T. rex’s unimaginable biting pressure got here from its stiff decrease jaw

The fearsome Tyrannosaurus rex might generate super bone-crushing chew forces due to a stiff decrease jaw. That stiffness stemmed from a boomerang-shaped little bit of bone that braced what would have been an in any other case versatile jawbone, a brand new evaluation suggests.

In contrast to mammals, reptiles and their shut kin have a joint dubbed the intramandibular joint inside their decrease jawbone, or mandible. New laptop simulations present that with a bone spanning the IMJ, T. Rex might have generated chew forces of greater than 6 metric tons, or concerning the weight of a giant male African elephant, researchers reported April 27 on the digital annual assembly of the American Affiliation of Anatomy. 

In at the moment’s lizards, snakes and birds, the IMJ is certain by ligaments, making it comparatively versatile, says examine creator John Fortner, a vertebrate paleontologist on the College of Missouri in Columbia. That flexibility helps the animals preserve a greater grip on struggling prey and likewise permits the mandible to flex wider to accommodate bigger morsels, he notes. However in turtles and crocodiles, for instance, evolution has pushed the IMJ to be somewhat tight and rigid, enabling sturdy chew forces.

Till now, most researchers have presumed that dinosaurs had decrease jaws with a versatile IMJ, however there’s a giant flaw with that premise, Fortner notes. A versatile jaw wouldn’t have enabled bone-crushing chew forces, however fossil proof — together with coprolites, or fossil poop, stuffed with partially digested bone shards — strongly means that T. rex might certainly chomp down with such forces (SN: 10/22/18).

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“There’s each cause to imagine that T. rex might chew actually exhausting, kinda off the charts,” says Lawrence Witmer, a vertebrate paleontologist at Ohio College in Athens who wasn’t concerned within the examine. “It’d be good to understand how they might carry off these chew forces.”

Utilizing a 3-D scan of a fossil T. rex cranium, Fortner and his colleagues created a pc simulation of the mandible that might be used to investigate stresses and strains, akin to the best way engineers analyze bridges and plane elements. Then they created two variations of the digital jawbone. In each of them, they lower in half a boomerang-shaped bone, referred to as the prearticular, that’s adjoining to however spans the IMJ. Then, in a single simulation, they joined the 2 sides of the IMJ with digital ligaments that rendered the jawbone versatile. In a second model of the simulation, the staff nearly rejoined the 2 items of the prearticular with bone somewhat than ligaments.

The staff’s simulations confirmed that when the severed prearticular was nearly rejoined with ligaments, stresses couldn’t be successfully transferred from one aspect of the IMJ to a different, says Fortner. In that state of affairs, the mandible grew to become too versatile to generate giant chew forces. However when the items of the prearticular had been rejoined with bone — much like having the bone stay intact — stresses might be easily and effectively transferred from one aspect of the joint to a different.

Two simulated T. rex jawbones reveal how a small bone (not seen) that spans a joint (white arrow) supplies for a robust chew. In a model the place that bone just isn’t intact (high), the jawbone flexes, which prevents stress induced by a chew at one tooth (black arrow) from transferring successfully throughout the joint. However in a jawbone during which that bone is unbroken (backside), the extra inflexible joint transfers stresses successfully, enabling higher chew forces.John Fortner

Two simulated T. rex jawbones reveal how a small bone (not seen) that spans a joint (white arrow) supplies for a robust chew. In a model the place that bone just isn’t intact (high), the jawbone flexes, which prevents stress induced by a chew at one tooth (black arrow) from transferring successfully throughout the joint. However in a jawbone during which that bone is unbroken (backside), the extra inflexible joint transfers stresses successfully, enabling higher chew forces.John Fortner

The staff’s findings “are probably attention-grabbing,” says Witmer. “The prearticular just isn’t a very massive bone, however it might be concerned within the chew,” he notes.

The T. rex mandible is an advanced association of assorted bones, however “the prearticular appears to lock the system collectively,” says Thomas Holtz, Jr., a vertebrate paleontologist on the College of Maryland in Faculty Park who wasn’t concerned within the examine. These simulations present “it supplies a demonstrable profit.”

Sooner or later, Fortner and his colleagues will conduct comparable analyses for the mandibles of different dinosaurs within the T. rex lineage to see how the preparations of constituent bones, and notably the IMJ, might need advanced over time.

The outcomes of such research might be fairly attention-grabbing, says Holtz. Dinosaurs close to the bottom of the T. rex household tree had jawbones that had been formed in another way, and so they didn’t have bones to brace the IMJ, he notes. These theropods, or bipedal meat-eating dinosaurs, additionally had bladelike enamel somewhat than the banana-shaped enamel of T. rex, so that they in all probability had a vastly completely different feeding model. In these ancestors, Holtz notes, a versatile IMJ could have served as a “shock absorber” when chomping down or throughout assaults on prey.

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