Sea Scorpion was a Gentle Giant

A fossil of A. cummingsi. Photograph by Division of Invertebrate Paleontology, Peabody Museum of Natural History, Yale University. Acutiramus cummingsi, a species of ancient sea scorpion that could reach more than 8 feet (2.4 meters) long and had claws the size of tennis rackets, was part of a group of sea scorpions that “were the monsters of the sea 400 million years ago,” said Ross Anderson, a paleontologist at Yale University. 

But new research suggests that this fearsome-looking creature was actually something of a pussycat.

Recent analyses of two fossilized sea scorpion species, A. cummingsi and a smaller, related species in the genus Eurypterus, suggests A. cummingsi had lousy vision compared with its less robust cousins. In fact, A. cummingsi’s eyes resemble those of modern-day horseshoe crabs, which make an inglorious living as bottom scavengers.

This means the predator probably couldn’t chase the large, swift fish and other animals once thought to make up its diet. (See “Giant Sea Scorpion Discovered; Was Bigger Than a Man.”)

“They’ve been interpreted as high-level predators … but it doesn’t seem likely that they were able to track and follow prey,” said Anderson, co-author of the new study, published July 8 in the journal Biology Letters. 

Gentle Giants

A. cummingsi, which thrived from roughly 423 to 410 million years ago, is the biggest arthropod—a group that includes lobsters and insects—that ever lived. The creatures’ name comes from the Latin acutiramus, which means “acute branch,” for the angle of the spines on its grasping claws.

“Those pincers are something else,” said Samuel Ciurca, a geologist who has written a book about sea scorpions and wasn’t involved in the new study. “A couple of the larger [pincers] had teeth on them, so it was teeth on teeth, like a saw.”

A model of an ancient giant sea scorpion at the Smithsonian’s National Museum of Natural History. Photograph by Jason Edwards, National Geographic Creative. 

Those pincers, its size, and its forward-facing eyes had led scientists to think that A. cummingsi was a top predator that could chase down and tear apart well-protected prey, such as nautilus-like cephalopods.

But recent research showed that those formidable-looking pincers were too weak to cut up armored animals. (See “First Tool Users Were Sea Scorpions?“)

Intrigued by that finding, study co-author Anderson and his colleagues studied the eyes of fossilized A. cummingsi in addition to those of the smaller sea scorpion, and discovered that its vision was more suited to a creature that ambushes its prey in subdued lighting.

Sea Slugs on the Menu? 

The study’s authors “have made a good case” for A. cummingsi being more of a gentle giant than thought, said Richard Laub, a geologist and paleontologist at the State University of New York’s University at Buffalo, who was also not involved in the new research. (See “New Sea Monster Found, Rewrites Evolution?“)

“The evidence suggests they ate soft objects that they were capable of shredding,” such as plants and dead animals, Laub said by email. Live prey “would have been primarily soft-bodied and unable to put up a significant struggle”—sea slugs being one example.

However, that vision depends partly on the workings of the brain, which can’t be studied in fossils. The new findings fit with his own in suggesting that A. cummingsi was less like a Tyrannosaurus and more like a giant plant-eating dinosaur, Laub added.

Added study co-author Derek Briggs, also of Yale University: “If you went swimming with these things … I don’t think there’s any chance that Acutiramus would take off after you and grab your leg.