A dog, when gnawing on a favorite bone, slices it with its molars. If the pooch instead had a crocodile's snout and gap teeth, crunching bones would be out of the question. But imagine if you will a croc-canine combo, one that is 40 feet long, 20 feet tall, walks on two legs and weighs about six tons. You'd end up with something strange, although not that far from a Tyrannosaurus rex. After all, when T. rex lived 67 million years ago, it was very fond of eating bones.

Yet it was far from your average dinosaur. “If you look at T. rex, it's a total anomaly compared to all other meat-eating dinosaurs,” paleontologist François Therrien told The Washington Post. Therrien, a curator at the Royal Tyrrell Museum in Canada, pointed to the mash-up of wimpy arms, massive jaws and teeth like “killer bananas.”

Those chompers allowed the T. rex to chew like a bone-crunching hyena despite its reptilian snout. Coupled with a massive bite force, as calculated in a study published Wednesday in the journal Scientific Reports, the conical teeth generated pressures at their tips of up to 431,000 pounds per square inch. The study authors said their report was the first to examine the pressure exerted by dino dentition.

The maximum pressure at the tip of a T. rex tooth was 28 times what is felt at the bottom of the deep-sea Mariana Trench in the western Pacific. It was enough to cause the toughest dinosaur bones to fracture.

Put another way, a bite from a T. rex could shatter bones like a “. 45-caliber bullet with a mushroom head,” said paleontologist Gregory M. Erickson, a co-author of the study and curator at Florida State University's Biological Science Museum.

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Scientists have long known that T. rex ate bones, as indicated by the fragments found in fossilized dinosaur dung. Bones, particularly the yolk-like marrow inside, are rich in nutrients.

As a rule, though, reptiles don't gnaw on bones. Their teeth aren't designed to. Even crocodiles, with their powerful jaws, tear chunks flesh away from a skeleton. 

Crunching bones is “a high-risk, high-reward system,” said Paul Gignac, a paleontology professor at Oklahoma State University, whose graduate work with Erickson in Florida led to the study. “You could damage teeth and not be able to feed, and starve to death.”

Mammalian crunchers such as wolves or hyenas use their specialized molars like hacksaws to slice through bones. Similar to these animals, T. rex gnawed repeatedly; one Triceratops pelvis, which Erickson studied in 1996, had been chewed about 80 times. But T. rex also used its teeth like a jackhammer to fracture bone.

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To figure out how the dinosaur managed to snap bones into fragments, Gignac and Erickson constructed a computer model of a T. rex jaw. They based the model on measurements of living relatives: birds, crocodiles and alligators, a group known as archosaurs.

“Our model is the first one to look at the musculature of archosaurs,” Erickson said. Other scientists had tried to measure T. rex bite force based in part on mammalian bites, or by extrapolating force from the dinosaur's body size, leading to a wide range of estimates. One 2012 Biology Letters study, for instance, reported that T. rex bit with 12,800 pounds of force. A few older estimates reported jaws four times as powerful.

With their model, Gignac and Erickson calculated that a T. rex could bite with about 8,000 pounds of force, fairly conservative in the realm of Tyrannosaur estimates. Still, it's a massive amount of force — equivalent to the creature dropping a hippopotamus on its prey.

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Therrien was impressed with the complexity of the model, which reconstructed the jaw down to individual muscles. It was a “great approach,” he said, “and the values are probably reasonable.” 

More important than bite force alone was how T. rex used it. To figure that out required calculating the pressure exerted on bones caught between the dinosaur's teeth. The two paleontologists used an engineering technique, picturing a bone held in a T. rex jaw as a three- or four-point flex test. The teeth, or the hardened roof of the mouth, acted as pressure points on a beam, which exerted a bending force until the bone snapped.

(You can try a three-point flex test at home, if you're feeling destructive: Grip the ends of a pencil in both hands and press inward on its middle with your thumbs. Now imagine your thumbs are 8-inch long teeth and your pencil is a Triceratops leg.)

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T. rex probably held whole limbs in its mouth as it gnawed. “It's not much different than when we bite down on a drumstick,” Gignac said, “though we go for flesh, not bone.” The jackhammer teeth allowed T. rex to crack into bones from fresh kills or scavenged corpses, a food source that other predators couldn't access.

The teeth also may have compensated for the dinosaur's puny arms when hunting. “These dinosaurs were literally headhunters,” Therrien said, “because everything had to be done with the head. When dealing with prey that’s your size, you have to kill it quickly.” In that case, a ferocious, bone-breaking bite goes a long way.

Read more:

Scientists believe they’ve found a ‘pregnant’ T. rex

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Scientists did an autopsy of a ‘T. rex’ on TV and it was awesome (and bloody)

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