guides for time traveling!"

In New Leaf
"Similar in appearance to a wood louse, the trilobite was an ancient sea-dwelling arthropod. It could range between 4 mm and 70 cm in length, depending on various factors. The trilobite was one of the first creatures to have eyes and the ability to detect enemies and prey."

In Real Life
The trilobite was a marine creature, but is now extinct. Its name meant three lobes. They were quite small and came from the class trilobita.