Long ago, gigantic marine turtles swam the Earth’s seas. Until recently, these prehistoric giants, reaching lengths of over 3 meters (10 feet) from head to tail, had been thought to be found only in waters surrounding North America.

Now, scientists have discovered a previously unknown species — the largest sea turtle found in present-day Europe ever to be identified.

Initially found by a hiker who stumbled upon the remains in 2016 in the Pyrenees mountains of northern Spain, the species has been given the name Leviathanochelys aenigmatica. “Leviathan” is the biblical term for a sea monster, an allusion to the creature’s large body size, while “chelys” translates to turtle and “aenigmatica” translates to enigma — in reference to the turtle’s peculiar characteristics, wrote the authors of a paper published Thursday in the journal Scientific Reports.

The unusual animal’s presence in this part of the prehistoric world revealed that giant turtles were more common than previously thought, according to the study

Before the discovery, the largest species uncovered in what’s now Europe measured at just 1.5 meters (5 feet) in length, similar to today’s leatherback sea turtles, which weigh an average of 300 to 500 kilograms (660 to 1,100 pounds) and measure 1 to 2 meters (or between 3 and 6.5 feet), according to the Smithsonian Institute.

The bone fragments of this newly identified species, however, have led scientists to estimate that Leviathanochelys had a 3.7-meter-long body (12.1 feet), almost as big as an average sedan.

“We never thought it was possible to find something like this. After quite a long study of the bone fragments, we realized that there were some features that were totally different, not present in any other fossil of a turtle species discovered so far,” said Albert Sellés, coauthor of the study and a postdoctoral researcher at the Autonomous University of Barcelona’s Miquel Crusafont Catalan Institute of Paleontology in Spain.

Originally, researchers believed the bones belonged to a different kind of animal, according to Sellés.

“It is quite common to find bone fragments, a lot of them. But most of them are uninformative,” Sellés said. “It is quite rare to discover something that really tells you a little bit of the life of the past.”

A local museum and Catalonia’s Ministry of Culture had originally collected the bone specimens, but they remained unstudied for nearly five years. When Sellés and the other researchers began their work studying the bones in 2021, they realized they were looking at a species of marine turtle completely new to science, and quickly went back to the Pyrenees site to perform more excavations.

There, more fragments of the specimen, including pieces of the turtle’s pelvis and carapace — the part of the shell that covered the creature’s back — were discovered. With these finds, the scientists observed more features not previously seen in any living or dead turtle species.

“The main differences of this new fossil are related to the pelvic region. More specifically, to a couple of bony bumps present in the anterior part of the pelvis, which we suspect are related to some kind of muscle that controls the movement of the abdominal region of the turtle,” Sellés said.

This feature or muscle most likely impacted the turtles’ breathing capacity, allowing them to hold their breath longer than other turtle species, in order to swim deep in the ocean to find food or escape predators, according to Sellés.

The research team estimated the ancient animal lived during the Campanian Age of the Late Cretaceous Epoch, making it at least 72 million years old.

The largest turtle on record, called Archelon, lived some 70 million years ago and grew to be about 4.5 meters (15 feet) long. Before this recent discovery, all prehistoric giant marine turtle discoveries were part of the same lineage as Archelon.

“We’re proving that turtles could achieve really gigantic proportions in different times, and also in different families,” Sellés said. “For the first time, we found a (giant) turtle that doesn’t belong to this family.”

The researchers hope to return to the fossil site again to look for more bones, as they are not certain that all fragments from this specimen have been discovered, according to Sellés.