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With blunt, serrated teeth and a bite powerful enough to crush the bones of its prey, Tyrannosaurus rex has earned a reputation as one of the largest and most terrifying predators of the dinosaur world. Scientists have estimated T. Rex had a maximum bite force of more than six tons (12,000 pounds), more than any other known terrestrial animal.

T. Rex’s bite is known to be so powerful, in fact, that scientists have long wondered how the massive therapod managed to bite down so hard without breaking its own skull. They assumed that, like many modern birds and reptiles, the dinosaur’s skull may have been flexible enough to move around and change shape as it bit down—a trait known as cranial kinesis.

But in a study published in The Anatomical Record in September 2019, scientists at the University of Missouri argue that T. Rex’s jaw was actually fused and stiff, much like the jaws of modern crocodiles, rather than flexible.

An artist’s rendition of the Tyrannosaurus rex with the 3D imaging showing muscle activation in its head.

An artist’s rendition of the Tyrannosaurus rex with the 3D imaging showing muscle activation in its head.

After studying how the jaws of two modern relatives of T. Rex, grey parrots and geckos, worked when they chewed, the researchers used computer simulations to apply those models to a 3-D model of a T. Rex skull. They observed that the joints and ligaments of the T. Rex skull did not respond well to being moved around in the way the bird and lizard skulls did, and that biting down with the force that T. Rex did would have broken its jaw if it had exhibited the flexibility associated with cranial kinesis.

“When you put a lot of force on things, there’s a tradeoff between movement and stability,” Casey Holliday, associate professor of anatomy at the MU School of Medicine and a co-author of the study, said in a university press release. “Birds and lizards have more movement but less stability. When we applied their individual movements to the T. Rex skull, we saw it did not like being wiggled in ways that the lizard and bird skulls do, which suggests more stiffness.”

The researchers concluded that T. Rex’s joints were likely tightly fused, making its skull extremely rigid and inflexible, able to withstand the tremendous force it used to bite down on its prey. This kind of skull resembles those of other powerful biters such as tigers and hyenas, as well as alligators and crocodiles, the biggest of which of have a bite force of around 3,700 pounds, the strongest that has been measured among living animals. Scientists believe the most powerful bite force ever belonged to now-extinct giant crocodiles, which measured 35 to 40 feet long and may have had a bite force of up to 18,000 pounds.

In addition to shedding new light on the anatomy of one of the prehistoric world’s apex predators, the new study may impact modern medicine (both animal and human) by providing a better understanding of how joints and ligaments work.

“In humans, this can also be applied to how people’s jaws work,” said Ian Cost, lead researcher on the study and an assistant professor at Albright College. “In animals, understanding how those movements occur and joints are loaded will...help veterinarians better understand how to treat exotic animals such as parrots, which suffer from arthritis in their faces.”

New Discovery

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