A diminutive tyrannosaur lived alongside Tyrannosaurus rex - Science | AAAS

Warm blood allowed T. rex to quickly hunt prey and live across a wide range of environments. Contrary to the assumptions of early paleontologists, most dinosaurs weren’t lumbering slowpokes. Centuries of evolutionary reevaluation now indicates the ancestors of present-day birds didn’t simply keep a steady pace—many of them boasted impressive speed. These active lifestyles also forced scientists to reconsider another anatomical detail. Once thought universally cold-blooded, a lot of dinosaurs eventually evolved warmer blood.

Tyrannosaurs sit at the top of the food chain in the popular imagination, but a newly described fossil from Alberta, Canada, suggests that a much smaller predator may have fed on the carcass of Albertosaurus sarcophagus, a tyrannosaur relative. Atrociraptor scavenge the carcass of Albertosaurus along the edge of a river floodplain. Image credit: Joshua Adam Doyon.