A small bone stiffened its lower jaw, bracing an otherwise flexible joint.
The fearsome Tyrannosaurus rex had a tremendous bone-crushing bite. What made this possible was a stiff lower jaw. And that stiffness came from a boomerang-shaped bit of bone. A new study finds that this small bone braced what would have been an otherwise flexible lower jaw.
Unlike mammals, reptiles and their close kin have a joint within their lower jawbone, or mandible. That lower jaw gives this joint its tongue-twister name — intramandibular (IN-truh-man-DIB-yu-lur) joint. Many scientists just call it the IMJ.
Using a computer model, scientists now show that with a bone spanning this IMJ, T. rex could have generated bite forces of more than 6 metric tons. That’s about the mass of a large male African elephant.
John Fortner is a vertebrate paleontologist at the University of Missouri in Columbia. He and his colleagues described their new analysis April 27. They presented their data at the virtual annual meeting of the American Association of Anatomy.
In today’s lizards, snakes and birds, ligaments bind the IMJ. That makes it relatively flexible, says Fortner. And this flexing helps animals maintain a better grip on struggling prey. It also allows the jaw to flex wider to accommodate larger morsels, he notes. But in turtles and crocodiles, for example, evolution has driven the IMJ to be rather tight and inflexible. And that has its own benefit: a more forceful bite.
Until now, most researchers had assumed dinosaurs had a flexible IMJ. But there was one big flaw to that idea, Fortner says. A flexible jaw would not have enabled a bone-crushing bite. And fossils strongly suggest that T. rex could indeed chomp down with such forces. Among those fossils were coprolites — fossil poop — filled with partially digested bone shards.
“There’s every reason to believe that T. rex could bite really hard, kinda off the charts,” says Lawrence Witmer, who wasn’t involved in the study. “It’d be nice to know how they could carry off these bite forces,” says this vertebrate paleontologist. He works at Ohio University in Athens.
Tech finds an answer
Fortner and his colleagues started with a 3-D scan of a fossil T. rex skull. From this, they used a computer model to simulation the mandible and how it would move. This allowed them to study stresses and strains on those bones in much the same way engineers analyze bridges and aircraft parts. Then they created two versions of the virtual jawbone. In both, they cut a boomerang-shaped bone in half. This bone, the prearticular (Pre-ar-TIK-yu-lur), is next to and spans the IMJ.
In one simulation, they joined the two sides of the IMJ with virtual ligaments. This would have left the jawbone flexible, the simulation showed. In a second simulation, the team virtually rejoined the two pieces of boomerang-shaped bone. Here, no ligaments were at play.
The computer model showed that when ligaments joined the severed prearticular, the jaw no longer could effectively transfer stresses from one side of the IMJ to another. Here, Fortner says, the mandible was too flexible to generate big bite forces. But when the pieces of the prearticular were rejoined with bone (similar to having the bone remain intact), the jaw smoothly and efficiently transferred stress from one side of the joint to the other.
The findings “are potentially interesting,” says Witmer. “The prearticular is not a particularly big bone, but it could be involved in the bite,” he says.
The T. rex lower jaw is a complicated group of joined bones. And “the prearticular seems to lock the system together,” says Thomas Holtz, Jr. He’s a vertebrate paleontologist at the University of Maryland in College Park who wasn’t involved in the study. The new model now suggests that prearticular “provides a demonstrable benefit.”
Fortner and his colleagues hope to conduct similar analyses for the mandibles of other dinos in the T. rex family. They want to see how the arrangements of jaw bones, and particularly the IMJ, might have evolved over time.
The results of such studies could be quite interesting, says Holtz. Dinosaurs near the base of the T. rex family tree had jawbones that were shaped differently, he notes. They also didn’t have bones to brace the IMJ. These theropods — or two-footed, meat-eating dinosaurs — had bladelike teeth. On T. rex, they’re banana-shaped. So the two types likely had a vastly different feeding style. In the T. rex ancestors, Holtz notes, when chomping down or during attacks on prey, a flexible IMJ could have worked as a “shock absorber.”