Ancient Sea Scorpion's Weaponized Tail Made Mincemeat of Our Aquatic Ancestors

A newly discovered eurypterid sported a weaponized tail that resembles an arrow and a sword, which it used to slash its prey to death.

Long before the evolution of sharks and barracudas, some of the scariest predators lurking in the primordial seas were eurypterids, better known as sea scorpions. Related to modern scorpions and horseshoe crabs, these animals included species that had sharp pinching claws and the ability to crawl out of water to hunt on land. They thrived around 430 million years ago, with certain species growing to over 10 feet long.

A newly discovered, well-preserved fossil eurypterid adds a weapon to the known arsenal of these ancient formidable predators: a slapping, slashing tail spine. Its wielder, Slimonia acuminata, represents a new species of eurypterid that once even feasted on our early fish ancestors. It is described in the journal The American Naturalist.

Slimonia would have lurked in the shallow waters of lagoons and lakes along the coast of primordial Europe, during the Silurian Period (443.7–416 million years ago),” lead author Scott Persons, of the University of Alberta’s Department of Biological Sciences, told Seeker. “At the time, our vertebrate ancestors were primitive fish.”

Persons and co-author John Acorn studied Slimonia’s remains, which were unearthed at the Patrick Burn Formation near Lesmahagow, Scotland. This sea scorpion was not one of the largest eurypterids, given that it measured about a foot and a half long. Its weaponized tail gave it an edge over most other predators, though.

One specimen preserves the sea scorpion’s serrated and spine-tipped tail, curved strongly to one side. The tail actually consists of two attached parts: a rounded area that looks a bit like an arrowhead, and then the actual knife-like spine. The researchers suspect that the former might have served double duty as a rudder when Slimonia was swimming.

As for how this predator dispatched prey, Persons said that Slimonia probably used its chelicerae, or segmented mouth parts, to hold onto our ancestors and other victims, “while repeatedly striking with sidelong blows from the tail spine.”

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Unlike today’s lobsters and shrimps, which can flip their broad tails up and down to help them swim, sea scorpion tails were vertically inflexible but horizontally highly mobile. A cool feature was that they could aggressively slap and slash sideways, while still meeting a minimum of hydraulic resistance. This helped to prevent them from propelling themselves away from an intended target with each forceful strike.

The other top ocean predators at the time were enormous cephalopods, meaning the relatives of modern squid, octopus and the chambered nautilus.

“These were real Lovecraftian monsters, with sharp beaks and lots of tentacles,” Persons explained, referring to the horror fiction of H. P. Lovecraft. 

The giant squid surely feasted on our ancient ancestors, but also probably took on Slimonia from time to time.

“A soft cephalopod body would not have wanted to get struck by Slimonia’s tail, but we know that octopus are capable of handling lots of spiky dangerous prey by dexterously immobilizing them with the strong tentacles,” Persons said.

He added that, unlike sea scorpions, whose fossilized shells reveal their full body size and shape, the remains of ancient cephalopods require more scientific guesswork. That's because their soft bodies do not easily fossilize.

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Clues to their existence are cephalopod shells, some of which were spiral shaped, like a snail’s, while others were swirl shaped, Persons said, “like a tall helping of soft-serve ice cream.”

“Of the swirl-shelled Silurian cephalopods — say that five times fast — a few grew to well over a meter (3.3 feet) in length,” he continued. “Like modern squid, they could have reached out with their tentacles, snagged a victim and then pulled it back into their mouths.”

It is a wonder then how many of our early vertebrate ancestors managed to dodge such predators, given Slimonia’s slashing tail and the Silurian cephalopod grabbing tentacles. Some of these ancient mammal ancestors did, however, later evolving limbs and the ability to live full time on land.

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