Researchers have discovered the fossil of a bizarre and previously unknown species of 'sea scorpion', which measured over 1.5 metres long, and may have been the Earth's dominant large predator 460 million years ago.
The fossil discovered in Iowa, US, is the oldest known species of eurypterid (sea scorpion) - extinct monster-like predators that swam the seas in ancient times and are related to modern arachnids.
Researchers have named the new species Pentecopterus decorahensis after the 'penteconter' - an ancient Greek warship that the species resembles in outline and parallels in its 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!" said lead author James Lamsdell, from Yale University in US.
"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," he said.
"This shows an amazing amount of detail, such as the patterns of small hairs on the legs," said Lamsdell.
The new eurypterid species is represented by more than 150 fossil fragments, excavated from the upper layer of the Winneshiek Shale in northeastern Iowa - a 27 metre thick sandy shale located within an ancient meteorite impact crater and mostly submerged by the Upper Iowa River.
Some large body segments suggest a total length of up to 1.7 metres, making Pentecopterus the largest known eurypterid from its era.
Pentecopterus is about 460 million years old, making it ten million years older than the previous oldest record of the eurypterid group.
Some features of Pentecopterus revealed in the fossils also allow the researchers to interpret the functions of certain body parts.
The rearmost limbs include a paddle with a large surface area, and joints that appear to be locked in place to reduce flex. This suggests that Pentecopterus used these paddles to either swim or dig, researchers said.
The second and third pairs of limbs may have been angled forward, suggesting that they were involved primarily in prey capture rather than locomotion.
The three rearmost pairs of limbs are shorter than the front pairs, suggesting that Pentecopterus may have walked on six legs rather than eight, the researchers said.
The exceptional preservation of the exoskeleton also helped the researchers to interpret the role of finer structures, including scales, follicles and setae (stiff bristles).
The study was published in the journal BMC Evolutionary Biology.
