T. rex’s unusual eye sockets helped it evolve a powerful bite

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Some large, meat-eating dinosaurs such as Tyrannosaurus rex had keyhole-shaped eye sockets, and reconstructions suggest this helped them bite with greater force

Life



11 August 2022

Tyrannosaurus rex with its original eye socket and eye (left) and a hypothetical reconstruction with a circular eye socket and enlarged eye (right)

Stephan Lautenschlager, University of Birmingham

The eye socket of Tyrannosaurus rex played an important part in how the giant carnivores evolved some of the most powerful bites of all time.

The skulls of some massive, meat-eating dinosaurs contain a puzzling feature: rather than being round, the bony eye socket, also called the orbit, is keyhole-shaped.

“Orbits and eyes are poorly studied in dinosaurs relative to other parts of their anatomy,” says Stephan Lautenschlager at the University of Birmingham, UK – particularly the variation in orbit shape between dinosaur groups.

After surveying the shape of eye openings across dinosaurs and their relatives, Lautenschlager found that most of the animals in the study had more or less round orbits. The main exceptions were big carnivores such as adult T. rex, Skorpiovenator and Cryolophosaurus. These distantly related carnivores all shared the keyhole orbit shape.

The unusual opening in their skulls may be the result of an evolutionary trade-off between larger eyes and more powerful bite forces. Computer models suggest that a smaller eye allowed for additional muscle attachments in the skull that would have allowed these dinosaurs to evolve stronger bites, whereas a larger orbit, and therefore a larger eye, wouldn’t have provided much additional benefit.

In stress tests of skulls with different eye shapes, the keyhole orbits were better able to dissipate the stresses of biting hard than round ones.

“Eyes are physiologically very expensive and there is likely a compromise between getting larger eyes, which could enable better sight, versus stress reduction,” says Lautenschlager, with the latter being more important for these carnivores.

The new study has major implications for research into how carnivorous dinosaurs matured, says Andre Rowe at the University of Bristol, UK, who wasn’t involved in the work. The juveniles of dinosaurs such as T. rex had circular orbits like most other dinosaurs. As T. rex grew up, however, its skull shape and anatomy changed dramatically.

“Juveniles preyed on agile, small prey, while adults dispatched large prey with a single bite,” says Rowe, and the orbit shape played a part in this menu shift.

Journal reference: Communications Biology, DOI: 10.1038/s42003-022-03706-0

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