Sea scorpions (eurypterids)

Eurypterids, often called sea scorpions, were predatory arthropods with a long segmented body, a tail spike or paddle, a pair of swimming paddles and often large pincers or spiny grasping legs. They were relatives of horseshoe crabs and spiders, not true scorpions. Pentecopterus decorahensis, described in 2015 by James Lamsdell and colleagues from the Winneshiek Lagerstatte in Iowa, is about 467 million years old and about 1.7 metres long, one of the oldest and largest early members.¹²

They reached their peak in the Silurian and Devonian, when they lived in lagoons, rivers and seas. The biggest was Jaekelopterus rhenaniae, which lived about 390 million years ago in what is now Germany. From a 46-centimetre claw found at Willwerath, Simon Braddy and colleagues estimated in 2007 a body length of 2.3 to 2.6 metres, making it one of the largest arthropods known, rivalled only by the Carboniferous land giant Arthropleura. Some species could probably also walk on land, as shown by large trackways in Scotland. Eurypterids declined through the Devonian and Carboniferous and disappeared in the Great Dying, about 252 million years ago.¹³