Europe's largest ever turtle species has been discovered

Giant turtles as long as a car once swam the seas of Europe over 70 million years ago.

Until recently, giant marine turtles of over three metres long, such as Archelon and Protostega, were only found in North America. In comparison, the largest European species measured just 1.5 metres in length.

But a new species discovered in Spain reveals that turtle gigantism wasn't a purely an American phenomenon. While Leviathanochelys aenigmatica's 3.7-metre-long body isn't quite as long as Archelon's, it was slightly wider, and ranks among one of history's largest turtles.

Dr Àngel Hernández Luján, who co-authored a description of the new species, says that it could be the first of more giant European turtles.

'We are optimistic, and believe that it is possible to find more giant turtle species in Europe,' Àngel says. 'Fragmentary remains of large marine turtles have previously been found throughout Europe, but none of them are as complete as Leviathanochelys.'

'It is just a matter of time before new species of large-bodied sea turtles from the time of dinosaurs are discovered.'

Dr Sandra Chapman, the Museum's former Curator of Fossil Reptiles and Birds who has studied fossil turtles, adds, 'I think this specimen is a really good find which has been well researched.'

'It adds another piece to the puzzle of trends in the evolution of turtles. It supports the idea that extreme sizes are more common prior to an extinction and smaller, but still substantial, sizes predominate afterwards.'

The description of the new species was published in the journal Scientific Reports.

What are the largest turtles?

While the evolution of turtles is shrouded in mystery, it is known that they can grow extremely large.

The largest living species is the leatherback sea turtle, which is generally found in the tropics but can occasionally venture as far north as Norway. The animals commonly reach around two metres in length, but one specimen held in the National Museum Cardiff approaches three metres long.

However, these species pale in comparison when measured against their ancient relatives.

'Compared to the turtles alive before the end of the Cretaceous, living turtles are not as large as their ancient relatives,' Sandra says. 'We've not seen their enormous sizes achieved again.'

The largest extinct freshwater turtle is Stupendemys geographica, which was discovered in South America in the 1970s. Its largest specimens have shells that are almost three metres long, with their full body stretching even further.

The largest extinct marine turtles, meanwhile, were first discovered more than a century earlier in the 1870s when Protostega gigas was described by Edward Drinker Cope.

This initial specimen measured around 3.9 metres long, with subsequent fossils showing that the species could grow to 4.2 metres in length, equal to two king-size beds.

Two decades later another species of giant turtle, Archelon ischyros, was also discovered. Though the first specimen was smaller than Protostega, a 4.6-metre-long Archelon fossil called 'Brigitta' subsequently claimed the title of the largest ever turtle.

For some time it was thought that these giant turtles were most closely related to the large leatherback sea turtles. However, more recent research challenges this relationship.

It is now thought that Protostega and Archelon instead branched off from the ancestors of modern marine turtles during the Late Jurassic over 145 million years ago, and lived in a body of water known as the Western Interior Seaway which divided what is now North America in two.

Like leatherbacks, however, the giant turtles would probably have eaten animals such as molluscs and jellyfish.

While giant marine turtles have been found outside North America, with fossils discovered in Morocco, Jordan and Russia, European finds have so far been limited and fragmentary. The discovery of L. aenigmatica provides firm evidence that they were present after all.

How was Europe's largest turtle discovered?

As with many other notable discoveries in palaeontology, the discovery of L. aenigmatica was completely accidental. A hiker walking in the Pyrenees mountains near the village of Coll de Nargó in northern Spain stumbled across bone fragments.

Members of the local museum, as well as the Catalonian Department of Culture, collected the bones from the site, but they would not be studied for several years.

In 2021, the lead author of the study, Oscar Castillo‑Visa, examined the remains and began new excavations at the site. This led to the discovery of the pelvis of a giant turtle, as well as the back portion of its shell, or carapace. Though the remains are fragmented, they contain enough distinct features for researchers to describe them as a new species.

The genus given to the turtle, Leviathanochelys, is derived from the name of a Biblical marine beast, Leviathan, and the ancient Greek for turtle. Its specific name, aenigmatica, refers to some of the animal's unusual characteristics.

'Leviathanochelys has an exceedingly large pelvis and shell reduction in the carapace,' Sandra says. 'As the species' shell has increased in size, the spaces between the bones have become larger to reduce its weight and bone density.'

In addition to these characteristics, one unique feature is known as an accessory pubic process, a section of bone bulging out from the side of the pelvis. No similar structure is known in any turtle living or dead, but marks on the bone suggest muscles were once attached to it.

'We speculate that this process served as an additional anchor point for muscles controlling the turtle's abdominal contraction,' Àngel says. 'This could mean it had a role related to the respiratory system, and could have helped the giant turtle to maximise its breathing capacity at great depths.'

Together, L. aenigmatica's features suggest it is an early member of the sea turtle family, with its closest relative being Allopleuron, a smaller species of extinct turtle found in Europe, the USA and Kazakhstan.

This means that L. aenigmatica is not closely related to Protostega and Archelon, suggesting that turtles grew to giant sizes multiple times throughout their evolution.

While L. aenigmatica was certainly big, the lack of its complete skeleton means its total length can only be estimated. Researchers used the size of its pelvis and comparisons with Archelon to calculate its potential body length, but as there is no firm link between pelvis size and body length in turtles L. aenigmatica could potentially be shorter or longer than thought.

'Using pelvis size to estimate body length is valid, but it has to be part of a wider set of measurements,' Sandra adds. 'Fortunately, the authors have put together a convincing argument that Leviathanochelys was this size.'

The new species nonetheless reveals that giant turtles were present on both sides of the Atlantic, with more of these enormous animals potentially waiting to resurface.