The fearsome jaws of Tyrannosaurus are famous. In museum halls, paleo art, and even feature films such as Jurassic Park, the Cretaceous carnivore has traditionally been depicted with banana-size fangs and a sinister grin. But paleontologists have now discovered that the living animal did not have a sharp-toothed smirk—T. rex, as well as many other carnivorous dinosaurs, had lips.
Reported today in Science, a multi-institution team of paleontologists propose that carnivorous dinosaurs like T. rex and Allosaurus had fleshy lips covering their teeth, much as modern-day lizards do. The hypothesis will likely alter both the public image of dinosaurs as well as how paleontologists study the ways these terrible lizards fed.
The new research got its start as discussions between co-authors Thomas Cullen, Kirstin Brink, and Derek Larson while all three were graduate students at the University of Toronto. Each had developed different expertise on dinosaurs and their anatomy, which began to meld together into tangible evidence for how dinosaurs like T. rex would have looked. (Read how billions of T. rex likely roamed Earth.)
Whether or not dinosaurs had lips has been a matter of debate among dinosaur fans and some experts for years. The arguments have often centered on whether the addition of lips, like fluffy body coverings, made dinosaurs less impressive and frightening to their adoring fans than the traditional, toothy renditions, and what evidence might justify such a change. The new study finally offers some tangible evidence to resolve the issue, moving beyond aesthetics to the biology of the extinct animals.
“There is a lot of interpretation in paleo art. It’s nice to offer some scientific data to back it up,” says Brink, now a paleontologist at the University of Manitoba.
Giving dinosaurs some lip
To determine whether dinosaurs like T. rex had exceptionally long teeth, Cullen, now at Auburn University; Brink; and colleagues examined the anatomy of living lizards and crocodiles, the microscopic structure of dinosaur teeth, and how tooth size compares with skull dimensions in species from T. rex to smaller carnivores such as Velociraptor and Coelophysis.
The star specimen, however, was the T. rex nicknamed Sue, the largest and most complete representative of the storied species (and on display at Chicago’s Field Museum). Despite Sue seeming to have extra-long chompers, the paleontologists found its teeth had the same relationship to skull size as seen in today’s monitor lizards, and therefore wouldn’t require any extraordinary lips to cover.
A critical part of the research was finding modern analogs. Today’s birds—descendants of dinosaurs—lack teeth, and crocodiles are specialized reptiles that live in water. Even though they’re not as closely related, however, reptiles such as monitor lizards are more useful in their relatedness to dinosaur anatomy. (Learn why T. rex couldn’t waggle its tongue.)
“This study is a great piece of paleontological forensic detective work,” says University of Edinburgh paleontologist Steve Brusatte, who was not involved in the new research.
Their overall analysis suggested T. rex and similar dinosaurs did not have extra long teeth for their size: A set of lips could have easily covered their teeth. More telling is that the enamel of dinosaur teeth is relatively thin, and would have been prone to drying out if constantly exposed to the air. Lips would have kept their teeth wet and functional, something that crocodiles don’t have to worry about as aquatic animals.
“Cullen and colleagues make an excellent case for the presence of extra-oral tissues in non-avian theropods,” says University of Pennsylvania paleontologist Ali Nabavizadeh, who was not involved in the new study.
Lips may not be unique to theropods, either. Recent studies on various herbivorous dinosaurs, such as the long-necked sauropods and horned ceratopsids, have found evidence of gums, cheeks, and other soft tissues covering their teeth.
And for those who might suggest that the change makes dinosaurs less impressive, Cullen notes that “having lips, or even feathers, has no real bearing on something being scary or fearsome”—just look at birds of prey or many mammalian carnivores.
Paleontologists sink their teeth in
Adding lips to T. rex and other dinosaurs creatures a fuller image of their anatomy and how they interacted with their environments, the scientists say.
“I’m very interested in the role soft tissues play when interpreting tooth pathologies,” Brink says, as gums, lips, and other tissues could experience many of the same injuries and diseases that affect living animals.
Nabavizdeh adds that lips likely made T. rex a more effective predator.
For one, reptiles don’t have the lip muscles that mammals do, so a T. rex couldn’t sneer (or do an Elvis Presley impression, for that matter). The lips, Nabavizdeh notes, were to protect the teeth from abrasion and to keep them wet.
Think of their lips as akin to the sheath on a knife, Brusatte adds, something that “helped maintain the murder weapons and ensued they were as deadly as possible.”
