The mighty T. rex had small eyes

The predatory dino has traded in its large eyes for more powerful jaws.

The famous Tyrannosaurus Rex could completely crush the bones of its prey. This dino could pulverize bones with a force equivalent to the weight of three toy cars (see sidebar). But, to get such a powerful bite, the T. rex cut elsewhere. And on his eyesight, say researchers in a new study. This is how the T. rex settle for smaller eyes.

Many animals, including most dinosaurs, typically have round eye sockets that house the eyeball. It’s different with some carnivorous dinosaurs. For example, it seems that large carnivores with a skull over a meter (like the terrifying T. rex) had oval, more keyhole-shaped orbits. A pressing question, however, is: what were these unusually shaped orbits used for?

The skulls of different types of dinosaurs with the shape of the eye socket shown in dotted lines next to them. As you can see, the eye sockets of herbivorous dinosaurs are much rounder than those of carnivores. Image: Dr Stephan Lautenschlager, University of Birmingham

To explain these rather curiously shaped eye sockets, researchers studied the eye sockets of 410 fossilized reptiles – including dinosaurs and their closest relatives such as crocodiles – from the Mesozoic era. The Mesozoic geological epoch is also known as the mighty age of dinosaurs, which spanned from 252 to 66 million years ago.

Oval ears
The researchers found that large carnivorous species developed keyhole-shaped eye sockets over time. However, they did not acquire such eye sockets until adulthood. In other words, as predatory dinosaurs grew, their eye sockets became more and more oval.

bite force
According to the researchers, this could be linked to the bite force of carnivorous dinosaurs. As a young T. rex as it grew, its prey also increased, which meant that its bite force must also increase. The researchers then ran several simulations to investigate how the shape of the eye socket influences bite force. And that goes to show that round eye sockets cause a lot more stress on the skull when biting. More oval orbits greatly reduce this tension. Narrower eye sockets also provide more space for the jaw muscles. Basically apex predators including T. rexthanks to more oval orbits, had a greater bite force, without compromising the stability of the skull.

Eyes
Although the jaws of T. rex were incredibly powerful, it was to the detriment of his eyes. In exchange for the greater bite force, the dino unfortunately also had to go through life with small eyes in addition to its short arms. The narrower eye sockets simply meant less space for the eyeballs.

eye size
The researchers also looked at what would have happened if eye size had increased at the same rate as skull length. In this case, the eyes of T. rex measure up to 30 cm in diameter and weigh nearly 20 kg (instead of the estimated 13 cm and 2 kg).

Tyrannosaurus rex skull with original orbit and eye (left) and hypothetical reconstruction with circular orbit and enlarged eye (right). Photo: Dr. Stephan Lautenschlager, University of Birmingham

The results point out the functional compromises dinosaurs had to make. And in this case, smaller eyes allowed for greater bite force. Not only is this an interesting new insight, but it finally explains why the terrifying T. rex equipped with such small eyes. But, in its place was the bite force of T. rex Impressive. However, the dinosaur is not the strongest in terms of bite force. The bite force of the giant crocodiles now extinct – which could reach more than ten meters in length – would have been nearly three times greater.