Giant Scorpion *Praearcturus gigas* May Be the Largest Ever Known

Had you travelled across Earth’s floodplains 415 million years ago, no other mammals would have been there to encounter. You would, however, have needed to watch out for a giant scorpion measuring more than a metre in length.

A major new examination of fossils has enabled UK researchers to verify the identity of Praearcturus gigas, a creature that could be the largest scorpion ever known.

Praearcturus gigas confirmed as a giant scorpion

The arthropod’s fossils were initially uncovered in the UK in 1870, yet scientists have debated its precise identity ever since. Using several sophisticated imaging methods, the research team now considers that question resolved.

As well as establishing that the animal was an enormous scorpion, the study offers fresh insight into the earliest stages of terrestrial life. At that point, land was home to small plants and fungi, while animals were only beginning to emerge from the oceans.

“Praearcturus lived when life on land was just starting out and the ancestors of reptiles, mammals and birds were yet to leave the water,” says lead author Richie Howard, a palaeontologist from the Natural History Museum in the UK.

“It suggests that this species might have grown so big because there weren't any other large predators, allowing it to dominate its environment.”

For the new assessment, researchers produced fresh camera lucida tracings, conducted computed tomography scans, and compared the specimens with several fossils from other UK locations dating from the Early Devonian period.

They also drew on fossils examined in Canada in 2015 that belonged to the ancient scorpion Eramoscorpius. Comparisons of their anatomy provide evidence that P. gigas was likewise a scorpion.

Its scale was remarkable. The scientists calculated that its pincers were 16 centimetres long, making each one longer than the entire body of many modern scorpion species.

The team additionally found ridged areas on the animal’s limbs, which were probably used to produce sound. This method, called stridulation, is consistent with that seen in other extinct scorpion species.

Evidence of life on land and in water

Although it would have ranked among the most formidable animals above the waterline, evidence indicates that the giant scorpion also spent part of its time in water.

“Without complex ecosystems to support Praearcturus on land, these animals probably spent part of their lives hunting in water,” says Howard.

“Some of the fossils found in Wales show that they had flap-like structures known as epimera that are similar to those found in lobsters and crabs.”

The conditions faced by other giant arthropods that appeared later on land would have been very different. These included millipedes as large as cars and dragonflies the size of today’s birds of prey. They had extensive forests through which to travel, as well as far more terrestrial animals to encounter - and consume.

How the giant scorpion may have survived

As competition for prey increased, the researchers propose that P. gigas may have persisted for a further 40 million years beyond the period represented by these fossils before vanishing from Earth.

Now that P. gigas has been confirmed as a scorpion, additional fossil research and analysis should help refine this timeline.

The results should also assist palaeontologists investigating the periods in which animals shifted from the oceans to dry land, particularly because the distinction is especially unclear among arthropods.

Determining which ancient animals lived on land, and when they did so, can improve our understanding of how species evolved, including the scorpions living on Earth today.

“Our best family trees from DNA sequences suggest that scorpions are closely related to other arachnids with which they share book lungs, such as the spiders,” says palaeontologist Greg Edgecombe, from the Natural History Museum in the UK.

“This predicts that they are descended from an air-breathing ancestor. If this is the case, then Praearcturus is an example of an animal that likely returned to the water after its ancestors moved onto land.”

The research has been published in Paleontology.