The largest known marine turtles such as the ancient extinct genus Archelon, which grew to sizes of 4.6 meters long and weighed up to 3.2 tons, lived in the seas surrounding the North American continent towards the end of the Cretaceous period. In contrast, no known European marine turtle—extinct or living—has exceeded 1.5 meters in shell-length.

Àngel H. Luján, Albert Sellés, and colleagues describe the remains of a new marine turtle specimen—which they name Leviathanochelys aenigmatica—found in the Cal Torrades locality, Northeastern Spain, which appeared nearly as large as Archelon. The remains, which were excavated between 2016 and 2021, consist of a fragmented but almost complete pelvis and parts of the upper shell (carapace), and date to the Campanian Age, between 83.6 to 72.1 million years ago.

The specimen possesses a distinctive prominence of bone that protrudes forwards from the front of the pelvis. This feature differs to other marine turtles, and indicates that Leviathanochelys represents a new taxon (group) of ancient marine turtles. This protrusion may have related to the respiratory system, suggest the authors.

Based on the size of the pelvis, the authors calculate that Leviathanochelys could have reached a body length of up to 3.74 meters. They estimate that the maximum width of Leviathanochelys' pelvis was 88.9 centimeters, which is slightly larger than the biggest estimates for Archelon's best known specimen (81.0 cm wide). The length of the pelvis from front to back was estimated to be 39.5 cm, slightly smaller than that of Archelon (46.0 cm long).