Circular or oval and keyhole-shaped: A study now sheds light on the different shapes of the eye sockets in dinosaurs. According to computer simulations, the narrow bone structures made the skulls more resistant to particularly strong biting loads. Similarly, elliptical or keyhole-shaped versions are typical for large carnivorous dinosaurs, while circular eye sockets are typical for herbivorous dinosaurs and smaller species and juveniles, the study shows.
Eyeballs are known to be spherical structures. The skull openings in which they sit are therefore usually adapted to this shape: As with humans, most animals have largely circular eye sockets. But there are exceptions and even in the living beings of the past there is a certain morphological diversity in the bone structures. So far, however, hardly anyone has dealt with this topic, writes Stephan Lautenschlager from the University of Birmingham.
That is why the paleontologist has now devoted a study to the distribution and possible meanings of the different eye socket shapes in dinosaurs and their relatives. Lautenschlager first recorded the characteristics of the eye sockets of 410 fossil animal species of the Mesozoic – from 252 to 66 million years. Among them were different representatives of the dinosaurs and some other archosaurs. As the paleontologist reports, a clear pattern was reflected in the evaluation results and the associations with known characteristics of these animals.
Unusual eye sockets in predatory dinosaurs
Most species, especially the herbivores, therefore had circular eye sockets. “The results show that only some dinosaurs had eye sockets that were elliptical or keyhole-shaped. These were carnivorous dinosaurs with skulls measuring a meter or more,” says Lautenschlager. Interestingly, however, their young animals usually still have rather round eye sockets, he found. In addition, evolutionarily older species tended to have more rounded eye sockets than younger ones: the large theropod carnivorous dinosaurs had more keyhole-shaped eye sockets than their ancestors, reports Lautenschlager.
These results thus suggested that larger carnivorous species adjusted their orbital shape in adulthood throughout evolutionary history. But why? The suspicion arose that the narrow forms were connected with the rough diet and size of these animals. Specifically, they could have offered advantages in terms of skull stability. In order to follow this lead, Lautenschlager carried out simulations and stress analyzes on the computer. Models of actual structures were used, as well as hypothetical versions of predatory dinosaur skulls with unnaturally shaped eye sockets.
Adaptation to high loads
The results made it clear that predatory dinosaur skulls with more circular eye sockets would be subjected to comparatively high stress when biting. In the case of the naturally elliptical or keyhole-shaped morphologies, on the other hand, this is significantly reduced: the stresses are distributed to stable parts of the skull behind the orbit, according to the simulations. This apparently allowed top predators like Tyrannosaurus rex to develop high biting power without compromising the stability of their skulls, Lautenschlager explains.
It was also shown that there was a relative reduction in the size of the eyes compared to the size of the skull in the giant carnivorous dinosaurs. If the size of the eyes had increased at the same rate as the length of the skull, the eyes of Tyrannosaurus rex would have reached a diameter of up to 30 centimeters and a weight of almost 20 kilograms. In fact, it was only about 13 centimeters and two kilograms. The reduction in size was influenced by the development of the keyhole-shaped bone structure, Lautenschlager explains: “In these species, only the upper part of the orbit was actually occupied by the eyeball,” says the paleontologist.
Source: University of Birmingham, professional article: Communications Biology, doi: 10.1038/s42003-022-03706-0