The terrifying Tyrannosaurus rex (T. rex) had scaly, lizard-like lips covering and sealing its mouth, and not permanently exposed teeth, unlike what was depicted in films such as Jurassic Park, a new study suggests. Other predatory dinosaurs, including Velociraptor, had similar lips.
The study, led by researchers at the University of Portsmouth, was published March 30 in the journal Science.
Flesh-eating dinosaurs had lips similar to those of modern lizards
For years, researchers and artists have debated whether theropod dinosaurs, the group of two-legged, flesh-eating dinosaurs, and birds, had lipless mouths where visible upper teeth hung over their lower jaws. Theropods included T. rex and Velociraptor, both top predators. Experts wondered if these carnivorous dinosaurs had jaws similar to the mouth of a crocodile.
However, the researchers who conducted the new study challenge these depictions of dinosaurs, and claim that the frightening creatures had lips similar to those of lizards, and their relative, the tuatara. This is a rare reptile found only in New Zealand, and appears similar to a stout iguana. Tuataras are rhynchocephalians. Rhynchocephalia is an order that diverged from lizards at least 230 million years ago. Rhynchocephalians thrived in the age of dinosaurs, and tuataras are the last survivors of this order.
Dinosaur mouth was more similar to that that of lizards than crocodiles
The new research is the most detailed study yet discussing the characteristics of the lips of theropods. As part of the study, the researchers examined the tooth structure, wear patterns, or the locations of tooth erosion, and the jaw morphology of lipped and lipless reptile groups.
The researchers found that the mouth anatomy and functionality of theropods resembles that of lizards more than crocodiles, implying that the teeth of theropods were covered with lizard-like oral tissues, including scaly lips.
Theropod lips were not muscular, but scaly
According to the study, the theropod lips were probably not muscular, like those of mammals.
While most reptile lips cover the organisms' teeth, they cannot be moved independently, or curled back into a scar. In other words, most reptile lips cannot make movements commonly associated with the lips in humans or other mammals.
In a statement released by the University of Portsmouth, Derek Larson, one of the co-authors on the paper, said palaeontologists often like to compare extinct animals to their closest living relatives, but in the case of dinosaurs, their closest living relatives have been evolutionary distinct for hundreds of millions of years and today are incredibly specialised.
Larson explained that it is quite remarkable how similar theropod teeth are to monitor lizards. The teeth of different monitor lizards, including the smallest dwarf monitor and the Komodo dragon, function in much the same way.
Therefore, Larson said, monitors can be compared quite favourably with extinct animals like theropod dinosaurs based on the similarity of teeth function, even though monitors and theropods are not closely related.
When did the idea of lipless dinosaurs become prominent?
Dr Mark Witton from the University of Portsmouth, another co-author on the paper, said dinosaur artists have gone back and forth on lips since scientists started restoring dinosaurs during the 19th century, but lipless dinosaurs became more prominent in the 1980s and 1900s. Lipless dinosaurs were then deeply rooted in popular culture through films such as Jurassic Park, Witton said.
Witton explained that there was never a dedicated study or discovery instigating the change in the depiction of dinosaurs from lipped to lipless in the 1980s and 1990s, which, to a large extent, probably reflected preference for a new, ferocious-looking aesthetic rather than a shift in scientific thinking.
Jurassic Park wrongly depicted T. rex
Witton said that the study authors are upending this popular depiction by covering the teeth of theropods with lizard-like lips. This means that a lot of the world's favourite dinosaur depictions are incorrect, including the iconic Jurassic Park T. rex, Witton concluded.
According to the study, tooth wear in lipless animals was significantly different from that seen in carnivorous dinosaurs. Another important finding is that dinosaur teeth were no larger, relative to skull size, than those of modern lizards. This implies that dinosaur teeth were not too big to cover with lips.
The study states that the distribution of small holes around the jaws, which supply nerves and blood to the gums and tissues around the mouth, were more lizard-like in dinosaurs than crocodile-like.
Dinosaur teeth had thin enamel
The researchers modelled the mouth closure of lipless theropods, and found that the lower jaw either had to crush jaw-supporting bones, or disarticulate the jaw joint to seal the mouth.
Kirstin Brink, a co-author on the paper, said saliva is important for maintaining the health of teeth, and teeth that are not covered by lips risk drying out and can be subject to more damage during feeding or fighting, as seen in crocodiles, but not in dinosaurs.
Brink explained that dinosaur teeth have very thin enamel, mammal teeth have thick enamel, except for some, and crocodile enamel is a bit thicker than dinosaur enamel, but not as thick as mammalian enamel.
Thomas Cullen, the lead author on the paper, said although it has been argued that the teeth of predatory dinosaurs might be too big to be covered by lips, the new study shows that, in reality, their teeth were not atypically large.
T. rex teeth are proportionally similar in size to those of living lizards
Cullen explained that even the giant teeth of Tyrannosaurus rex are proportionally similar in size to those of living predatory lizards when compared to skull size. This rejects the idea that the teeth of predatory dinosaurs such as T. rex were too big to be covered with lips.
Significance of the study
The study is important because it provides new insights into how researchers reconstruct the soft tissues and appearance of dinosaurs and other extinct species, which, in turn, can give important information on how dinosaurs fed, and how they maintained their dental health.
Witton said that the new study, and other studies like it, show that scientists now have an increasingly good handle on many aspects of dinosaur appearance, and are far from being clueless.
The authors note that the study does not claim that no extinct animals had exposed teeth. Some animals, such as the sabre-toothed carnivorous mammals, or marine reptiles and flying reptiles with extremely long, interlocking teeth, had exposed dentition.
The authors conclude that analyses of dental histology from crocodilians and theropod dinosaurs, including T. rex, indicate that the most likely condition of theropods was complete coverage of the marginal dentition with extraoral tissue when the mouth was closed. Not only do the findings change the world's perceptions about the appearance and oral configuration of these iconic predators, but also have important implications for researchers' interpretations of other terrestrial animals with large teeth.