Trilobite

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

Donating Trilobite 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!"

Donating Trilobite 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!"

Completing Trilobite 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."