A giant turtle the size of a great white shark roamed the oceans around 80 million years ago, say scientists.
It was one of the largest that ever lived - measuring more than twelve feet long and weighing about two tons.
The remains dug up in the Southern Pyrenees, northeastern Spain, consist of a fragmented but almost complete pelvis and parts of the upper shell, or carapace.
They date to the Campanian Age, between 83.6 to 72.1 million years ago. The new species has been named Leviathanochelys aenigmatica.
It is the biggest marine turtle ever to be discovered in Europe - second only to Archelon that lived around the ancient North American continent.
Corresponding author Dr Angel Lujan, of the Autonomous University of Barcelona, said: “The large body size could have evolved as a response to the unique habitat conditions of the European Cretaceous archipelago seas.”
The specimen possesses a distinctive bone that protrudes forwards from the front of the pelvis.
This feature differs to other marine turtles - indicating Leviathanochelys represents a new group of ancient marine turtles.
Dr Lujan said: “This protrusion may have related to the respiratory system. It supports the hypothesis Leviathanochelys had an open marine pelagic lifestyle.”
Based on the size of the pelvis, the authors calculate Leviathanochelys could have reached a body length of up to 12ft 3in (3.74 metres).
They estimate the maximum width of its pelvis was nearly three feet (89cm) - slightly larger than Archelon’s. The length was a bit smaller at 1ft 4in (39.5cm).
Dr Lujan said: “This makes Leviathanochelys the largest marine turtle ever discovered in Europe, and one of the largest found worldwide.”
Archelon lived around 70 million years ago towards the end of the Cretaceous. It grew to 15ft long (4.6 metres) and weighed up to 3.2 tonnes.
In contrast, no known European marine turtle, extinct or living, has exceeded five feet in shell-length - until now.
Dr Lujan said: “To date, it was thought that the largest marine turtles to ever sail the oceans, such as Archelon, were restricted to North America during the latest Cretaceous.
“The discovery of the new gigantic and bizarre Leviathanochelys from the Middle Campanian marine deposits of the Southern Pyrenees, which rivals in size to Archelon, sheds a light on the diversity of marine turtles and on how the phenomenon of gigantism in these groups was also occurring in Europe.
“Despite the scarcity and fragmentary nature of the individual, the new evidence not only increases the taxonomic diversity of the Late Cretaceous marine turtle biota in Europe, but also opens a new line of exploration and raises new questions, in order to solve the evolutionary mechanisms and ecological pressures that could have favoured the independent evolution of colossal marine turtles in multiple lineages, especially during the Late Cretaceous.”
He added: “These findings indicate that gigantism in marine turtles developed independently in different lineages in both North America and Europe.”
Male Great White sharks average 11 to 13 feet although females are bigger - reaching 15 feet on average.
Leviathanochelys is described in the journal Scientific Reports.