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Not just tiny arms: T. rex also had super small eyes to accommodate its big bite

illustration shows t. rex skull with its true eye socket shape compared to skull with circular eye socket, which can hold a far larger eyeball
This illustration of Tyrannosaurus rex compares the dino's original eye socket and eye (left) with a hypothetical reconstruction with a circular eye socket and enlarged eye (right). (Image credit: Dr Stephan Lautenschlager, University of Birmingham.)

The powerful jaws of Tyrannosaurus rex snapped together with such force that they would splinter the bones of the dinosaur's prey. But to gain such a powerful bite, the king of the dinosaurs had to make an evolutionary trade-off: It had to settle for smaller eyes.

Based on an analysis of 410 fossilized reptile specimens from the Mesozoic period (252 to 66 million years ago), a scientist concluded that T. rex and other flesh-eaters of similar ilk evolved smaller, narrower eyes over time, likely to compensate for their bites becoming more and more forceful. In particular, carnivores with skulls longer than 3.2 feet (1 meter) tended to have elongated, keyhole-like eye sockets — or orbits — as adults, while the carnivores' young offspring and herbivores of all ages had circular eye sockets. 

"This makes sense, of course; as the predators grew larger they would have switched to larger prey, which needed larger bite forces to tackle," Stig Walsh, the senior curator of vertebrate palaeobiology at National Museums Scotland, who was not involved in the study, said of the juvenile carnivores. 

The new research, published Thursday (Aug. 11) in the journal Communications Biology (opens in new tab), lends support to the idea that the brain and sensory organs, such as the eyes, must adapt to accommodate animals' primary feeding strategies, Walsh told Live Science in an email. And in the case of T. rex, that feeding strategy centered around a bone-crushing bite.

Related: 'Bold theory' that Tyrannosaurus rex is 3 species gets stomped to pieces 

Here we see computer simulations of hypothetical dinosaur skulls. The colors indicate skull stress and demonstrate how high stresses occur in the skull with a round eye socket (top), while lower stresses occur in a skull with a keyhole-shaped eye socket (bottom). (Image credit: Dr Stephan Lautenschlager, University of Birmingham)

For his analysis, the study's author Stephan Lautenschlager, a vertebrate palaeontologist at the University of Birmingham in the U.K., scoured the existing literature to find descriptions of dinosaur and reptile skulls that dated back to the Mesozoic. From these, he selected hundreds of skulls with pristinely preserved eye sockets, as well as a handful of incomplete skulls whose eye sockets could be reconstructed with a "large degree of confidence." 

The 410 specimens included a wide range of species, from crocodilians to hulking herbivores like Triceratops to flesh-eating theropods such as T. rex and Tarbosaurus bataar, a tyrannosaur relative with a similarly huge frame and puny arms.

In comparing all these skulls, Lautenschlager spotted several patterns: Most of the creatures, but especially herbivores, had circular eye sockets. However, as you move through the Mesozoic, the orbit shapes of large-headed carnivores began morphing into oval- and keyhole-shaped openings. 

Juvenile specimens of some of these carnivores — including T. rex and T. bataar — suggested that the dinos developed these squashed eye sockets in adulthood, whereas they retained circular sockets in their youth. "Obviously we don't have growth series for many species, but for the ones we have, for me this makes the case much stronger that the reason for the shape variation we see is related to feeding," Walsh noted. So as a young T. rex grew larger, so too would its prey, and its bite force had to increase to adjust. 

To understand how these eye socket shapes might affect a dinosaur's ability to munch through bones, Lautenschlager devised three computer models, each more complex than the last. 

The first and simplest model was of a flat plate with the various eye socket shapes carved into it — think of how rivets would help distribute forces through a solid steel plate. "The position and shape of the hole has an influence [on] how stress and deformation propagate" through the plate, Lautenschlager told Live Science in an email. The final and most complex model was a digitized T. rex skull. "As the stereotype of a large carnivorous theropod with an extremely constricted orbit, [T. rex] was ideal to test the effect of orbit shape in an actual dinosaur species," Lautenschlager said.

These models revealed that, during a simulated bite, keyhole-shaped eye sockets deformed far less than the circular ones because they directed the force of the bite toward robust bones behind the eye socket. "The keyhole shape reduces and redirects stresses in the skull during bite a lot better than a circular orbit would," Lautenschlager said. "This is clearly an adaptation found in many large carnivores across different groups. Something that evolved independently." 

Related: As many as 2.5 billion Tyrannosaurus rexes once stalked Earth

If, in an alternate timeline, T. rex never evolved elongated orbits and instead had circular ones, the dinosaur's eye would have weighed nearly 44 pounds (20 kilograms) and measured 11.8 inches (30 centimeters) across, instead of weighing an estimated 4.4 pounds (2 kg) and measuring 5.1 inches (13 cm) across, the models suggested. So circular sockets could have supported eyes about seven times the volume of eyes that could fit in keyhole sockets. 

Having such massive eyes would have been metabolically costly to T. rex and wouldn't match what we know about the dinosaur's brain, Walsh said. "The retina is an outgrowth of a region of the brain called the diencephalon, and the one thing we know about large predators like T. rex is that the size of their brains did not keep pace with the size of their bodies as they grew during their lifespans," he said. So if T. rex's eye size kept pace with its overall skull size, the regions of the brain that deal with vision would have also needed to grow larger. 

It's key to note that, while the new study provides strong hints about dinosaurs' overall eye sizes, fossilized skulls can't reveal fine details of the anatomy of the eye or associated soft tissues, such as nerves and muscles. 

"This is [where] we reach an impasse in palaeontology as we can tell little about actual eyeball shape based on the fossilised bones," Lautenschlager said. "Some dinosaur species may have had specialised eyes similar to modern birds." For example, owls have elongated, barrel-shaped eyes, and this shape affects how light hits their retinas; for now, we can't discern the exact shape of T. rex's eye or how that may have impacted the species' vision, he said. 

In follow-up studies, it would be interesting to expand Lautenschlager's orbit shape analysis to include birds, dinosaurs' only living descendents, as well as mammals with powerful bites, Walsh said. "Perhaps mammals with high bite forces evolved a different way to dissipate stresses that reptiles — this would bear investigation," he said.

Originally published on Live Science.

Nicoletta Lanese
Staff Writer

Nicoletta Lanese is a staff writer for Live Science covering health and medicine, along with an assortment of biology, animal, environment and climate stories. She holds degrees in neuroscience and dance from the University of Florida and a graduate certificate in science communication from the University of California, Santa Cruz. Her work has appeared in The Scientist Magazine, Science News, The San Jose Mercury News and Mongabay, among other outlets.