The bone-shattering bite of a Tyrannosaurus rex could have crushed a car, delivering up to six tons of pressure to its hapless victims. But while multiple lines of evidence support this estimate of the dinosaur’s mighty bite force, debate has swirled about how it got the job done with what seems to be a loosely jointed skull.
The answer is, it didn’t, according to a new model of all the stresses and strains that moved across a T. rex skull as it chomped down. The results, presented this month in the journal The Anatomical Record, show that the skull bones of T. rex must have been held fixed and rigid for the animal to have had such a fearsome bite. (Find out about the world’s biggest T. rex, which was recently found in Canada.)
“T. rex is just one of those very optimally built animals,” says study coauthor Casey Holliday, a paleontologist at the University of Missouri School of Medicine. “It has all these giant jaw muscles, and it’s very efficient at taking that muscle force and putting it into its prey item because it has a stiff skull.”
No wiggle room
The idea that the joints between some of the bones in a T. rex skull might have been mobile has been pervasive, Holliday says. That’s in part based on the appearance of the fossils, and in part because some living relatives of dinosaurs, including parrots and snakes, have flexible skulls with bones that move. Reptiles in particular have a series of bones that link their braincases with their palates and then to their lower jaws, or mandibles.
“This is very different than mammal skulls, like ours, in which there are only two parts: the part that holds the brain and the mandible,” Holliday says.
However, the idea that T. rex also had a flexible skull presented a problem mathematically. (Also see how T. rex’s tiny arms may have actually been vicious weapons.)
“When you have this giant thing like a T. rex skull that is six feet long and four feet wide and bites with a huge amount of force … if you have flexibility built into the system, you’re going to be set up for a lot more failure,” Holliday says.
“You want to take all the force from the muscles and put it into the prey through your teeth, and not have it leak out through a bunch of wiggly joints.”
To test the idea, Holliday and his former graduate student Ian Cost, now an assistant professor at Albright College in Reading, Pennsylvania, created digital models of T. rex skulls with palates that were able to flex out to the sides like those of geckoes, or ones that moved up and down like those of grey parrots. The researchers then modeled the biomechanics of these skulls in action.
The team found that the carnivore would have been able to apply pressure most effectively when the joints in its upper skull remained largely immobile, although a tiny amount of flexibility would have helped the skull resist the incredible forces applied to it.
“The face and cranium of T. rex weren’t capable of movement … this supports our conclusion that the [palate] bones of the mouth didn’t move when T. rex bit its prey,” Cost says. This means that the species could better use the full force of its jaw muscles than its ancestors or relatives with mobile palates could.
“The results presented in this study, which has been carried out with tremendous attention to detail, not only demonstrate that the skull of T. rex could resist very high bite forces, but precisely how it did so,” says Laura Porro, an expert on fossil biomechanics at University College London. Porro adds that the work will now help researchers determine the flexibility of skulls belonging to other fossil animals.
Eric Snively, a paleobiologist at Oklahoma State University in Tulsa who has also studied the feeding mechanics of T. rex, says that the research “helps answer how T. rex could bite with the highest forces of any land animal.”
Tyrannosaurs are unusual, he argues, because their teeth are strongest at the front of the mouth, unlike predators such as crocodiles, which have their crushing teeth at the back. (Here’s how modern crocodiles may rival T. rex’s bite force.)
“Their snouts were fused up with interlocking bones on the bridge of the nose, but until the current study, we didn’t understand how the rest of the cranium functioned,” he says.
“We’re now gaining a complete picture of their skull anatomy, which is great for figuring out how T. rex bit with enough force to lift several big pick-up trucks.”