ST MAUGHANS GEOLOGIC FORMATION — Researchers reported findings on Praearcturus gigas on June 2 in the journal Palaeontology. Fragments of the species were discovered in 1870 in the St Maughans geologic formation in England.

Praearcturus gigas is approximately 415 million years old. The arthropod was approximately one meter long and possessed a 15-centimeter-long pincer with a movable claw facing away from its fixed claw. In crustaceans like lobsters, the movable claw points toward the fixed claw. Praearcturus gigas had flaplike structures similar to the appendages lobsters use for swimming.

The species had ridges on its pedipalps. Several scorpion species rub ridges on their pedipalps against their body to make a hissing sound. Researchers proposed in the 1980s that Praearcturus gigas was a scorpion.

Praearcturus gigas had an elongated, nearly triangular sternum with a groove down the middle. This characteristic was also present in Eramoscorpius brucensis, a 430-million-year-old scorpion species discovered in 2015. Modern scorpions have a small, usually pentagonal sternum.

Richard Howard, a paleobiologist at the Natural History Museum in London, discussed the 2015 discovery. He said, "E. brucensis was not discovered until 2015, so that key parallel was not evident beforehand."

Praearcturus gigas lived at a time when there were no land vertebrates. It was four to five times the size of Gigantometrus swammerdami, the largest modern scorpion species. Giant insects appeared in the Carboniferous period at least 55 million years after Praearcturus gigas. The rocks containing the fossils were deposited by rivers.