Ancient scorpion of the sea is the granddaddy of arachnids

A newly described species of sea scorpion measures over 1.5 metres long and lived 460 million years ago.

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Michelle Starr
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Artist's impression of Pentecopterus decorahensis.

Patrick Lynch/Yale University

The granddaddy of arachnids (so to speak) has been found between ancient layers of shale. The massive sea scorpion, or eurypterid, is related to modern arachnids and hails from the Ordovician Period some 460 million years ago, before the evolution of air-breathing animals. It's also some 10 million years older than the previously recorded oldest eurypterid.

The predator, found in the upper layer of northeastern Iowa's Winneshiek Shale in an ancient meteorite crater, has been named Pentecopterus decorahensis after the ancient Greek galley called the penteconter. According to the research team that described the animal, it resembled the ship in both appearance and predatory behaviour.

"The new species is incredibly bizarre. The shape of the paddle -- the leg which it would use to swim -- is unique, as is the shape of the head. It's also big -- over a metre and a half long," explained lead author James Lamesdell, a postdoctoral associate at Yale University.

"Perhaps most surprising is the fantastic way it is preserved -- the exoskeleton is compressed on the rock but can be peeled off and studied under a microscope. This shows an amazing amount of detail, such as the patterns of small hairs on the legs. At times it seems like you are studying the shed skin of a modern animal -- an incredibly exciting opportunity for any paleontologist."

Pentecopterus is also the largest known eurypterid from the Ordovician, with some of its body segments suggesting it reached up to 1.7 metres (5.6ft) in length. It's not the largest sea scorpion ever discovered, however. That honour goes to Jaekelopterus rhenaniae of around 390 million years ago, during the Devonian Period, measuring up to 2.5 metres (8.2ft) long.

The shapes and features of the 10 legs of Pentecopterus suggest various functions.

The rearmost pair are paddle-shaped with joints that appear locked, which suggests swimming or digging. They're also covered in dense bristles, like the legs of a swimming crab. The bristles in swimming crabs are used to increase the surface area of the leg, which helps the crab swim. Pentecopterus' bristles have smaller follicles, though, which suggests maybe they were used as feelers as well.

The three rearmost pairs of limbs are shorter than the front two pairs, which suggests that the animal walked on six pairs of legs, rather than eight as modern scorpions do. Its second and third pairs of limbs may also have been used to catch prey.

The full paper can be found online in the journal BMC Evolutionary Biology.