After Decades of Debate, Scientists Say These Fossils Belong to the Largest Known Scorpion, Which Lived 415 Million Years Ago

Researchers have wondered whether Praearcturus gigas was a giant crustacean called an isopod or some other creature. A new analysis of museum specimens suggests that it was a scorpion that stretched more than three feet long

Eli Stark-Elster | AAAS Mass Media Fellow

July 1, 2026 2:55 p.m.

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About 415 million years ago, the floodplains of what’s now England and Wales were haunted by a nightmarish predator: a baseball bat-size scorpion, armed with pincers the size of table knives.

That’s the finding of a study published on June 2 in the journal Palaeontology, based upon a set of fossils that have been housed in the Natural History Museum in London for more than a century. In 1870, scientists named the specimens’ species Praearcturus gigas, which they described as a giant isopod, a type of crustacean. But now, the creature has been reclassified as a different arthropod—the largest known scorpion to have ever walked the Earth.

“That is a chonky-looking organism,” Russell Bicknell, a paleobiologist at Flinders University in Australia who was not involved in the work, tells CNN’s Shraddha Chakradhar. “You would not want to run into this thing in a dark alley. It would be an absolute beast.”

After P. gigas was discovered, debate ensued about what the animal was. Researchers have proposed it was a millipede relative or a sea scorpion, among other possibilities, without ever breaking the deadlock.

So, paleobiologist Richard Howard of the Natural History Museum and his colleagues joined the identity search. They re-photographed the handful of P. gigas specimens, made detailed tracings of them, CT-scanned them and compared them to remains of other ancient arthropods.

One helpful clue came from the creature’s claws. The pincers of many arthropods include a movable “finger” and a fixed one, but the arrangement differs between animal types. In crustaceans, the movable and fixed fingers face toward each other. But P. gigas’ mobile finger pointed away from the fixed claw—the setup seen in scorpions.

The clincher, however, was a strangely long sternum with a groove down the center on the underside of the specimen’s shell. It shared this odd feature with a recently described fossil scorpion, Eramoscorpius brucensis, which was found in Canada. 

“E. brucensis was not discovered until 2015, so that key parallel was not evident beforehand,” Howard tells Science News’ Sahas Mehra.

The giant scorpion is striking not only for its size but for its place in evolutionary history. Usually, giant arthropods—like the small car-size millipede Arthropleura—are associated with the lush Carboniferous rainforests that appeared tens of millions of years after the scorpion’s time.

“Praearcturus lived when life on land was just starting out, and the ancestors of reptiles, mammals and birds were yet to leave the water,” Howard says in a statement. “It suggests that this species might have grown so big because there weren’t any other large predators, allowing it to dominate its environment.”  

The team suspects that the scorpion spent time hunting in fresh water. The fossils come from rocks deposited in rivers, and the animal has flaps resembling the swimming appendages on lobsters. Additionally, there probably wasn’t enough land prey available at the time to sustain the beast.

The debate over P. gigas’ identity might not be settled, however, as some scientists are skeptical that it’s undeniably a scorpion. “The problem I have, and to be fair to the authors, they acknowledge this … [is] we only have bits and pieces of the original animal,” says Jason Dunlop, scientific director of the arachnid, myriapod and stem-group arthropod collection at the Museum of Natural History in Berlin, to CNN.   

Still, the new work suggests that there might be other valuable paleontological discoveries waiting for us in dusty museum drawers.

“Specimens collected over a century ago can still hold entirely new insights,” Howard says in a separate statement. “By revisiting them with modern techniques, we can uncover discoveries that reshape our understanding of life on Earth.”

Eli Stark-Elster | Read More

Eli Stark-Elster is the AAAS Mass Media Science & Engineering Fellow at Smithsonian magazine. He is a PhD student in evolutionary anthropology at the University of California, Davis. His research focuses on documenting the methods used to induce altered states of consciousness across cultures—he conducts fieldwork in southern Africa, where he studies the shamanic use of hallucinogenic plants and fungi. His writing and research has been featured on NPR, Psyche and the Conversation. He also writes on his Substack, Unpublishable Papers.