All animals likely feared the carnivorous Tyrannosaurus rex in Cretaceous times -- even their own kind.
A 66-million-year-old fossilized tyrannosaur bone unearthed in Wyoming's Lance Formation in June is providing new evidence that T. rexes were sometimes cannibalistic.
A recent analysis of the bone revealed tooth marks in the pattern of a typical T. rex's chompers, according to Loma Linda University paleontologist Matthew McLain, who presented the findings at the annual meeting of the Geological Society of America in Baltimore on Sunday.
"We were very surprised to see tons of these tooth scores all over part of it," the lead researcher told The Huffington Post. "What's unique about this bone is you're not just seeing one bite, it looks like it was dragging its teeth several times. ... So we can be very confident about what animal was biting this."
The researchers noticed that the teeth that gnawed the bone must have been serrated, a feature common to theropod dinosaurs, a diverse group of bipedal dinosaurs that T. rex belongs to. In fact, the serrated structure of the teeth of T. rex and other theropod dinos is what allowed them to rip through the flesh easily.
They measured the distances between the serrations, and matched them with the teeth of the two large theropod dinosaurs that lived in the Lance Formation some 66 million years ago: the Tyrannosaurus rex and the Nanotyrannus lancensis. This eliminates all interpretations but cannibalism, McLain said in a press release, since both animals are in the same family.
The paleontologists examined the direction of the teeth markings, which suggested that the feeding cannibal was getting flesh off the bones of a tyrannosaur that was already dead, instead of biting it to kill. They were unable to determine whether the cannibal was scavenging or also killed the tyrannosaur first.
"We're sure it was feeding, these are feeding traces," McLain said. "The marks were made either near death or after death, because there is no sign of healing on them. If the animal was bitten and survived then the spots would heal."
McLain pointed out that if the cannibal had killed the tyrannosaur, it more likely would have bitten into its prey's upper body -- not its leg area. The bite marks on the leg suggest scavenging, he said.
Scientists have previously unearthed clues that tyrannosaurs and other large carnivorous dinosaurs engaged in cannibalistic feeding behavior. A 2010 study about four T. rex specimens also uncovered teeth marks that suggested the animals ate their own kind.
Dr. Nick Longrich, who was the lead author of the 2010 study but was not involved in the new research, said T. rexes probably routinely ate each other.
"Cannibalism seems to be surprisingly common in these guys," Longrich, a senior lecturer of biology and biochemistry at the University of Bath in England, told HuffPost. "The main source of mortality for many predators is other members of their species -- alligators eat baby alligators, brown bears kill juvenile brown bears."
McLain said that the previous study and now the new leg bone analysis both provide unprecedented insight into what -- and whom -- tyrannosaurs were eating.
"It really reinforces that cannibalism was going on," he said. "Now we're saying, yeah, it definitely was going on."
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