For years, paleontologists have wondered: Did the fierce Tyrannosaurus rex eat its own? A 66-million-year-old fossil unearthed in eastern Wyoming provides some of the strongest evidence yet that it did, researchers say. The fossil—a tyrannosaur leg bone fragment the size of a human forearm—is scarred with deep grooves left by a large meat-eater as it tore flesh from the ancient carcass. One of the grooves is particularly telling because it contains tiny channels left by tooth serrations called denticles. Those channels rule out smooth-toothed predators like crocodiles, instead pointing to bipedal meat-eating dinosaurs called theropods. And since the only large theropods known to be living then in the region—T. rex and Nanotyrannus lancensis—both belong to the tyrannosaur subgroup, the evidence bolsters the idea that tyrannosaurs were, in some cases, cannibals. Presenting next week at the annual meeting of the Geological Society of America in Baltimore, Maryland, the researchers report that the spacing of the grooves, approximately three every 2 millimeters, strongly suggests that the meat-eater was a large animal, probably larger than the victim. It’s not clear whether the groove-maker killed the victim or merely scavenged its bones, but the shape of the grooves clearly suggests the victim was dead when the tooth marks were made.