Giant fossil sea scorpion discovered
The fossil is of the oldest known species of eurypterid (sea scorpion). The extinct monster-like predators swam the seas in ancient times and are related to modern arachnids.
The authors of the report, published in the open access journal BMC Evolutionary Biology, named the new species Pentecopterus decorahensis after the "penteconter" - an ancient Greek warship that the species resembles.
"The new species is incredibly bizarre," said lead author James Lamsdell from Yale University.
"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!"
He said the fossil was extremely well 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 palaeontologist," he said.
More than 150 fossil fragments were 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.
At around 460 million years old, the fossil is 10 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.
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.