This Prehistoric Sea Scorpion Was the Size of a Person
Concept drawing of Pentecopterus. Image: Patrick Lynch/Yale University

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This Prehistoric Sea Scorpion Was the Size of a Person

Want to know what 467-million-year-old sea scorpion hair looks like? Today’s your lucky day.
September 1, 2015, 12:00am

The fossilized remains of an enormous sea scorpion have been found in a Iowan fossil bed at the bottom of an ancient impact crater. (Take a moment to let all that sink in.)

Named for a Greek warship called the penteconter, Pentecopterus decorahensis is like some kind of evolutionary fever dream. The newly-discovered species was decked out with lethal clawlike appendages and an idiosyncratic, paddle-shaped leg that was likely used for locomotion.

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Measuring almost six feet long, Pentecopterus was a veritable giant in the seas of the Ordovician period, some 467 million years ago.

"It was probably the largest animal in its ecosystem," paleontologist James Lamsdell told me. Lamsdell is the lead author of a paper describing the animal, published today in the journal BMC Evolutionary Biology.

"From what we know, there was nothing else around that would have been likely to consider Pentecopterus prey," he added. "It seems that Pentecopterus was the dominant animal in its ecosystem."

This is even more impressive considering this species is the oldest eurypterid—the scientific term for sea scorpion—ever found in the fossil record. Eurypterids were a very diverse group of creatures that flourished for over 200 million years, before dying off at the dawn of the Triassic period. Among their ranks were the largest arthropods that ever roamed the Earth, and their close relatives live on today in the arachnid family.

The discovery of Pentecopterus pushes the evolutionary timeline of these influential animals back about nine million years, though Lamsdell said the eurypterid family tree's roots run even deeper.

"We know that Pentecopterus is actually a relatively advanced eurypterid," he told me. "The exciting thing about this is that it means that there must have been a number of other eurypterid groups around at the time too that we have yet to discover."

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"It is clear however that Pentecopterus was one of the earliest large predators in these complex ecosystems," he added.

What's more, Pentecopterus left behind the kind of beautifully fossilized remains that most paleontologists only dream about. "It is very rare to find such exquisite preservation in fossils of this age," Lamsdell said. "I have never seen anything like this before in a eurypterid."

The fine state of the specimens is due to the unique nature of Iowa's Decorah crater where the fossils, which include both adult and juvenile members of the species, were excavated.

The crater was formed about 470 million years ago, when a 200-meter-wide meteorite impacted the Earth. The Ordovician oceans flooded the deformation, creating a shallow marine environment of brackish water. Here, Pentecopterus communities flourished, and when individuals died, they were etched into geological history by the crater's deoxygenated seafloor, which provided perfect conditions for fossilization.

Indeed, according to Lamsdell, some of the fossils have even retained the creature's hair and skin patterns. "The really exciting thing is that fine details like hair patterns can tell us a lot about the animals' ecology," he said.

"For animals with an external exoskeleton, hairs are the primary way in which they sense the world around them, as eyes can only be looking at one place at any one time," he continued. "From looking at hair patterns we can see which parts of the animal were particularly sensitive."

"For example, there are many hairs on the margins of the swimming paddle, meaning that it would have been very sensitive to changes in current flow, which would have helped it as a swimmer," Lamsdell said.

So, to sum up: Paleontologists have inferred intimate details about a monster species of sea scorpion, the oldest yet found, which lived in the cozy fallout of a planetary collision nearly half a billion years ago. If that doesn't rate high on your wow meter, you need to get it fixed.