Archelon

The Archelon is a two-part fossil that can be donated to the museum in.

Completing the Archelon in New Leaf
"With a name that literally means "ruler turtle," the archelon was an ancient species of sea turtle. Reaching 13 feet in length and two metric tons in weight, it was a giant that lived up to its name. It likely ate jellyfish and squid, though theories say it may have also preyed on ammonites. Unlike with modern turtles, its shell was too soft to allow it to pull in its limbs for a defensive position."

In Real Life
An archelon was a herbivorous marine turtle around in the late Cretaceous period and weighed 2200kg. They did not have a solid shell but a skeletal framework supporting them.