World's Oldest Sea Scorpion Found, Measuring Nearly 6 Feet Long

An artist's impression of the world's oldest sea scorpion, Pentecopterus decorahensis Patrick Lynch / Yale University

Scientists have found the world's oldest—and one of the largest—sea scorpions. The giant oceanic predator measured up to 5.5 feet in length and roamed the seas 460 million years ago, according to a study published August 31 in the journal BMC Evolutionary Biology.

This ancient species predates the oldest previously-known sea scorpion by about 10 million years, says James Lamsdell, a Yale researcher and the study's lead author.

It also appears relatively advanced anatomically, which is strange since it's the earliest one to be discovered. Roy Plotnick, a scientist at the University of Illinois at Chicago who wasn't involved in the study, says the findings suggest that sea scorpions were not a new species at that time, and had already been around for a while.

Also known as eurypterids, sea scorpions were the largest arthropods to ever appear on Earth, Plotnick says. They resemble modern-day scorpions—they have bodies with 12 segments with a head and tail on each end—and the two are thought to be distantly related. But sea scorpions (obviously) lived in the ocean. And this particular species, with its bizarre paddle-like appendage used for swimming and "large grasping appendages with long spines," was clearly a predator, Lamsdell says.

An up-close view of one of the sea scorpion's fossilized appendages James C. Lamsdell et al / BMC Evolutionary Biology

The scientists named this species Pentecopterus decorahensis, after the ancient Greek warship called the penteconter, which roughly resembles it in shape and its (likely) aggressive behavior.

Sea scorpions didn't preserve well, so it's remarkable that this fossil is in such good shape, Plotnick says. "The preservation of the animal is fascinating in and of itself—the fossils look like they could be fresh molts" of living specimens, Lamsdell says.

The shape of these animals' jaws suggested they ate soft wormlike creatures. When asked if they could harm a human, Plotnick says it's unlikely, although they "could probably give you a nice nasty pinch."

At the time this species lived, in the Ordovician period, the Earth was a very different place. Oceans covered much of North America, and most of life was concentrated in the seas. Sea scorpions would've been one of the largest animals on the plant at the time, Plotnick adds.

A scientific sketch of Pentecopterus James C. Lamsdell et al / BMC Evolutionary Biology