Researchers Take A Bite Out of Tyrannosaurid Eating Theory

FAYETTEVILLE, Ark. - Popular movies have shown Tyrannosaurus rex snapping up prey and swallowing it in great, ghastly gulps. But a close look at the teeth of tyrannosaurids revealed to University of Arkansas researchers that these large carnivores may have eaten more like mammals than their reptilian cousins - by slicing and processing food with their teeth before swallowing it down. This is the first time researchers have documented reptilians with eating habits that more closely resemble those of a mammal.

University of Arkansas anthropology professor Peter Ungar and former graduate student Blaine Schubert worked together under a grant from the Jurassic Foundation to reconstruct the diets of tyrannosaurids. These giant carnivores - classified as reptilians, a term that encompasses both bird and reptile species - could have been scavengers or hunters. Ungar and Schubert originally started their research to settle that question.

In the process, however, Ungar and Schubert uncovered evidence that tyrannosaurids may have processed their food like mammals, rather than swallowing it whole like other reptilians. A paper detailing their findings has been accepted for publication in an upcoming issue of Acta Palaeontologica Polonica (APP), a paleontology journal headquartered in Poland.

Modern reptilians eat differently from mammals, the researchers explained. For the most part, they swallow food whole, without chewing. They don't have the same degree of muscle differentiation and jaw control that mammals have.

"They use their teeth to kill and capture animals, or to pull plant material into the mouth," Ungar said. "They don't break stuff down in the mouth at all. All digestion happens inside the body."

Mammals, on the other hand, have complex jaw joints and well-separated jaw muscles, with teeth of different shapes to allow precise chewing. The act of chewing starts the digestion process by breaking the food down to be digested further in the stomach.

"Mammals start with a big chunk of food and end up with lots of little pieces," Ungar said.

Reptilians and mammals process their food differently because they must maintain different metabolic rates. Reptilians are cold-blooded and rely on the sun to heat them so they can function. Because they don't derive their own heat, their metabolic rate is about one-tenth that of mammals.

"They don't require the same amount of energy from food to function," Ungar said.

As warm-blooded creatures, mammals have to increase the efficiency with which their bodies break down food. Part of that process involves breaking food down in the mouth so stomach acids can work on it more quickly, and the body can get more nutrients from it.

Scientists have yet to determine conclusively whether dinosaurs were warm-blooded or cold-blooded creatures. Since tyrannosaurids are reptilians, however, logic would seem to dictate that they would swallow their food whole.

"This makes me pause and say 'Wow they're breaking things apart in the mouth.' They're either eating something big or eating something more efficiently," Ungar said. "They're doing something different than what the typical crocodile does, for whatever reason."

Ungar and Schubert started their research at the Museum of Natural History in Gainesville, Fla., where they studied the teeth of modern reptilians, including caimans, iguanas and Komodo dragons. They used microscopy to look at the scratches, pits and grooves on the teeth and compared these wear features to the known diets of the reptilians. None of the modern reptilian teeth showed the tell-tale wear facets that occur with tooth-to-tooth contact, indicating that the reptilians were not using their teeth to process food before swallowing.

Next Ungar and Schubert's research took them to the Royal Tyrrell Museum of Palaeontology in Alberta, where they took high-resolution impressions of tyrannosaurid teeth. On these teeth, the researchers found wear facets with microscopic scratches all running in the same direction - something before only found in mammals.

"We believe at least some of these facets indicate tooth-to-tooth contact," Ungar said. "It's intriguing because the tyrannosaurids are the only group of reptilians we've seen this in."

"We are now excited about looking for similar patterns in other carnivorous dinosaurs," added Schubert.

Ungar and Schubert's findings indicate that T-rex and other tyrannosaurids may have adapted specific physical features for slicing flesh. Other meat-eating dinosaurs don't show the same features, making tyrannosaurids a specialized carnivore.

Ungar and Schubert's research was just the first step in determining that they may have processed their food differently. Next, they plan to test the hypothesis that these creatures were specialized, bone-crushing carnivores.

"We're looking at years of further research," Ungar said.