Praearcturus gigas & Devonian Sea Monsters: Giant Scorpion and Aquatic Predators That Ruled Early Devonian Britain 415 Million Years Ago

In the shallow rivers and floodplains of what is now England and Wales, two very different “monsters” dominated 415 million years ago: the world’s largest scorpion, Praearcturus gigas, and formidable placoderm fish that patrolled the same waters.

New research published in Palaeontology (June 2026) by the Natural History Museum and University of Manchester confirms Praearcturus gigas as the largest scorpion ever known. Measuring over one metre long with 16-centimetre pincers, this giant arachnid stalked the Old Red Sandstone floodplains. CT scans and re-examination of 150-year-old fossils reveal it was a powerful ambush predator that likely spent much of its life in water, preying on primitive fish and other arthropods.

At the same time, the rivers and coastal shallows teemed with armoured placoderms such as Coccosteus and early arthrodires — the first major vertebrate predators. These “sea monsters” had heavy bony armour and shearing jaws, setting the stage for the Age of Fishes.

Praearcturus thus bridged the emerging land and aquatic worlds, while placoderms ruled the water column. Together they represent the dawn of large terrestrial and freshwater predators just as complex ecosystems were forming.

The fossils, housed at the Natural History Museum, show how Britain’s Devonian landscapes hosted some of the strangest and most formidable creatures in Earth’s history.