Like the theropod dinosaur T. rex, Chilesaurus stood upright, with strong hind legs and shorter front limbs. But its flat teeth were more closely related to ornithischians like triceratops and stegosaurus. That's just one thing that irked scientists when Chilesaurus was named a theropod following its discovery in Chile in 2004, per the Guardian. After examining 450 anatomical characteristics of early dinosaurs, scientists now understand why it wasn't a neat fit. They say Chilesaurus isn't a theropod at all. "A perfect half-and-half mix" of theropods and ornithischians, the "Frankenstein" species is actually the first "transitional" dinosaur to suggest the two groups are related, Cambridge University researcher Matthew Baron tells the BBC.
Researchers previously had no evidence to suggest theropods and ornithischians diverged on the dinosaur family tree, according to the study in Biology Letters. But this split "seems to have happened because of [a] change in diet for Chilesaurus," which researchers now believe is one of the earliest ornithischians at about 150 million years old, Baron explains in a release. Its inverted, bird-like hip structure allowed for a complex digestive system, and over time, as it apparently "became more advantageous for some of the meat-eating dinosaurs to start eating plants," this characteristic reappeared in other ornithischians before a distinctive beak. Theropods, meanwhile, went their own route, scientists suggest. (T. rex apparently couldn't run.)