In films, predatory dinosaurs with huge teeth that protrude even from a closed mouth attract attention. But the picture is still wrong, reports a Canadian research team in the journal Science.
“Predatory dinosaurs are normal animals and not primitive monsters”, says Maximilian Kellermann. The paleontologist researches dinosaurs in the Bavarian State Collection for Paleontology and Geology, but was not involved in the study. “Because reconstructions often have skin stretched over the skulls of finds, without considering the soft tissue that shapes many parts of the face, many extinct animals look dangerous,” he says. Millions of years after death, usually only hard parts of the body remain. The teeth of predatory dinosaurs are remarkably long, but any skin that might have been covering them has long since disintegrated into its component parts. From this, it was previously concluded that the teethers of Tyrannosaurus rex and its relatives, similar to their closest relatives, crocodiles, protrude from the closed mouth.
lip protection
Predatory dinosaurs are shown as primitive monsters. – © Mark P. Witton
Moreover, even birds, which from the point of view of evolutionary biology even directly belong to dinosaurs, do not have lips. The groups are only suitable for a limited comparison: “Today’s birds don’t have teeth – and crocodiles adapted to life in the water over 250 million years of development”, explains Kellermann.
Unlike air, teeth do not need to be protected from drying out in water: their outermost layer, called “enamel”, contains small amounts of water, which stabilize this hard substance. To avoid damage, the lips of land vertebrates retain moisture in their mouths. “Only some terrestrial vertebrates, such as crocodiles and porpoises, which have returned to the water, are left without this protection”, says Kellermann.
The suspicion that predatory dinosaurs could also have protected their teeth from drying out with a skin had been brewing for several years. But only now the team led by Robert Reisz, from the University of Toronto, presents three valid arguments. In addition to the extinct predatory dinosaurs, there is another group of reptiles that live on land, with the monitor lizards that live today, which have remarkably long teeth compared to their skulls. Despite this, none of the species of monitor lizards have teeth protruding from their closed mouths.
The teeth of Papuan monitor lizards, which are often over two meters long and live in New Guinea, are particularly large and strong. “From the looks of these animals’ skulls, you wouldn’t expect the teeth to be covered by the lips,” explains Kellermann.
However, when the Canadian team compared predatory dinosaurs with monitor lizards, dentition and skull size did not differ much. So T. rex and its relatives didn’t use their huge gaping teeth like crocodiles? This is indicated by the tooth of a Daspletosaurus from the Royal Tyrrell Museum in Drumheller. These animals were closely related to T. rex, but smaller, measuring eight to nine meters in length.
The examined tooth had been used for a long time, but showed significantly less signs of wear than a crocodile tooth, whose enamel had been severely worn away on the outside. This is also an indication of sufficient moisture in a closed mouth, which stabilizes the tooth enamel. There are also holes in the jawbone for blood vessels that supply external soft tissues such as the lips, which are similar in layout to predatory dinosaurs and monitor lizards, while both differ in design from the crocodilian. Therefore, it seems that the undoubtedly dangerous bipedal dinosaurs seem less martial than they are often portrayed.