During one of his post-graduate archeology classes, Buster's team discovered some fossilized bones in a rock. He took it home as homework.
Buster's team found the rock, which he believed to contain remains of a pterodactyl but it appears to be humanoid. Buster accidentally crushed the skull with his hammer, but blamed gravity. He left the rock in the backseat of the family's Mercedes. ("Charity Drive")