A sea-dwelling reptile the size of a bus has been identified for the first time from a giant fossil found in Russia.

Scientists say the find has “profoundly changed” our understanding of the evolution of plesiosaurs who lived in the sea when dinosaurs roamed Earth around 130 million years ago.

Plesiosaurs possess an unusual body shape not seen in other marine vertebrates, with four large flippers, a stiff trunk, and a highly varying neck length.

Pliosaurs are a kind of plesiosaur, characterised by a large, two-metre-long skull, enormous teeth and extremely powerful jaws, making them the top ocean predators during the dinosaur age.

An international research team described in the journal Current Biology a new, “exceptionally well-preserved” and highly unusual pliosaur from about 130 million years ago.

It was found in Autumn 2002 on the right bank of the Volga River, close to the city of Ulyanovsk, by Gleb Uspensky, of Ulyanovsk State University, one of the co-authors of the paper.

The species is called Luskhan itilensis – meaning the “master spirit from the Volga river”.

Its skull is 1.5 metres in length, indicating it was a very large animal, but its rostrum (beak) is extremely slender, resembling that of fish-eating aquatic animals such as gharials or some species of river dolphins.

Study lead author Doctor Valentin Fischer, a lecturer at Liege University in Belgium, said: “This is the most striking feature, as it suggests that pliosaurs colonised a much wider range of ecological niches than previously assumed.”

He said the findings have ramifications regarding the final extinction of pliosaurs, which took place several tens of million years before that of all dinosaurs, except some bird lineages.

Dr Discher added: "The new results suggest that pliosaurs were able to bounce back after the latest Jurassic extinction, but then faced another extinction that would - this time - wipe them off the depths of ancient oceans, forever."