Giant, creepy and crawly: using the fossilised claw as a benchmark, scientists believe the ancient sea scorpion was about 2.5 metres long.

Credit: Royal Society/Simon Braddy

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SYDNEY: A fossilised giant claw belonging to a 2.5-metre sea scorpion has been discovered in Germany – it is the biggest arthropod ever known.

The 46-centimetre claw was wielded by a species of sea scorpion called Jaekelopterus rhenaniae that lived between 460 and 255 million years ago. Using the claw as a benchmark, the scientists believe its owner was between 2.33 and 2.59 metres long.

"These claws are incredibly rare, particularly big ones. This is one-of-a-kind," said palaeontologist Simon Braddy from the University of Bristol in England, co-author of a paper describing the claw which was published today in the British journal Biology Letters.

Size matters

The claw was uncovered in a quarry near the western German border town of Pruem. According to the researchers, its owner was half a metre longer than all other sea scorpions in the fossil record, making it the largest known arthropod.

Arthropods are a group of animals that have a hard external skeleton and include spiders, insects and crabs. The largest living arthropod is the Japanese spider crab, which has a body size of up to 37 cm.

During certain periods in the Earth's history, much larger arthropods existed on land and in the oceans. Some scientists believe higher levels of oxygen in the atmosphere contributed to their evolution, while others cite an evolutionary arms race with their armoured prey.

Jaekelopterus rhenaniae lived at a time when arthropod gigantism was widespread, but it was rivalled in size only by the giant millipede, which reached lengths of over two metres. Equipped with a long, flat, jointed carapace, it stalked warm shallow sea waters, using its claws to grasp hold of prey.

Arthropod size reached a peak at about the same time our vertebrate ancestors developed jaws and gained the ability to eat other animals. "The most exciting aspect to the find is really that it shows that the arthropods were not giving over their role as top-level predators without a fight," said co-author Erik Tetlie from Yale University in Connecticut, USA.

"The largest animals show us what was possible in the past, and it is interesting to ask why things are different today," said Braddy. "We can learn more about patterns and processes in evolution by understanding the limits of life."