Picture a scorpion a metre-and-a-half long, with hairy legs and a wielding a large paddle – if you were in the ocean 460 million years ago, you wouldn't need to use your imagination.
Researchers in Iowa have found 150 fossil fragments of the ancient predator in a meteorite crater.
Pentecopterus decorahensis, named for a Greek war vessel it "resembles in outline and parallels in its predatory behaviour", is the earliest known example of a eurypterid, or 'sea scorpion', 10 million years older than the previous record-holder.
Related to modern-day spiders, Pentecopterus had a large paddle it used to swim, and possibly dig.
"The new species is incredibly bizarre," says lead author of the study James Lamsdell of Yale University. "The shape of the paddle - the leg which it would use to swim - is unique, as is the shape of the head."
The differing size of its limbs suggests two of them were used to capture prey, while it walked on the other six.
The largest of the Pentecopterus specimens found suggest it could grow to a whopping 1.7m long, making it the biggest sea scorpion of the era.
"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," says Dr Lamsdell.
"This shows an amazing amount of detail, such as the patterns of small hairs on the legs."
It's believed the hairs were used as sensors, or possibly to expand the surface area of its limbs to aid in swimming.
"At times it seems like you are studying the shed skin of a modern animal – an incredibly exciting opportunity for any palaeontologist," says Dr Lamsdell.
The eurypterids were already the biggest arthropods known to science, and the discovery of Pentecopterus extends that lead even further.