What did Tyrannosaurus rex eat? To some extent, say researchers, they ate each other.
The less-than-appetizing news, that T. rex may have been a cannibal, comes from Nicholas Longrich, a paleontologist doing his post-doctoral work at Yale University. He reports that he was sorting through 65-million-year-old fossils from Montana when he found the toe bone of a Tyrannosaurus rex with a gouge mark in it -- a small but clear sign that something else had been feeding on it.
"It was a V-shaped gouge, which is a classic bite mark," said Longrich in a telephone interview with ABC News. "I thought, OK, it was a big carnivorous dinosaur that did this, and there was only one big carnivore at the time.
"It was not only the dominant carnivore -- it was the only carnivore."
That was two years ago. Since then, Longrich has combed the specimen collections of universities and museums and found three more fossils, all of T. rex arms or feet, all with the same kinds of markings.
"The marks are interpreted as feeding traces, and these fossils therefore record instances of cannibalism," he and three senior colleagues write in the online edition of the journal PLOS One. "Cannibalism seems to have been a surprisingly common behavior in T. rex, and this behavior may have been relatively common in carnivorous dinosaurs."
The scientists say they were not terribly surprised. "Big carnivores are designed to eat other big animals, and one of them is likely to be your own species," Longrich said. Modern alligators and other meat-eaters often feed on each other. But until Longrich stumbled across the gouge marks, there was no proof of how that greatest of the meat-eating dinosaurs behaved.
"It takes a kid like Nick to do this," said Gregory Erickson of Florida State University, who looked at the evidence and agreed to sign on as a co-author of the new paper. "He's one of those people with the patience to pull drawers in the back rooms of museums, which not enough people do."
With practice, paleontologists say they get pretty good at reading fossils. They can tell teeth marks apart from, say, random accidents. They can also compare them to modern animals. A lion, for instance, leaves fairly distinctive teeth marks when it bites down on its prey.
"Sometimes you can match the marks to the tooth itself," Longrich said. "Some, though not this one, go so far as to have scrape marks from serrations on the teeth."
Erickson joined in: "T. rex was just so powerful, with those teeth like railroad spikes, that it left deep puncture marks."
The researchers say they wanted to be careful, though. They spent time ruling out other explanations. They held off until Longrich had found several different examples of the same thing.
"It adds another piece to what we know about what was arguably the most famous animal in the world," said Erickson. "It makes sense, but we needed empirical evidence."
Does our view of the king of the dinosaurs change now? It is new evidence that the world of the Cretaceous period, 65 million years ago, could probably have been pretty brutal.
"This is not consistent with just biting," Longrich said. "They were definitely feeding on each other. Whether they killed each other, or just fed on each other's dead carcasses, we don't know. But it's not like one of these animals would have just sat there while another one fed on his toe."