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What do you picture when you think of the Tyrannosaurus Rex? Giant teeth, huge legs, little arms?

Well, what about lips...?

Scientists now reckon that in front of the T.rex's scary smile was a set of thin lips.

Researchers from the University of Portsmouth think some of the ideas we have of what dinosaurs look like are very out of date.

Can't get enough of dinos? Find out which one you are in our quiz further down the page.

The T.rex was a theropod, which means it was a dinosaur with hollow bones and three claws on each limb.

For a long time, people thought theropods just had big teeth coming out of their mouths, like a crocodile.

Crocodiles and alligators are the closest living relative of the theropod - so people assumed that the T.rex's face would look similar.

However, the new research finds that they actually looked a bit more like lizards, with thin lips over their teeth.

The experts looked at a dinosaur tooth - a very big one - and tried to work out how worn it was.

They thought that the less damage it had on the outside, the more likely it was to have had some layer of protection.

The tooth they looked at - that had been in a theropod's mouth for 500 days - was found to not have that much outside damage.

Tyrannosaur teeth lasted around a year before being replaced - which is much longer than crocodiles, which grow new teeth quite regularly.

By looking at the theropod teeth and comparing to lizard teeth, researchers think the theropod's teeth were fully covered.

They also found another similarity. There are small holes in lizards' jaws, which blood vessels go through to bring blood to the lips - and the theropod teeth had a similar pattern.

The teeth also lean in the same direction as a lizard's - whereas a crocodile has teeth that are angled in a different way.