New York: The fossil of a previously unknown “sea scorpion” – an extinct monster-like predator that swam the seas some 467 million years ago – has been discovered in Iowa in the US.

Measuring over 1.5 metres long, Pentecopterus is the oldest known species of eurypterid (sea scorpion) and is related to modern arachnids.

It has the sleek features of a penteconter, one of the first Greek galley ships.

According to a Yale University research team, Pentecopterus could grow to nearly six feet, with a long head shield, a narrow body, and large, grasping limbs for trapping prey.

It is the oldest described eurypterid — a group of aquatic arthropods that are ancestors of modern spiders, lobsters and ticks.

“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 is also big – over a metre and a half long!” said lead author James Lamsdell from Yale University.

This shows that eurypterids evolved some 10 million years earlier than we thought.

“The relationship of the new animal to other eurypterids shows that they must have been very diverse during this early time of their evolution,” said James Lamsdell, post-doctoral associate at the university.

“Pentecopterus is large and predatory, and eurypterids must have been important predators in these early Palaeozoic ecosystems,” he added.

Geologists with the Iowa Geological Survey at the University of Iowa discovered the fossil bed in a meteorite crater by the Upper Iowa River in northeastern Iowa.

Fossils were then unearthed and collected by temporarily damming the river in 2010.

Researchers from Yale and the University of Iowa have led the analysis.

The fossil-rich site yielded both adult and juvenile Pentecopterus specimens, giving the researchers a wealth of data about the animal’s development.

In addition, the researchers said, the specimens were exceptionally well preserved.

Spines are also present on some limbs and appear similar to those found on horseshoe crabs where they aid in processing food.

The discovery has been detailed in the open access journal BMC Evolutionary Biology.