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Tyrannosaurs competed by biting each other's faces, research suggests.
The bites could have been part of competition for mates, food and dominance between some of the largest land predators to have ever lived.
Gouges on the skulls of sexually mature tyrannosaurs suggest the dinosaurs attempted to bite each other's faces and lower jaws.
The paper, published in Paleobiology, suggests that these behaviours reflect those of some of their closest living relatives - the crocodiles and alligators - and may provide further clues as to the behaviour of the extinct reptiles.
A team of Canadian researchers from the Royal Tyrrell Museum of Palaeontology and the University of Alberta also found that the marks were mostly inflicted between what are likely to have been males, and by those which were at or near the age of sexual maturity.
Dr Paul Barrett, a researcher in palaeontology at the Museum and who was not involved in the study, says, 'This adds a great new insight into the evolution of bird behaviour by tracking changes among their close dinosaur relatives.
'The team shows convincingly that while earlier-evolving, meat-eating dinosaurs still had crocodile-like behaviour when fighting each other, this disappears when feathers evolve, which allowed display to take over from biting, as occurs in living birds.
'It's amazing that we can get these direct insights into fossilised behaviours, which have been frozen in time as marks on their bones.'
Intraspecific aggression - where members of the same species compete with one another - is common in the animal kingdom. From rutting deer to elaborate mating displays by birds of paradise, animals compete in a variety of ways.
Previous studies of tyrannosaurs have found lesions and scars on their skulls, but these have generally focused on individual bones or skeletons.
It has been suggested that these were the result of intraspecific aggression, so researchers examined a range of skeletons, skulls and skull fragments from tyrannosaurs to build up a wider picture of the inflicted injuries.
The Canadian team looked for marks that had partially healed, showing the dinosaur remained alive after the injury was inflicted. From the different lesions, they could assess how the bite was made from the varying depth of the wound.
They found that the bites were mostly on the bottom of the upper jaw and top of the lower jaw, with the shapes suggesting tyrannosaurs were trying to bite each other in the mouth. The clustering of these marks further suggest it was a specific behaviour, rather than random injury from throughout their life.
Analysis of skulls of the American alligator show similar wounds from intraspecific aggression in similar places on the skull. Before biting in this way, alligators will often make displays of aggression. This suggests tyrannosaurs may have done something similar.
The scars weren't found in young or small dinosaurs, and only began to crop up after the individual was about half its full size. From that point onwards, they recurred on a regular basis throughout the individual's life.
The researchers suggest this means that the bites only occurred after a tyrannosaur reached sexual maturity, with the smallest specimens with marks estimated to be around six years old.
The findings suggest that male tyrannosaurs may have bitten each other's faces as part of contests with similarly sized competitors for food, resources, dominance and territory once they reached adulthood. Given the occasional marks on what might be females, the bites may also have been part of courtship behaviour.
The findings potentially have implications beyond the tyrannosaurs, with other dinosaur clades also having similar bite marks that may be a result of similar behaviours. In particular, there is a trend for larger predators to have similar marks on their fossils.
However, in dinosaur species closer to birds, these marks tend to disappear. This occurs around the time feathers start to evolve, suggesting that the more violent behaviours gradually changed to displays.
Changes in their body structure may also have contributed, with weaker bones and beaks making it more difficult for them to leave lasting marks. Instead, birds have taken up other violent behaviours, including eye pecking, wing beating and kicking, which are less likely to leave marks.