The monstrous Tyrannosaurus rex was not only a danger to other dinosaurs, but to itself.
Scientists have now found signs T. rex could have been a cannibal - gouges on the bones of these giants that could only have been made by another T. rex.
As vertebrate paleontologist Nicholas Longrich of Yale University was searching through fossil collections for mammal toothmarks on dinosaur remains, he discovered a bone with gashes up to nearly a half-inch (1.3 cm) deep in it. Given the age and location of the fossil, the marks had to be made by T. rex.
"They're the kind of marks that any big carnivore could have made, but T. rex was the only big carnivore in western North America 65 million years ago," Longrich explained. [See " The World's Deadliest Animals "]
It was only after he identified the culprit that Longrich realized the victim was another T. rex. After combing through a few dozen T. rex bones from several different museum fossil collections, he discovered a total of one arm bone and three foot bones, including two toes, that showed evidence of T. rex cannibalism.
"It's surprising how frequent it appears to have been," Longrich said. "It could mean that they were really thorough at cleaning up after animals died in the environment. Or it could mean that they were killing and eating each other fairly often."
The U- and V-shaped gouges are of the kind T. rex made when it was stripping meat from a carcass, Longrich said. The marks appear to have been made some time after death, Longrich said - the culprit might have picked at the smaller foot and arm bones after eating most of the meat off the more accessible parts of the carcass.
However, Longrich is uncertain as to whether these marks are the result of fighting or scavenging, adding that if two T. rex battled to the death, the victor might have made a meal out of his adversary.
"Modern big carnivores do this all the time," Longrich said. "It's a convenient way to take out the competition and get a bit of food at the same time." Cannibalism is widespread in the animal kingdom, including female spiders that eat their mates, chimpanzees that consume infant chimps, and great white sharks that eat their siblings in the womb.
Although only one other dinosaur species, the bipedal predator Majungatholus, was a known cannibal, Longrich said the practice was probably more common than we think.
"Until now we had only one documented example of cannibalism in dinosaurs, so you'd tend to assume it's pretty rare," Longrich told LiveScience. "All of a sudden, we have four examples of cannibalism in T. rex. So that's definitely a surprise."
The scientists detailed their findings online Oct. 15 in the journal PLoS ONE.