Carnivorous dinosaurs like Tyrannosaurus rex had eyes that appear tiny relative to their head circumference. There were solid reasons for this, as paleobiologist Stephan Lautenschlager from the University of Birmingham has discovered. The bony eye socket (the cranial fossa in which the eye lies) was narrowed and reduced in size in these animals, which was associated with a higher load capacity of the skull. As a result, they were able to bite enormously powerfully. Lautenschlager reports on this in the journal Communications Biology.
The researcher examined fossils of various dinosaur and reptile species of the Earth's Middle Ages. It turned out that the early species had predominantly circular or elliptical eye sockets. However, in the late Middle Ages, carnivores appeared, in which the cranial pits were shaped like a keyhole. One of them was T. rex.
Huge and not vegetarian
Apparently, this change in shape had something to do with body size: it occurred in animals whose heads were more than one meter long. In addition, it was related to the diet, because almost all species with keyhole-like eye sockets were carnivores.
With the help of computer models, Lautenschlager calculated how the forces in the bone are distributed when compressive and tensile loads act on the structure of the dinosaur skull. He took different shapes for the eye sockets and calculated the mechanical stresses that occurred in each case. Skulls with keyhole-shaped openings performed better in virtually all stress tests – regardless of whether they were squashed or bent vertically or horizontally.
Calculations especially for the body of the T. Rex showed: thanks to its narrow eye sockets, the mechanical stress in the bone tissue decreased when the animal bite, and the head deformed less. In everyday life at the time, this was probably urgently needed: T. Rex closed his pine eight to ten times more powerful than today's crocodiles, as studies showed. This corresponded to a weight of several tons per tooth.
Conditional -shaped eye sockets can only be demonstrated in adult tyrannosaurs: the young animals still had circular openings. With increasing age, increasingly powerful stature and hunting for ever larger prey, however, the biting forces who acted on the skull are likely to have risen immensely. This would explain why the advantage of the changed bone shape only really came into play when fully grown - and it only then developed.
In the adult T. rex, the eyeball only claimed the upper bulge of the eye socket. Therefore, it was much smaller in relation to the head than in a juvenile. If the carnivores had retained a circular cranial pit throughout their lives, and if this – including the eyeball – had grown proportionally to the rest of the organism, then a fully developed tyrannosaur would have possessed 30-centimeter-sized visual organs that would have weighed around 15 kilograms, Lautenschlager calculates. The animal would then have benefited from better vision, but the energy cost of maintaining such huge eyes would have "almost certainly negated" this advantage, as the researcher notes.
