Did Praearcturus live in water?

Several specimens preserve flap-like structures on the abdomen resembling those of modern lobsters and other crustaceans. Those anatomical features hint that the animal could move between water and land, an unusual habit for a scorpion of this size.

"The boundary between land and sea was much less defined at this time," said Dr. Greg Edgecombe, Merit Researcher at the Natural History Museum and a co-author. "Praearcturus gives us a fascinating glimpse into how early animals adapted to these changing environments. It may even represent a lineage that returned to the water after earlier ancestors had already begun living on land."

A broader analysis of the arachnid fossil record, led by Dr. Russell Garwood of the University of Manchester, also found that scorpions appear unusually often in Early Devonian rocks compared with other arachnid groups. That pattern supports the possibility that freshwater habitats preserved their remains more readily than dry land did.

Why did it take 150 years to identify it?

Scientists first described Praearcturus in 1871, but they classified the remains as a giant crustacean resembling a woodlouse. The original fossils lacked a tail, the key feature that distinguishes scorpions from other arthropods, leaving the identification uncertain for more than a century.

The reinterpretation became possible after researchers compared those historical specimens with better-preserved fossils found more recently. Those newer remains showed clearly recognizable scorpion anatomy, giving the team the evidence needed to reassign the species.

"Praearcturus has puzzled us paleontologists for more than a century," Garwood said. "By bringing together material from several collections and using cutting-edge imaging techniques, we've been able to build a clearer picture of the animal than was previously possible, which is really exciting."

Garwood added: "What makes Praearcturus so interesting is that it became enormous at a time when life on land was otherwise very small. But it was a world that could somehow support a giant predator."

Howard stressed that older museum specimens still hold major scientific value. "Specimens collected over a century ago can still hold entirely new insights," he said. "By revisiting them with modern techniques, we can uncover discoveries that reshape our understanding of life on Earth."

What does this mean for the story of giant arthropods?

The finding pushes the history of arthropod gigantism back by at least 50 million years, before the Carboniferous Period, the era long associated with oxygen-driven arthropod gigantism. Until now, that oxygen-rich atmosphere had been the leading explanation for oversized insects, sea scorpions, and millipedes of the deep past.

Praearcturus shows that giant body plans could emerge under different conditions. Low oxygen, sparse competition for prey, and access to freshwater habitats may all have helped the species reach meter-scale proportions.

The study appears in Palaeontology, Volume 69, Issue 3, 2026. Its DOI is 10.1111/pala.70064.