Seismosaur

The Seismosaur is a four-part fossil that can be donated to the museum. It was introduced in and also appears in.

Completing the Seismosaurus in Wild World
"The seismosaur is, quite simply, a stunning creature. Of the dinosaurs that we know about now, this is the absolute longest. Its body was rather small and quite light for its length, to be honest. Ah. Hoo. Terribly sorry. Who am I to judge its weight, eh wot?"

Completing the Seismosaurus in City Folk
"...Wait! Did the earth just move? Because the seismosaurus is complete! Hoo, what a mighty moment this is! The seismosaurus is the longest of all known dinosaurs, eh wot? Its body was small for its length, though, and rather light, as well... Proportionally, its brains were also rather light! HOO! I... beg your pardon! I was so enthralled, I... I spouted hot air like a boiling teapot, eh wot?"

In the Animal Crossing Movie
Ai, Yu, Sally, Bouquet, and Halberd stumble upon a full skeleton of a seismosaur after finding ammonite fossils. Yu decides to get the fossil to donate to Blathers, but to no avail. The cave begins to collapse, and after they escape, the cave is sealed, inhibiting them from entering again.