Archelon was a prehistoric sea turtle that weighed as much as a small car

Nature · Published 3 October 2026 Swipe more like this →

In short: The largest turtle to ever exist, Archelon, once patrolled the shallow Cretaceous seas of North America. Measuring nearly five meters long and weighing up to 3.2 tonnes, this ancient giant was a master of its environment, swimming alongside formidable predators like the Mosasaurus before vanishing 74 million years ago.

Archelon belonged to the extinct family Protostegidae, a lineage distinct from modern sea turtles. Unlike the fused, rigid shells of today’s turtles, Archelon possessed a frame-like shell structure. It also retained nasal bones above the nostrils, a feature lost in all living sea turtle species. The largest known specimen, nicknamed Brigitta, reached 4.6 metres in length with a flipper-to-flipper span of four metres.

These giants inhabited the Western Interior Seaway, a vast, shallow, and warm body of water that once bisected North America. During the Late Cretaceous, between 80 million and 74 million years ago, this seaway provided an abundant food supply, including molluscs, jellyfish, and large fish. Archelon’s massive size likely offered a survival advantage, helping it avoid predation by other marine reptiles and increasing its egg-laying capacity.

The extinction of Archelon coincided with the end of the Cretaceous Period, approximately 66 million years ago, when an asteroid strike triggered a global crisis. However, researchers suggest that Archelon may have been uniquely vulnerable. While other large marine turtles survived, Archelon’s specialized adaptation to the shallow seaway environment and its high caloric needs may have left it unable to cope with the dramatic loss of food and habitat that followed the mass extinction event.

Today, Archelon remains a subject of intense paleontological interest. Because fossilized stomach contents are rare, scientists rely on skull morphology to infer its diet, comparing its crushing beak to those of modern loggerheads or hawksbills. Finding a nest or hatchlings would be a breakthrough, as it would reveal whether these massive creatures shared the open-ocean childhoods typical of modern sea turtles or if they remained in the shallows throughout their lives.

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