The ancient crocodile that traded its armor for a life in the open ocean

Nature · Published 3 October 2026 Swipe more like this →

In short: Metriorhynchids were the only archosaurs to fully abandon land for the sea. By evolving hydrofoil-like limbs and shark-like tails, these prehistoric predators became masters of the pelagic realm. A newly discovered species from Spain reveals that the specialized traits defining their success first appeared millions of years earlier than expected.

During the Mesozoic era, the Metriorhynchidae family achieved a feat unique among archosaurs: they returned to a fully pelagic lifestyle. Unlike their modern crocodilian relatives, these creatures shed their heavy armor scutes and developed smooth skin, paddle-like forelimbs, and tail flukes to maximize swimming efficiency. Their transition to the water was so complete that they were likely incapable of walking on land, with evidence suggesting they even gave birth to live young at sea.

For years, the evolution of the subclade Rhacheosaurini—a group of increasingly specialized piscivores—remained a mystery. These animals were known for their elongate, tubular snouts and high tooth counts, which allowed them to hunt fast, small-bodied prey. While these features were common in the Late Jurassic and Early Cretaceous, they were curiously absent from the well-sampled Middle Jurassic fossil record of England. This led researchers to believe that these marine adaptations might have taken a long time to provide a selective advantage.

The discovery of Maledictosuchus riclaensis in the Middle Callovian deposits of Spain has rewritten this timeline. As the basal-most member of the Rhacheosaurini, this specimen proves that the craniodental toolkit required for marine success was already present in the Middle Jurassic, roughly 164 to 161 million years ago. Although Maledictosuchus riclaensis remains the only known representative from this period, its existence confirms that the evolutionary innovations of the Rhacheosaurini did not emerge suddenly in the Late Jurassic, but were instead established much earlier in the group's history.

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