Tyrannosaurus rex and at least one other carnivorous dinosaur were capable of opening their jaws up to 90 degrees.
Plant-eating dinosaurs, on the other hand, were limited to a narrower jaw gape, suggesting that feeding style and diet of dinosaurs were closely linked to how wide they could open their mouths.
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"Theropod (carnivorous) dinosaurs, such as Tyrannosaurus rex or Allosaurus, are often depicted with widely-opened jaws, presumably to emphasize their carnivorous nature," author Stephan Lautenschlager from the University of Bristol's School of Earth Sciences said in a press release.
"Yet, up to now," he continued, "no studies have actually focused on the relation between jaw musculature, feeding style and the maximal possible jaw gape."
The study, published in the journal Royal Society Open Science, involved digital models, based on fossils, and computer analyses to recreate the muscle strain that likely occurred as dinosaurs opened their jaws.
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The research looked at not only T. rex, but also another huge carnivore, Allosaurus fragilis, and plant eater Erlikosaurus andrewsi. Erlikosaurus andrewsi was an interesting choice, since this dino was closely related to the meat-eating dinosaurs, but was known to mainly eat plants.
The herbivore's gape was just 45 degrees, equivalent to that of humans.
"All muscles, including those used for closing and opening the jaw, can only stretch a certain amount before they tear," Lautenschlager said. "This considerably limits how wide an animal can open its jaws and therefore how, and on what, it can feed."
Human evolution clearly benefited from cooking and other food processing innovations that allow us to eat a wide variety of foods without having very impressive jaw skills.
Among dinosaurs, T. rex appears to have had the best-sustained bite force, which would have allowed the dino to rip through flesh and skin, and to crush bone.
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"We know from living animals that carnivores are usually capable of larger jaw gapes than herbivores, and it is interesting to see that this also appears to be the case in theropod dinosaurs," Lautenschlager said.
In terms of living animals, hippos have an even wider bite than any dinosaur ever had, given that they can open up their jaws to an incredible 150 degrees. That means a 4-foot-tall individual could actually stand upright in a hippo's open mouth.
Hippos are considered to be herbivores, but they have been observed scavenging on meat when their preferred food sources are scarce.