Researchers found a part of a T. rex tooth wedged between two tailbones of a duckbill dinosaur unearthed in northwestern South Dakota. The tooth was partially enclosed by regrown bone, indicating the smaller duckbill had escaped from the T. rex and lived for months or years afterward.
Since the duckbill was alive and not just a carcass when it met the T. rex, the fossil provides definitive evidence that T. rex hunted live animals, researchers say in Monday's issue of the Proceedings of the National Academy of Sciences.
The fossil, from around 67 million years ago, indicates the T. rex bit the duckbill from behind and "intended to take it for a meal," said David Burnham of the University of Kansas, an author of the report.
It's not clear whether there was a chase involved, he said.
Experts who didn't participate in the study said there was already ample evidence that T. rex went after live animals as well as scavenging carcasses. It brought a bone-shattering bite and teeth up to a foot long to each task.
The new fossil is the first to include a T. rex tooth embedded in the bones of its prey, giving "extremely strong physical evidence that the attacker was a tyrannosaur," said Thomas Holtz, Jr., of the University of Maryland.
"It's one other bit of evidence (for hunting) fully consistent with the other data already established from lots and lots of lines of evidence," Holtz said.
You might think a T. rex would take down anything in sight, but Jack Horner of Montana State University said it apparently preyed on the weak, the sick and the young instead.
It makes sense that T. rex also scavenged, said Kenneth Carpenter, curator of paleontology at the Utah State University East Prehistoric Museum.
"If there's a free meal, why not?" he asked. But decay can make carcasses toxic after a while, he said, and "at that point, T. rex is going to have no choice but to hunt."
Proceedings of the National Academy of Sciences: http://www.pnas.org
Malcolm Ritter can be followed at http://www.twitter.com/malcolmritter
Copyright 2013 The Associated Press.