It was one of the most fearsome sea monsters of all time, a vast sea scorpion that lurked below the surface and ambushed its prey with razor-sharp claws.

A newly discovered fossil has revealed that the predator, which lived 390 million years ago, grew to at least 2.5 metres (8ft 3in), making it among the largest arthropods known to science. Jaekelopterus rhenaniae would have lain in wait in shallow water for passing fish, seizing them with its 46cm claws in a sudden movement. After slicing its hapless prey into smaller pieces, it would have diced it up, using saw blades on its legs, devouring it with mouth parts in its belly.

While it would normally have fed on smaller prey such as fish, it was perfectly capable of taking larger animals, and sometimes turned cannibal, consuming smaller specimens of its own kind, scientists behind the discovery said.

“It would certainly have made a meal of anyone going for a swim 390 million years ago,” Simon Braddy, of the University of Bristol, said.

“These sea scorpions would have been truly terrifying creatures. They were like the crocodiles of the Devonian period, ambush predators that were very much top of the food chain.” J. rhenaniae takes its name from a German scientist who first described the creature in the early 20th century after it was found in the Rhineland. Now a fossilised claw has shown that it grew far larger than had been thought previously.

The giant claw was discovered in a quarry near Prüm by Markus Poschmann, of the German Cultural Heritage Directorate in Mainz. Dr Poschmann and Dr Braddy were able to calculate that the sea scorpion grew to at least 2.5m long, some 40cm longer than the previous largest specimen.

“I was absolutely amazed by the size of this fossil,” Dr Braddy said. “We knew that arthropods from this period were large, but not that they were this large. It must be one of the largest arthropods ever to have evolved.”

Dr Poschmann said: “I was loosening pieces of rock with a hammer and chisel when I suddenly realised there was a dark patch of organic matter on a freshly removed slab. After some cleaning I could identify this as a small part of a large claw. The pieces had to be cleaned separately, dried and glued back together. It was then put into a white plaster jacket to stabilise it.”

Arthropods have an exoskeleton and segmented bodies with appendages on each segment. The ancient sea scorpions, which were arachnids, are ancestors of modern scorpions. During the Devonian period, from 416 to 359 million years ago, arthropods had yet to be challenged by vertebrates. The supersized sea scorpions lived alongside dragonflies with a wingspan of 75cm, and huge cockroaches.

Details of the discovery appear in the journal Biology Letters.

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