Something strange happened to Tyrannosaurus rex dinosaurs towards the end of the 1970s.
Until then, the prehistoric predator was depicted in images and movies with lips, said Thomas Cullen of Auburn University in Alabama.
"If you look back at artistic depictions of dinosaurs way back in the 1920s and 30s ... they always had lips," Dr Cullen said.
T-rex not only had lips in the early movies, it had them in real life millions of years ago, according to research led by Dr Cullen published today in the journal Science.
"It's not lips like you or I or any other mammal have. It's more like when you think of a lizard," he said.
That's right ... instead of the terrifying toothy tyrant in Jurassic World, T-rex had a gob that looked more like a goanna or Komodo dragon.
"They can't curl their lips back and snarl or anything like that," Dr Cullen explained.
Instead, he said, the skin and scales on the side of the dinosaur's head would have extended down far enough to make a tight seal when its mouth was closed.
The story of shrinking lips
Dr Cullen and his colleagues were often asked to consult on museum displays and documentaries, so they decided to investigate what T-rex and its carnivorous cousins looked like.
"In the 1960s and 70s, there was push to reconstruct T-rex as more active animals that were sort of less overtly lizard-like," Dr Cullen said.
During this time, T-rex slimmed down to a more sinewy shape and began to lose its lips.
"People had been saying since the '70s that T-rex's teeth were too long to fit in its mouth," Dr Cullen said.
Then, in the 90s, Jurassic Park came along.
"The T-rex from Jurassic Park is so iconic that it has influenced everything since," Dr Cullen said.
"They depict their T-rex with no lips.
"But oddly enough, they depict every other dinosaur in the movie as having lips."
Dinosaurs vs crocs and alligators
The idea that T-rex had teeth poking out of its closed mouth may have stemmed from its distant cousins.
Crocodilians (crocodiles and alligators) share a distant ancestor with theropod dinosaurs, and are one of the few animals alive today that have teeth on the outside of their mouth.
"When we're looking at extinct animals, we don't have direct evidence of some soft tissue structure, so the most common thing to do is to look at their living relatives," Dr Cullen said.
But theropod dinosaurs have thinner enamel on their teeth than crocodiles and alligators.
This meant, the team hypothesised, that the teeth would lose their pointy edge and wear down if they were exposed to air.
To test their theories, they evaluated the anatomy of the dinosaur's jaw and skull, as well as other reptiles', both living and dead.
First, they looked at the size of the tooth versus the size of the skull in a number of theropod species compared with living species of reptiles such as monitor lizards.
"Monitor lizards have teeth that are proportionately larger to their skulls than even T-rex, but every single one of them is fully lipped," Dr Cullen said.
That ruled out the idea the dinos' skulls were too large for their teeth.
Next, they looked at the microscopic structure of the teeth of a tyrannosaur compared with those of an alligator.
If one side of the tooth was exposed to the air and the inner side of the tooth sat along the edge of the jaw, there should be different patterns of wear.
"In our alligator samples we see exactly that, but our tyrannosaur has an even thickness of the enamel," Dr Cullen said.
The tyrannosaur also had a series of pits on its upper jaw bone for blood vessels and nerves, which were absent in the alligator.
"[Crocodilians] don't have lips. Instead they have a series of much finer and way more numerous pits over the entire part of their snout that relates to the very tight skin that they have around their face," Dr Cullen said, adding that cells in these pits may help crocodilians sense vibrations in water.
Finally, the team looked at the biomechanics of the bite. Jaws of animals that have lips have a looser fit than those that don't to create a seal around their mouths.
Dr Cullen said their analysis showed alligators had a tighter fit than the tyrannosaur.
"To make a lipless fit in a thing like a T-rex, you have to hyper-extend the lower jaw almost to full dislocation, whereas if they had lips, and you had a bit of space, then it articulates perfectly well."
Palaeontologist Steve Wroe of the University of New England said the study formed a "reasonably compelling" case of how carnivorous theropods such as T-rex once looked.
"The comparisons of theropod skulls and teeth with those of goannas suggest when T-rex had its mouth closed, you wouldn't have seen its teeth," said Professor Wroe, who was not involved in the research.
"I guess this would have made T-rex appear a little less scary ... at least until it opened its mouth."
What's old is new again
The lips/no lips debate has been going on for decades, particularly in the palaeoart world, said Steve Salisbury, a palaeontologist from the University of Queensland, who was not involved in the study.
"I'm glad this paper has cleared up comparisons with crocodilians," Dr Salisbury said.
He said there had been speculation for some time that carnivorous dinosaurs were beaked or had some kind of structure growing around the teeth based on the presence of pits.
"A lot of people have been proposing the nerves and blood vessels housed in these pits were supplying a keratin-like structure around the mouth like birds."
Last year, a lipped T-rex made it back onto our screens in David Attenborough's documentary Prehistoric Planet.
"I imagine right now there's a small community of palaeoartists that are rejoicing," Dr Salisbury said.
"They've finally been vindicated with their lipped dinosaur reconstructions that they've been producing.
"Many people over the years said [these] weren't anatomically correct because crocodiles don't have lips."
Now the rest of us, and Hollywood, have to catch up.
"Just when we think we know what a dinosaur looks like ... science throws a curve ball and we have to re-imagine it," he said.
