Naturally, this left paleontologists scratching their heads when, in 2007, they discovered the remains of a family of small ornithopods—named Oryctodromeus—in what seemed like an underground den.

The ornithopods are a group of dinosaurs that include Iguanodon, Hypsilophodon and their relatives, including the rare rhabdodontids. Ornithopods first appeared in the Middle Jurassic but were ...