The Oldest Species Of Sea Scorpion

Chordates have dominated the ocean for most of the animal kingdom's duration on Earth. During the Mesozoic giant marine reptiles were the top predators while during the Cenozoic and for most of Palaeozoic sharks and their relatives patrolled the reefs and the ranges of the deep sea. Previously, however, it was not the chordates but the arthropods which ruled the ocean. During the Cambrian, when most animal phyla evolved, the chordates were few and far between compared to the myriad of arthropods forms.

The chordates only began to gain ground when the first fish evolved and even then such forms were small. It was only when the placoderms evolved, possessing jaws and thick armour plates, that chordates became the dominant marine lifeforms. The arthropods' last stand, however was by no means small. Properly known as eurypterids, sea scorpions were the top predators in many marine ecosystems prior to the rise of the chordates. With some species coming in at over two metres, and all species possessing segmented armour and powerful claws, they were formidable creatures. Recently a new discovery has pushed their evolutionary history back in time.

A reconstruction of Pentecopterus decorahens

Temporary damming of the Upper Iowa river in 2010 gave researchers access to a fossil bed which had originally formed in a meteorite crater. Here they discovered fossils of a new eurypterid species which they named Pentecopterus decorahens.

'What's amazing is the Winneshiek fauna comprise many new taxa, including Pentecopterus, which lived in a shallow marine environment, likely in brakish water with low salinity that was inhospitable to typical marine taxa,' said Huaibao Liu from the Iowa Geological Survey and the University of Iowa. 'The undisturbed, oxygen-poor bottom waters within the meteorite crater led to the fossils' remarkable preservation.'

A limb of Pentecopterus. Scale bar is 10 millimetres

Hairs on the body of Pentecopterus. Scale bar is 1 millimetre

The 467 million year old fossil displays remarkable detail, including articulated limbs and the hairs which covered the body. The most incredible feature, however, is its size. Pentecopterus came in at a staggering 1.8 metres in length, making it the size of a human being.

'Eurypterids of comparable size are known, but occur much later in the group's history,' said James Lamsdell from Yale University. 'This shows that eurypterids evolved some 10 million years earlier than we thought, and the relationship of the new animal to other eurypterids shows that they must have been very diverse during this early time of their evolution, even though they are very rare in the fossil record.'

A meteorite striking the Earth during the Ordovician produced a five kilometre wide crater which was flooded by the sea to produce a shallow marine environment. Its size, combined with a series of walking and swimming limbs and a pair of spiked claws would have made it a fearsome predator in the ecosystem. The name itself is derived from a type of Ancient Greek galley ship, the penteconter.

A range of specimens of different sizes even yielded insight into its growth and development; large individuals would have hunted prey directly whilst juveniles would have sifted through sediment at the bottom of the crater sea.

Its age and size are important because they suggest that the evolutionary history of eurypterids may extend further back in time still, establishing a great antiquity for their future success. As the Palaeozoic progressed, placoderms took the eurypterids' place as top predators in the oceans. The eurypterids persisted at the top of food chains in freshwater environments which they had colonised tens of millions of years previously, but gradually they were ecologically marginalised. Like so many groups of Palaeozoic organisms they did not survive through the Permian extinction 250 million years ago - a diverse and successful lineage wiped from the oceans.