It grew up to six metres long and its toothy mouth and crocodile-like body was the terror of ancient rivers and shorelines many millions of years ago.

Just don't call it a dinosaur.

"Mosasaurs are not dinosaurs," said University of Alberta biologist Michael Caldwell.

The University of Alberta department of biological sciences and the Hungarian Natural History Museum's department of paleontology and geology teamed up to excavate a bauxite mine in Hungary where they discovered the new type of the long-extinct marine lizard.

Mosasaurs, unlike dinosaurs, were true lizards, meaning they were able to dislocate their jaw at will and swallow anything they could get their mouths around. This, it turns out, is what makes Caldwell's mosasaur — called Pannoniasaurus — so interesting.

Most mosasaurs were giant undersea predators, some growing up to 16 metres long, which breathed air but were full-time, fearsome sea creatures complete with paddle-like limbs similar to those of a whale. They lived around the same time as the dinosaurs and have been called the T. Rex of the sea.

"They were much bigger than T. Rex," said Caldwell, an expert in mosasaurs. "They really were sea monsters."

'They really were sea monsters.'—University of Alberta biologist Michael Caldwell

Pannoniasaurus, however, wasn't.

About 84 million years old, it is the first mosasaur ever found that lived in freshwater and retained the long, skinny legs of a land-based lizard. Judging by the shape of its skull and the abundance and type of teeth, it probably hunted much like a modern crocodile, lurking just under the surface of the water to suddenly pounce on fish, or frog, or anything that moved.

But, even though Pannoniasaurus didn't have the marine lifestyle or the seal-like flippers of its sea-going cousins, it still shared with them one essential mosasaurian characteristic — that little bone at the back of its skull that allowed its jaws to gape so impressively.

"This is kind of new stuff for us in the mosasaur world," said Caldwell. "Up until about five to 10 years ago, we treated the group as though it had a common ancestor with paddle-like limbs. We're beginning to recognize that the story is remarkably more complex than that."

'Evolution at work'

"It's an elegant and exciting example of evolution at work," Caldwell said. "If the ancestors of all mosasaurs started out on land, the existence of Pannoniasaurus shows that the move from land to ocean took place at different times and in different ways, depending on what evolutionary pressures were at work.

"This is really exciting new news in the business of evolutionary biology. It's almost like saying hominids evolved more than once, and having the fossil evidence to say so."

The bones Caldwell and his co-authors write about in a paper published Wednesday come from a wide variety of individuals of different ages and sizes, so it's unlikely they came from a single mosasaur that somehow found its way up an ancient river. But the team still lacks a complete skeleton, so drawings of what the creature may have looked like remain speculative.

Think, however, of a large, thin-bodied crocodile with a remarkably big bite.

"This is a particularly unique lineage of mosasaurs — true lizards — that were very successful in these freshwater ecosystems," said Caldwell.

"He was the big guy in his ecosystem."