The king of the dinosaurs , Tyrannosaurus rex, had very small arms compared to the rest of his body. For many animals, and most dinosaurs, the eye socket is just a circular hole in the skull that houses the eyeball; but this is very different in the large carnivores.
It seems that having developed narrower eye sockets than their ancestors allowed them to bite more forcefully.
“The results show that only some dinosaurs had elliptical or keyhole-shaped eye sockets,” says Stephan Lautenschlager of the University of Birmingham and leader of the work published in the journal Communications Biology. “However, these were all large, carnivorous dinosaurs with skulls 1 meter or more.”
The researcher compared the eye sockets of 410 fossilized specimens from the Mesozoic Era , including dinosaurs and their close relatives such as crocodiles. The ancient reptiles came from the Mesozoic, between 252 and 66 million years ago, and ended with that annoying meteorite that marked the end of the Age of the Dinosaurs.
Observation of the eye holes revealed that there were certain trends between the eyes of dinosaurs and ancient reptiles. Herbivores had circular eye sockets, as did juveniles of certain predatory species. However, adult predators seemed to develop keyhole-shaped eye sockets, which house the tiny eyes we see in T. rex (complementing their tiny arms). Circular eyes were also more common in early Mesozoic species whose later ancestors were more likely to have smaller, less circular eyes.
Using computer simulations and stress analysis, Lautenschlager investigated what purpose those unusual eye socket shapes might serve and found that a skull with circular eye sockets was more prone to high biting stresses. However, if these were replaced with elliptical or oval eye sockets, the stress during the bite would be greatly reduced, allowing top predators to develop tall bite sources that would not compromise the stability of their skull.
“In these species, only the upper part of the eye socket was occupied by the eyeball. This also led to a relative reduction in the size of the eyes compared to the size of the skull.” “ These morphologies are beneficial in mitigating and dissipating feeding-induced stress and require only a small investment to reinforce the bony structure of the skull. In contrast, developing and maintaining large circular orbits and corresponding eyes would be physiologically costly and would likely outweigh the potential benefits for visual acuity.”
The study also showed that most plant-eating species and large juvenile dinosaurs retained a circular eye socket. Only large adult carnivores adopted other morphologies, such as elliptical, keyhole-shaped, or figure-of-eight eye sockets.
The researchers also looked at what would have happened if the size of the eyes had increased at the same rate as the length of the skull. In this scenario, Tyrannosaurus rex eyes would have been up to 30 cm in diameter and weighing almost 20 kg, instead of an estimated 13 cm and 2 kilos in weight.
So it’s possible that the evolution of T. rex ‘s ancestors gradually changed the circular eye sockets and large eyes so they could become more efficient carnivorous predators.
Referencia: “Functional and ecomorphological evolution of orbit shape in mesozoic archosaurs is driven by body size and diet” by Stephan Lautenschlager, 11 August 2022, Communications Biology.