Tyrannosaurus rex and close relatives exchange large eyes for big bites

Scientists have found that Rex tyrannosaurs (or Tyrannosaurus rex) evolved narrower eye sockets than their ancestors or helped them and similar large carnivorous dinosaurs — also known as theropods — bite harder. The study was recently published in Communications-Biology.

Stephan Lautenschlager of the University of Birmingham in the United Kingdom compared the eye sockets of 410 Mesozoic (252 million to 66 million years ago) reptile fossil samples, including close relatives of dinosaurs such as dinosaurs and crocodiles. He found that most of the samples had round eye sockets, especially in herbivores.

However, large carnivores with skulls longer than one meter usually have oval or keyhole-shaped eye sockets in adulthood, although their eye sockets are more rounded when they are juvenile. The eye sockets of older specimens appear to be rounder than those of the closer samples, and the eye sockets of large theropods are closer to the shape of the keyhole than their ancestors. These observations suggest that large carnivorous species have evolved keyhole-shaped eye sockets over time, but this shape occurs only in adulthood, not in adulthood.

To investigate the effect of eye socket shape on skull structure and function, the authors compared the force on bites of reptile skull theoretical models with five different eye socket shapes. The authors also compared the maximum eyeball size that a model of a tyrannosaurus skull with round or keyhole-shaped eye sockets could carry. The keyhole-shaped eye socket disperses force during the bite to the harder part of the skull behind the eye socket, making the eye socket less deformed and helping to reduce pressure on the skull. However, the Tyrannosaurus rex model with round eye sockets could carry 7 times the size of the eyeballs as the keyhole-shaped eye socket model.

Different shapes of eye sockets (outlined with dotted lines) can be seen on different dinosaur skulls. Image credit: Stephan Lautenschlager

A computer-modeled virtual dinosaur skull with colors indicating the pressure on the skull. The pressure on the skull in the round eye socket (upper) is greater, and the pressure on the skull in the keyhole-shaped eye socket (lower) is smaller. Image credit: Stephan Lautenschlager

The skull of the Tyrannosaurus rex’s original eye socket eye (left) and the virtual round eye socket eye (right) is reconstructed with art. Image credit: Stephan Lautenschlager

The authors suggest that theropod dinosaurs evolved narrower eye sockets, which may have reduced the eyeball space in their skulls, while leaving this space to the jaw muscles and increasing the hardness of the skull. This may allow them to trade larger eyes for greater bite force, which previous studies have suggested can enhance visual perception. The findings highlight this functional trade-off that determined the evolution of dinosaurs. (Source: China Science Daily Feng Weiwei)

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