The largest scorpion to walk the Earth was the size of a baseball bat

By Shraddha Chakradhar, CNN

(CNN) — Imagine a huge scorpion the size of a baseball bat, scrambling over mossy rocks and around large, treelike structures before slipping into a nearby stream.

That’s how a team of scientists describes what the largest ever known scorpion would have looked like as it prowled its environment roughly 415 million years ago in what’s now Great Britain.

To arrive at this fascinating new understanding, experts revisited fossils that had been in London’s Natural History Museum for more than 100 years. Piecing together those specimens along with more newly discovered fossils allowed the group to form a more complete picture of an organism that was once thought to be a crustacean, much like lobsters and other shellfish.

Praearcturus gigas was roughly 1 meter — a little more than 3 feet — in length, the scientists estimated in a study published June 2 in the journal Palaeontology.

“That is a chonky-looking organism,” said Russell Bicknell, a paleobiologist and research fellow at Flinders University in Adelaide, Australia, who wasn’t involved with the new report. “You would not want to run into this thing in a dark alley. It would be an absolute beast.”

Previous work on the scorpion, first identified in the 1870s, had suggested that it might have been part of a group of crustaceans known as isopods. It wasn’t until the 1980s, as scientists learned more about P. gigas and related animals, however, that the field also began considering it may have been another type of arthropod, or an invertebrate with an exoskeleton and jointed appendages — specifically a scorpion.

The study underscores the importance of revisionary science, said Elizabeth Dowding, chair of paleoenvironmental analysis at Friedrich-Alexander University of Erlangen-Nuremberg in Germany. She was not involved in the new research.

“How we think about extinction and evolutionary biology comes from the ability of scientists to work over the same ground, by repetition,” Dowding said. “It’s just amazing that this story itself is one of revision and consistent curiosity over the same set of rocks. … It’s demonstrative of the way science works.”

Identifying an early giant scorpion

Working with eight fossils excavated over the years from three sites, the study team used CT scans and other tools to take a closer look at the specimen in the Natural History Museum’s collection. The researchers also worked with an artist to create renderings of what the animal might have looked like in the environment of that time.

The “smoking gun” that the fossilized remains belonged to another species, said lead study author Richard Howard, curator of fossil arthropods at the Natural History Museum, was a study from 2015 that described a scorpion in Canada.

That creature, Eramoscorpius brucensis, had a key feature that, to Howard and his colleagues, was the teller. Its sternum, which is the plate on the underside of the scorpion between the bases of its legs, was long and triangular and had a groove down its middle just like P. gigas’ sternum, Howard said.

“It’s exactly the same in the two scorpions. So, we can infer from that that these are two closely related animals,” he said.

P. gigas lived in what’s known as the early Devonian Period, when life on Earth was still primarily aquatic. The presence of such a scorpion during that time, therefore, is somewhat of a surprise, according to Howard.

“That’s much older than we would expect to find giant arthropods,” he said. Scorpions and other giant arthropods, including early versions of dragonflies and millipedes, lived some 50 million years later, Howard explained. Jungles and trees during that time created an influx of oxygen that made giant terrestrial life possible, he said.

But during the early Devonian, when there wasn’t much oxygen, “The lines between what is a land-living animal and what is an aquatic animal are much more blurred,” Howard said.

Apart from its giant size, P. gigas was a creature whose legs, claws and head were covered in rough bumps, a characteristic trait of scorpions, according to scientists. Although there are no eyes preserved in the museum’s fossil samples, the study authors think that P. gigas, like modern scorpions, also had eyes on the front of its head.

Notably, P. gigas likely had roughly 6-inch-long (16-centimeter) pincers, about the length of a dollar bill. “It’s like four times the length of a modern, large scorpion,” Flinders University’s Bicknell said. In comparison, the giant forest scorpion, considered the largest modern-day species of scorpion, is usually between 4 and 5 inches (10 and 13 centimeters) in length.

The scorpion also seems to have had flaplike structures on its abdomen called lateral epimera. “No other scorpion has those that we know of,” Howard said. Scientists usually associate those body parts with marine arthropods such as horseshoe crabs. The flaplike features might have helped P. gigas swim, according to Howard.

The new work also allowed the team to classify two other arthropods from the same time period. Those organisms, one of which was also likely a giant scorpion, were not previously thought to be related to P. gigas, but in the current study, the authors suggest that these other species are likely also P. gigas.

Land or sea creature?

The authors also considered some of the animal’s behaviors. One theory for why the scorpion got to be so big is to avoid being eaten, as one of the earliest terrestrial beings of its kind, according to the study.

That great size would have also posed a problem as its food sources on land were all tiny creatures such as mites and other much smaller arachnids. “Surely something that’s the size of a dog can’t go around eating all these tiny, tiny things,” Howard said. “I don’t know how it would even catch them.”

The team instead hypothesize that P. gigas lived an amphibious lifestyle, feeding on primitive jawless and armored fish that inhabited the waters at that time.

Not everyone is convinced that P. gigas is a scorpion, however. “The problem I have, and to be fair to the authors, they acknowledge this … we only have bits and pieces of the original animal,” Jason Dunlop, scientific director of the arachnid, myriapod and stem-group arthropod collection at the Museum of Natural History (Museum für Naturkunde) in Berlin, said in an email to CNN.

Two key features of scorpions — the sting at the end of the tail and comblike sensory organs called pectines on scorpions’ underside — have not been found, said Dunlop, who was an author on the 2015 paper and a reviewer of the new study. “Things like big pincers might also be seen in some crustaceans,” he said.

Howard acknowledges his team was working with an incomplete specimen, but there is no reason to assume the tail wouldn’t have ended in a sting, he said. “If you discover a dinosaur skeleton and it’s got no head, you don’t assume it didn’t have a head,” he said.

To Dunlop, the skepticism demonstrates how difficult it can be to work with fossils — specimens are rarely unearthed intact as depicted in popular films such as “Jurassic Park.” “Real fossils are often broken, messy and incomplete, and the challenge is then to interpret what we are seeing using the evidence we have available,” he said.

Revisiting fossils

There are several implications of the work. “It sets the scene for reinvigorating how we think about animals from this time period,” Bicknell said. “I think what we may see over the next five to 10 years might be an increased rate in which new scorpions from this time period are documented.”

Revising P. gigas to be a scorpion also has practical implications, according to Dowding.

“Because of this revision, every single paleobiology database will have to update its information to incorporate this new data,” Dowding said, especially since the authors also used their understanding of P. gigas to shed light on two other organisms.

“The ramifications of this work potentially change global understandings of the diversity of this group.”

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