A giant scorpion that once roamed what is now England and Wales has been confirmed as the largest of its kind ever to exist, thanks to new research by scientists at The University of Manchester and the Natural History Museum.
Measuring around a metre in length and armed with pincers over 16 centimetres long, Praearcturus gigas would have been a formidable predator stalking floodplains around 415 million years ago. Remarkably, the fossils used to identify Praearcturus have been held in the Museum's collection for more than 150 years.
The study, published in the journal Palaeontology, used modern analytical techniques and comparisons with newly described fossil species to suggest that Praearcturus is a scorpion, and a distinct species.
Dr Richard J. Howard, Curator of Fossil Arthropods at the Natural History Museum, London, and lead author of the study, said: "When we think of giant arthropods, people often picture Carboniferous rainforests with giant millipedes or dragonfly-like insects from later in Earth's history. But Praearcturus lived at least 50 million years earlier, well before the evolution of trees, when life on land was only just getting started.
"Confirming that this animal is a scorpion fundamentally changes our understanding of how and when these creatures evolved to such extraordinary sizes."
Dr Russell Garwood, Palaeontologist at The University of Manchester, added: "Praearcturus has puzzled us palaeontologists for more than a century. 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.
"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. To try and better understand this ancient world we compared the size of fossil scorpions with other animals alive at the time. To reach such extraordinary sizes, and conclude that perhaps it lived in water, where life was bigger."
Praearcturus gigas lived during the Early Devonian. Small plants and fungi had only recently begun to spread across the landscape, and complex terrestrial ecosystems like forests had yet to evolve. This means that, unlike later giant arthropods, Praearcturus did not benefit from the high atmospheric oxygen levels associated with the rise of forests. Instead, its enormous size may reflect a world with relatively little competition from other large predators. This suggests that Praearcturus might have grown so big simply because there weren't many other large animals around meaning it could dominate its environment in a way that wouldn't be possible later on.
The fossils also hint that this giant scorpion may have led a partly aquatic lifestyle. Some specimens show flap-like structures on the abdomen similar to those found in modern crustaceans such as lobsters, suggesting it may have been capable of moving between water and land. Quantification of the wider arachnid fossil record, led by Dr Garwood and the team, shows that scorpions are unusually abundant in rocks of this age compared with other arachnids, supporting the idea that some early forms may have lived in freshwater environments where they are more likely to survive as fossils. This places Praearcturus at a pivotal moment in Earth's history when animals were first experimenting with life outside the oceans.
This places Praearcturus at a pivotal moment in Earth's history when animals were first experimenting with life outside the oceans.
Dr Greg Edgecombe, Merit Researcher at the Natural History Musuem, London, and co-author of the study said: "The boundary between land and sea was much less defined at this time. 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."
First described in 1871, Praearcturus gigas was originally thought to be a giant crustacean, similar to a woodlouse. The known fossils fragmentary nature lacked key features such as a tail making it difficult to classify with confidence for more than a century.
The breakthrough came through comparison with better preserved fossils discovered in recent years, which revealed key anatomical features unique to scorpions. The discovery highlights the continuing scientific importance of museum collections.
Dr Howard added: "Specimens collected over a century ago can still hold entirely new insights. By revisiting them with modern techniques, we can uncover discoveries that reshape our understanding of life on Earth."
The discovery of such a large scorpion so early in the history of life on land challenges assumptions about why prehistoric arthropods reached gigantic sizes. Rather than being driven solely by environmental factors such as oxygen levels, the findings suggest that ecological opportunity such as a lack of competition may have played a crucial role.
