Meet Pentecopterus: The (newest) oldest known species of sea scorpion

A previously undiscovered species of eurypterid, or sea scorpion, a more than 460 million year old creature representing the oldest known species of its kind, has been discovered by scientists from the University of Iowa and Yale University.

The new species has been given the name Pentecopterus decorahensis and measured more than 1.5 meters long, which also makes it the largest known eurypterid of its time, lead author James Lamsdell, a postdoctoral associate from Yale’s Department of Geology and Geophysics, and his co-authors reported in the September 1 edition of the journal BMC Evolutionary Biology.

Pentecopterus is an absolutely bizarre animal and working on it was a constant string of surprises,” Dr. Lamsdell told redOrbit via email. “Finding such a large eurypterid so early in their evolutionary history was completely unexpected, and its appearance is completely unusual. The most obvious strange feature is the head, which has this elongate projection at the front that makes it look unlike any other eurypterid we’ve found.”

“Possibly the strangest thing about it though are the swimming paddles, which are formed by the last pair of appendages on the head,” he added. “Many eurypterids have swimming paddles, but in Pentecopterus they are incredibly complex structures that allow the animal to change the surface area of the paddle, giving it greater maneuverability. This paddle is so unusual that when it was first found parts of it we thought they were from a totally different animal.”

Creature named in honor of ancient Greek warships

It wasn’t until they discovered additional intact specimens that he and his fellow investigators realized that they all belonged to the same creature. The sea scorpion is represented by at least 150 fossil fragments excavated from the upper layer of the Winneshiek Shale, a 27 meter thick sandy shale located in an ancient impact crater in northern Iowa, in 2010.

The extinct predator, which is related to modern arachnids, is said to be roughly 460 million years old, which means that it predates the oldest previously discovered representative of the eurypterids by at least 10 million years. The new species has been named the Pentecopterus decorahensis in honor of an ancient Greek galley known as the penteconter.

“The penteconter was one of the first large Greek warships, while Pentecopterus is the earliest large eurypterid predator that we have discovered,” Dr. Lamsdell told redOrbit. “There is also a superficial physical resemblance; Pentecopterus had a long, narrow body and a long head shield that resembles the prow of the penteconter.”

In addition to the paddles, the fossils included several features that allowed the research team to discern the function of other body parts. For instance, the creature’s second and third limb pairs might have been angled forward, suggesting that they were used primarily for capturing prey and not for locomotion, and the three rearmost pairs were shorter than the front ones, indicating that the creature may have walked on just six legs instead of eight.

Pentecopterus is an incredibly exciting discovery that can tell us much about the early evolution of eurypterids along with details of how they lived,” said Dr. Lamsdell. “The preservation is exceptional – the exoskeleton is fossilized in such a way that it feels like you a studying an animal that has just shed its skin. This sort of exceptional preservation lets gives us important insight into what eurypterids were like as living animals and allows us to explore patterns in evolution and ecology throughout the groups’ history.”

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Feature Image: This is an artist’s impression Pentecopterus. (Credit: Patrick Lynch/Yale University)