Tyrannosaur Teens Didn’t Have the Bone-Crushing Bite of Adults

Teeth from the T. rex specimen “Stan.”
Teeth from the T. rex specimen “Stan.”
Image: Mary Altaffer (AP)

Adult tyrannosaurs were basically bone crushing machines on two feet. New research suggests juvenile tyrannosaurs couldn’t match the same level of brutality, resulting in a distinctive bite that set them apart from their adult counterparts.

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Nothing like tyrannosaurs exist today, and thank goodness for that. As the quintessential apex predator of the Mesozoic, these megatheropods featured bite forces that boggle the imagination. The modern lion exerts a respectable 1,300 Newtons of force when chomping down on prey , but T. rex—with its wide and deeply set jaw—exerted a whopping 60,000 Newtons of force with each horrific bite.

“Adult T. rex is estimated to have possessed some of the highest impact bite forces recorded in any animal, with enough power to crush a car based on previous studies,” Andre Rowe, a geology PhD student at the University of Bristol, wrote in an email. “While the animal’s massive size and huge muscles were key in delivering its staggering bite, there is also significance in its shape, as we found the slender mandible of the younger forms were less capable of withstanding the stresses of such a high impact bite.”

Such is the main finding of a new study led by Rowe, published on Tuesday in the science journal Anatomical Record. The new paper provides further evidence showing that juvenile tyrannosaurs were strikingly different from their adult versions, and that they occupied—and likely dominated—a separate ecological niche.

The point of the new study was to explore how the feeding techniques of large predatory dinosaurs changed during their various growth stages. Previous research showed that the juvenile T. rex had a slender jaw, which eventually grew into the iconic deeply-set jaw seen in adults. Rowe and his colleagues “wanted to test the functional significance of this change,” he said.

To do so, the researchers ran 3D biomechanical models based on CT scans taken of complete T. rex skeletons, and inferences about muscle size, shape, and placement. For a juvenile T. rex model, the researchers tested a young tyrannosaur from Mongolia named Raptorex kriegsteini.

“The main method we used in our study was an engineering technique called finite element analysis, which reveals stress and strain in solid structures,” explained Rowe. “Areas of structures tested that are at high risk of breakage will display ‘hot’ colors like red and white, whereas ‘cold’ colors like green and blue indicate points where breakage risks are most minimal.”

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Adult T. rex jaw with biting stress indicated by color.
Adult T. rex jaw with biting stress indicated by color.
Image: Andre Rowe

Tests of the juvenile and adult tyrannosaurs at equal jaw lengths showed that the juveniles endured higher stresses, indicating the need for a deep, broad jaw shape in adults. Key to this finding was an analysis of the T. rex pterygoid muscles, which are located at the lower back end of their mandibles.

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“Since we were applying virtual muscles to these dinosaurs to get accurate biting results, we had the opportunity to test for the significance of individual muscles during feeding,” said Rowe. “The pterygoid muscles were found to decrease the bending stresses near the front of the jaw, where T. rex may have applied its highest impact bites using its massive, conical teeth.”

As an interesting aside, modern crocodiles exert their highest bite forces towards the back of their jaws, whereas in tyrannosaurs, it’s towards the front.

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Skeletons of tyrannosaur specimens tested in the study.
Skeletons of tyrannosaur specimens tested in the study.
Image: Clockwise from above left: adult Tyrannosaurus rex “Sue” (FMNH PR 2081) (Field Museum of Natural History, Chicago, IL; photo by the Field Museum), juvenile Tyrannosaurus rex “Jane” (BMRP 2002.4.1) (Burpee Museum of Natural History; photo by A. Rowe), adult Tarbosaurus bataar (Dinosaurium exhibition, Prague, Czech Republic; photo by R. Holiš) and Raptorex kriegsteini skeletal reconstruction (LH PV18) (Long Hao Institute of Geology and Paleontology, Hohhot, Inner Mongolia, China; photo by P. Sereno). Final image by Andre Rowe.

Rowe’s new paper suggests the slender jaws of adolescent tyrannosaurs would’ve been susceptible to serious damage had the mandible shape remained the same throughout maturity into adulthood. But that’s not the case, as the wider adult T. rex mandible is capable of absorbing the tremendous forces required to crush bone. In terms of specifics, a juvenile T. rex with a slender jaw—but with the length of an adult jaw—would’ve had to endure 3.3 times the stress, which wouldn’t have worked. Hence the dramatic physical differences seen in juvenile and adult tyrannosaurs.

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“Adult tyrannosaurs are noted for their bone-crunching bite, which they used to swallow chunks of flesh and bone whole, but the juveniles were not equipped for such a bite and hence may have used a slashing bite that it would eventually outgrow,” explained Rowe.

So instead of hunting massive herbivores like Triceratops and Edmontosaurus, juvenile T. rexes likely hunted smaller dinosaurs, and possibly even the small mammalian species that existed during the Late Cretaceous, a period that ended some 66 million years ago.

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“While the adult T. rex remains an iconic apex killer, it is quite remarkable to think that it began as a slender-bodied animal that pursued scurrying mammals,” said Rowe.

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Kat Schroeder, a PhD candidate at the University of Mexico who was not involved with the new study, said the new research supports an idea presented in her recent paper, that “juvenile tyrannosaurs were physically different enough from their parents to act as their own ‘morphospecies,’ and may have out competed other medium-sized theropods.

Schroeder said the relatively small sample size used for the new study was “limiting,” but the “science is well done,” and it “fits with the hypothesis that tyrannosaurs changed their hunting style drastically as they grew.”

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Looking ahead, Rowe would like to see further work having to do dinosaurs, and other extinct animals, and how their feeding styles change over time.

“CT-scanning and 3D modelling techniques have given us wonderful insights concerning dinosaur biology, and the methods are applicable to animals both living and extinct,” he said.

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George is a senior staff reporter at Gizmodo.

DISCUSSION

Fun fact - you’d be in a lot more danger if a juvenile T-Rex was nearby and hungry, versus an adult. Adult Rexes mostly ate larger prey and had a top speed of around 12 MPH based on biomechanical modeling, whereas young Rexes could hit 33 MPH and hunted stuff that was much more comparable to humans in size and weight.