Sea scorpions grew to 2.5 metres and many never lived in the sea
Eurypterids, nicknamed sea scorpions, include the largest arthropods that ever lived: Jaekelopterus reached 2.5 metres. Yet the nickname misleads twice over. Only the earliest species were marine, many later ones lived in brackish or fresh water, and none of them were true scorpions.
The oldest known fossils date to about 480 million years ago, early in the Ordovician, and the group probably arose in the late Cambrian. With around 250 species it is the most diverse chelicerate order known from the Paleozoic era. Eurypterids flourished in Silurian seas, when most known species lived, and the Silurian genus Eurypterus alone makes up more than nine in ten of all specimens found. They kept diversifying in the Devonian but were hit hard by the extinction late in that period and dwindled through the Permian. Whether they vanished in the middle of the Permian or in the great extinction 251.9 million years ago is still argued.
Size varied enormously. Most species were under 20 centimetres, and the smallest, Alkenopterus, was just over 2 centimetres. Fossils have turned up on every continent, but most come from North America and Europe, since the animals lived chiefly in waters around the old landmass of Laurasia; only a few genera, such as Pterygotus and Adelophthalmus, spread worldwide. Some studies propose a dual breathing system that might have let them venture briefly onto land.
The body followed the chelicerate plan: a head section under a shield carrying compound eyes and simple eyes, and a twelve-segment abdomen ending in a tail spine called the telson. Six pairs of limbs sprouted from the head. The first, the chelicerae, matched spider fangs and carried small pincers for pushing food into the mouth, though in the pterygotids they became long toothed claws. The other five pairs bore toothed plates for chewing and served mostly as walking legs, growing larger toward the rear.
In the larger suborder, the sixth limb pair broadened into a swimming paddle, which explains the name Eurypterida, from Greek words for broad and wing. Several lineages had flattened telsons that may have worked as rudders, and some members of one group, notably Eusarcana, had a tail like a true scorpion's, possibly able to inject venom.
Source: Eurypterid
