Plesiosaurs were not dinosaurs, but they ruled the oceans for 140 million years.

Nature · Published 3 October 2026 Swipe more like this →

In short: Often mistaken for dinosaurs, these marine reptiles were actually distant cousins that thrived in the open ocean. While land-dwelling dinosaurs faced extinction, plesiosaurs survived for over 140 million years, evolving into diverse forms ranging from small, long-necked swimmers to massive, ferocious predators with bone-crushing bite forces.

Plesiosaurs belong to the sauropterygian group, a lineage of reptiles that returned to the sea from land. They first appeared in the fossil record around 240 million years ago, though the earliest confirmed member, Rhaeticosaurus, dates to 204 million years ago. Unlike their land-bound dinosaur relatives, plesiosaurs were fully aquatic, utilizing four wing-shaped flippers to navigate the water. Experiments with robotic models suggest they generated 60% more thrust by coordinating the movement of these limbs, a unique swimming style that allowed them to dominate marine environments.

The group split into two main lineages: the long-necked, small-headed plesiosauroids and the short-necked, large-headed pliosaurs. As they evolved, some species reached staggering proportions. Elasmosaurus, for instance, grew to 13 metres long with a neck containing 76 vertebrae. Pliosaurs were equally formidable; Pliosaurus funkei, discovered in 2009, reached an estimated 15 metres in length and 45 tonnes in weight. These predators possessed immense power, with some jaws capable of exerting a bite force of 33,000 psi.

The discovery of these creatures fundamentally changed scientific understanding of extinction. When the first complete Plesiosaurus skeleton was presented by Mary Anning in 1824, it helped confirm that species could vanish entirely. While other sauropterygians perished during mass extinctions at the end of the Triassic Period, plesiosaurs survived, likely because their adaptation to the open ocean allowed them to travel vast distances to find food when local environments collapsed. Their legacy remains a cornerstone of paleontological history, marking a pivotal moment in our grasp of Earth's ancient life.

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