Contrary to popular belief, the fearsome Tyrannosaurus Rex probably did not have lips and Triceratops most likely did not have cheeks.

"I almost expect a backlash as a result of our findings. There is a sense that we are changing the way a lot of dinosaurs look," says Lawrence Witmer of Ohio University, who carried out the research.

"When you work on extinct animals, there is a pressure and a sincere desire to know what these animals looked like.

"We draw these pictures and they look right to us because they remind us of animals we see today. But these pictures may be wrong."

For example dinosaurs may not have had fleshy cheeks. The idea that they had cheeks was based on scientists' comparison of these dinosaurs to modern-day mammals, such as sheep.

But Mr Witmer's studies have found this comparison to be false. Modern mammals with muscular cheeks do not have the same sort of excavated area on their lower and upper jaw that is found in dinosaur fossils.

A more likely conclusion is that these jaw features supported an extended beak, similar to the beaks on eagles or crocodiles.

It also appears scientists have made a similar mistake with the Tyrannosaurs Rex, which has been likened to modern-day lizards with muscular lips.

Lizards have scales that hang down along the edges of their jaws and hide their teeth when their mouths are closed.

However, this is not necessarily so with the Tyrannosaurs. These dinosaurs had skin that probably extended to the margin of their jaw but did not extend to cover their teeth.

"Lips on a Tyrannosaur are important if you are making a movie or a toy for a child, but it is really not a big deal if you are trying to figure out what these animals were like," said Mr Witmer.

"With or without lips, Tyrannosaur was a vicious hunter."