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 discoverer of a new type of the long-extinct marine lizard in a bauxite mine in Hungary.
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 mosa-saur - 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."
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 its 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 Pannonia-saurus didn't have the marine lifestyle or the seal-like flippers of its seagoing 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.
"Mosasaurness is really about the skull and about habits, as opposed to everything but the head being focused on swimming adaptations."
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.