Imagine a scorpion the size of a car, armed with claws bigger than your arm and hunting through the darkness of ancient seas. This was Jaekelopterus — the largest arthropod predator ever known and one of the most terrifying creatures to have ever stalked Earth's oceans. But despite its fearsome appearance, this giant ‘sea scorpion’ wasn’t actually a scorpion at all.
The nightmare-inducing sea scorpion was actually a eurypterid - a diverse group of primarily marine arthropods that lived for more than 200 million years between the Ordovician (480 million years ago) and the Permian (267 million years ago).
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Reaching lengths of up to 2.6m, Jaekelopterus was a giant of its kind - most other eurypterids were less than 20cm-long. Like other eurypterids, though, Jaekelopterus was a predator that used spiky, grasping claws - known as chelicera - to grab and rip apart its prey. The claws of Jaekelopertus alone measured 45.5cm in length and were large enough to grab most things that lived in Early Palaeozoic oceans, from fish to other eurypterids.
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As well as sporting a pair of massive pincers, Jaekelopterus also had keen eyesight. A 2019 study found Jaekelopterus possessed unusually large photoreceptors (around 70 μm), almost 70 times larger than those of humans (1-2 μm). This would have given Jaekelopterus excellent visibility in low-light environments, such as the silty estuaries and lagoons where it lived and hunted.
Jaekelopterus was highly agile too, as is suggested by studies on the hydromechanics of its swimming paddles
Top image: Junnn11, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
