Published online 21 November 2007 | Nature | doi:10.1038/news.2007.272

News

Giant sea scorpion discovered

Two-and-a-half metre arthropod prowled ancient rivers.

The claw alone is 46 cm long.Markus Poschmann

Those who are unnerved by creepy crawlies should stop reading now — a newly discovered two-and-a-half-metre monster arthropod, the largest yet discovered, is being unleashed on the world.

Luckily for those of a nervous disposition, the release of this long-extinct relative of sea scorpions is limited to the pages of the journal Biology Letters. There, Simon Braddy of the University of Bristol, UK, and colleagues report finding a 46-centimetre claw from a Jaekelopterus rhenaniae— from which they infer the existence of a giant example of this species1.

Although called a 'sea' scorpion, the 400-million-year-old beast probably terrorized lakes and rivers, and rarely, if ever, ventured into the oceans.

"This huge monster lived alongside other sea scorpions and fish," says Braddy. "They would probably lie in wait. When another animal went in front of it, it would lurch forward and capture it. ... These things would tear their prey to shreds and then eat the little pieces."

Scaled up

At 2.5 metres, this monster was bigger than a man.Simon Powell

The J. rhenaniae claw was found near Prüm in Germany. To work out the size of the arthropod it belonged to, Braddy and colleagues collected information on other sea scorpions and the ratio between their claw size and body length. This turned out to be relatively constant, leading the researchers to conclude that a creature with a 46-centimetre claw probaby had a body length of between 233 and 259 centimetres, or 333 and 359 centimetres including the claws and arms.

"I was amazed," says Braddy. "Whenever I’ve shown the picture to people they’re equally amazed. ... When you work out how big this beast was you’re staggered by the size of it."

Unlike some crabs, sea scorpions are not known to grow a single enormous claw, so extrapolation is reasonably straightforward, says Braddy. Greg Edgecombe, a palaeontologist at the Natural History Museum in London, said the researchers were "on pretty safe grounds" with their calculations.

A previous sea scorpion find had been reported as being 250 centimetres long based on a similar extrapolation from its claw size, Braddy says. But Braddy's new analysis indicates that this beast was actually a puny 210 centimetres long, leaving J. rhenaniae the arthropod crown-holder for size.

"It changes our ideas about what is possible," says Edgecombe of these creatures. "It forces us to modify our ideas about how large arthropods can be."

Big boys

Exactly how or why this animal grew to the size it did is unclear.

Giant arthropods, including huge millipedes and dragonflies, are known to have existed on land during the Carboniferous, some 359 million to 299 million years ago. Their existence is sometimes explained by an increase in the levels of atmospheric oxygen at that time. This could have allowed creatures with breathing systems that relied on the diffusion of oxygen into tissues, rather than a full-blown respiratory system, to grow much larger.

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But giant aquatic sea scorpions existed before this oxygen boost, notes Braddy.

Another theory holds that the size of these animals is the result of an evolutionary arms race with their prey, which included armoured fish.

Braddy speculates that they grew so big because of a lack of competition from vertebrates. When vertebrates arrived, it could have spelled the end for the giant scorpions. "It’s a case of the vertebrates coming along and spoiling the arthropod party," he says. 

  • References

    1. Braddy, S. et al. Biology Letters doi:10.1098/rsbl.2007.0491.
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