The fossil of a previously unknown species of eurypterid that lived 460 million years ago (Middle Ordovician period) has been discovered in Iowa.

The creature, Pentecopterus decorahensis, is the oldest known species of eurypterid (sea scorpion) – a group of aquatic arthropods that are ancestors of modern spiders, lobsters, and ticks. The discovery published in the journal BMC Evolutionary Biology was made by paleontologists from Yale University and the University of Iowa.

“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,” said lead author Dr James Lamsdell of Yale University.

Dr Lamsdell and co-authors named the ancient monster after the ‘penteconter’ – an ancient Greek warship that the species resembles in outline and parallels in its predatory behavior.

“Pentecopterus decorahensis is large and predatory, and eurypterids must have been important predators in early Palaeozoic ecosystems,” Dr Lamsdell said.

The creature had a long head shield, a narrow body, and large, grasping limbs for trapping prey.

“Pentecopterus decorahensis 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 6 feet (1.5 m) long,” Dr Lamsdell added.

“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.”

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James C. Lamsdell et al. 2015. The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa. BMC Evolutionary Biology 15: 169; doi: 10.1186/s12862-015-0443-9