A new species of ancient turtle that swam the shores of Europe millions of years ago may have been one of the largest sea turtles to have ever lived, scientists have said.

Fossilised remains of the turtle – measuring nearly four metres in length – were discovered in the Cal Torrades locality in north eastern Spain.

Named Leviathanochelys aenigmatica, it is thought to be the largest marine turtle ever discovered in Europe, and one of the largest found worldwide.

The largest known sea turtle is the extinct Archelon, which lived in the seas surrounding the North American continent somewhere between 66 to 100 million years ago.

The Archelon was around 4.6 metres long and weighed up to 3.2 tonnes.

The Leviathanochelys, meanwhile, measures 3.74 metres.

No known European marine turtle – extinct or living – has exceeded 1.5 metres in shell-length, the researchers said.

A team of scientists led by Angel H Lujan of the Autonomous University of Barcelona analysed the remains of the turtle, which were excavated between 2016 and 2021.

They found a fragmented but almost complete pelvis and parts of the upper shell, which date to the Campanian Age – somewhere between 83.6 to 72.1 million years ago.

One key feature that caught the researchers eyes is a distinctive bone that protrudes forwards from the front of the pelvis – something that is not seen in other marine turtles.

This indicates that Leviathanochelys represents a new taxon (group) of ancient marine turtles, the experts said.

The researchers believe this protrusion may be related to the creature’s respiratory system.

Based on the size of the pelvis, the scientists calculate that Leviathanochelys could have reached a body length of up to 3.74 metres.

They estimate the maximum width of Leviathanochelys’ pelvis was 88.9 centimetres, making it slightly larger than the biggest estimates for Archelon’s best-known specimen (81cm).

These findings indicate that gigantism in marine turtles developed independently in different lineages in both North America and Europe, the researchers said.

The study is published in the journal Scientific Reports.