Fossilized Tooth In Dinosaur Tailbone Proves T. Rex Was A Predator
July 16, 2013

Fossilized Tooth In Dinosaur Tailbone Proves T. Rex Was A Predator

Lawrence LeBlond for - Your Universe Online

A recent discovery of a fossilized tooth from the Hell Creek Formation in Montana is bringing new evidence to the table Tyrannosaurus rex was indeed a hunter.

The tooth was found embedded in the healed tailbones of a duckbill dinosaur, suggesting the animal survived a very close encounter with the tyrant lizard some 65 million years ago. The embedded tooth is the first concrete proof T. rex was not just a scavenger, but also a predator of the time that hunted and killed prey, according to researchers involved in the discovery.

Peter Larson, a paleontologist with the Black Hills Institute of Geological Research (BHIGR) and study coauthor, said this is "not just the smoking gun--we've actually found the bullet."

The tooth discovery adds to evidence revealed earlier this year in the journal Cretaceous Research that showed T. rex was more than just a scavenging reptile. Robert DePalma II, leader of the latest research from the Palm Beach Museum of Natural History, discovered a facial scar in a duckbill dinosaur that was the result of a close encounter with a T. rex. Much like the latest discovery, that dinosaur luckily survived the encounter to live another day.

A popular assessment of T. rex has been that it was not a hunter, but rather a scavenger. This revelation has not been without debate, however, as numerous researchers have weighed in on the subject.

While it is clear most scientists believe T. rex had a bite stronger than any other known predator, there had not been enough proof to show that a big bite translated to a big killer. With the latest find, the proof is in the pudding, according to the researchers.

"The animal was attacked, survived and escaped," paleontologist David Burnham at the University of Kansas Biodiversity Institute, who helped to analyze the fossil, told Robert Lee Hotz of The Wall Street Journal. "Until we found this specimen, people could say that T. rex was a scavenger; here is evidence it attacked living things."

The tooth, stuck in a pair of fused hadrosaur vertebrae, was discovered by DePalma during a 2007 field expedition. "It was a very odd bone at first glance," he said. "As soon as I could see that tooth lodged in the side, I knew it was an extremely significant piece."

To be sure the fused vertebrae did hold a tooth, the team examined the fossil with a medical X-ray CT scanner at Lawrence Memorial Hospital in Lawrence, Kansas. Then, to identify the species from which the tooth derived, they analyzed the distinctive serrations under a microscope.

"We were able to conclusively identify the tooth as coming from a Tyrannosaurus rex and no other dinosaur," DePalma said to WSJ.


It has been hotly debated T. rex only fed on dead carcasses. Despite findings of dinosaur bones scarred by tooth marks, skulls punctured by bites and fossilized footprints recording predatory pursuits, there has never been conclusive evidence T. rex was indeed a hunter. That is, of course, until now.

Still, not everyone is convinced.

Jack Horner, a superstar paleontologist at Montana State University, who has long argued T. rex was an "obligate scavenger" -- surviving solely on the flesh of dead carcasses -- doesn't buy into the new evidence.

"It's one data point, and that's the least amount of data you can have," said Horner, who was not involved in the research.

With most tooth marks uncovered in the past, it has been difficult to prove the bites had come from eating an already-deceased animal or a fresh kill. "If a T. rex bit a bone of an animal that was already dead, that mark looks identical to what would be made on an animal that it just killed," said DePalma.

So, in the past, scientists would try to determine feeding behavior by analyzing physical and sensory attributes of T. rex.

Horner, et al, have concluded T. rex was not only too slow to chase prey, its arms were too puny and its eyesight too poor to make successful attacks on healthy adult animals. At best, T. rex may have been able to chase down old, weak, or sickened animals, but mainly relied on carrion, said Horner.

"My argument has always been that T. rex is [an opportunist] like a hyena," he added.

But DePalma and colleagues refute Horner's idea. This should "put the debate to rest," DePalma said.

The fact the prey, in this case a hadrosaur, managed to escape attack from the T. rex suggests it was a healthy adult capable of fending off the pursuer. The vertebrae fossil proves this animal was attacked by a T. rex while it was alive because the bones had a chance to heal and fuse together, which would not have happened if the animal was already deceased, according to the researchers.

"It's unmistakable that the tooth was in there before the animal died," said DePalma.

And because the location of the injury was the tail bone, it lends credence to the fact T. rex was pursuing its prey when the bite occurred. "The fact that the injury is on the hindquarters of the animal is actually not surprising," DePalma said. "If you examine Kalahari lions, for example, they will attack the hindquarters of their victims to immobilize them and then they'll go in for the kill."


While DePalma is convinced the attack was predatory in nature, he said there are several ways the failed attack may have happened.

One possibility could be that T. rex was prowling a watering hole where duckbills gathered. Another scenario is that it was hiding out in nearby foliage waiting for the right moment to make an ambush. The region where these fossils are found -- the dry, rocky, desert-like hills of the Hell Creek Formation -- was once a lush, temperate forest environment crisscrossed by rivers filled with life.

It is likely T. rex opted for the closest hadrosaur within its reach, "and in this case it proved too much of a match for it," said DePalma to National Geographic's Ker Than.

DePalma's colleague Larson, imagines it somewhat differently. In his scenario, a small pack of T. rexes join forces to take down the prey. Multiple T. rex individuals have been found buried together, which suggests they may have lived and hunted together.

"A scenario might be a family of two or three related individuals that hunted together," Larson said. "One animal would distract the prey or perhaps send a herd of duckbills scattering, while the other Tyrannosaurs would lie in wait and eventually take up the chase for the weaker individual."

Once the T. rex managed to run down the prey, it would then use its weight to muscle it to the ground and then try to tear open the throat with its powerful jaws to secure the kill, according to Larson.

However, Horner argues such imagined scenarios are just that; imaginations unsupported by the evidence.

The fossil evidence only "shows that a T. rex bit a live duckbill dinosaur. There's no evidence that it was chasing it," Horner said.

In keeping with scenarios, Horner added his speculative theory: "A T. rex could have walked up to a sleeping duckbill dinosaur and bit it, realized it was alive, and then backed away," he said. "That's just as plausible as saying that it's chasing it because there's no evidence for either one."

Apart from the heated debates, DePalma believes the tailbone evidence will help paleontologists reconstruct the ecology of the Hell Creek Formation 65 million years ago.

"When you have a giant carnivore that composes a significant portion of the ecosystem, identifying its role as either a predator or a scavenger is going to have significant implications for paleoecological reconstructions," DePalma said.

DePalma and colleagues' research is published in this week's issue of the Proceedings of the National Academy of Sciences (PNAS).