In the days before Tyrannosaurus Rex roamed the Earth, prehistoric animals lived in fear of its ancestor, the Raptorex. But this petit progenitor of the Tyrannosauri would have been a mere mouthful for the mighty T-Rex.

Paleontologists recently found the remains of the Raptorex in a lake bed in Northern China. Like the Tyrannosaurus Rex, Raptorex sports an over-sized head, itty bitty forearms, strong jaws, and a runner's build — "jaws on legs" as Paul Sereno of the University of Chicago described it — but it was only one hundredth the mass of the T-Rex and stood a mere 3 meters in length.

It's the combination of its small stature and its classically T-Rex features that has paleontologists excited. Other smaller relatives of the Tyrannosaurus have lacked that dinosaur's tiny forearms and comparatively large head, which has long caused paleontologists to suspect those adaptations were due to the Tyrannosaurus's large size. But finding them on a smaller relation means they will have to rethink the T-Rex's evolutionary development.

Raptorex – a prototype T. rex [New Scientist]

Meet T-Rex's Diminutive Ancestor

Raptorex Rendering

Meet T-Rex's Diminutive Ancestor

Raptorex Skeleton

Meet T-Rex's Diminutive Ancestor

Meet T-Rex's Diminutive Ancestor

T. Rex vs. Raptorex

Meet T-Rex's Diminutive Ancestor

Skull Comparison

Meet T-Rex's Diminutive Ancestor