What is the common name for extinct arthropods of the order Eurypterida?

The common name for extinct arthropods of the order Eurypterida is sea scorpions.

Eurypterids were aquatic chelicerate arthropods that lived from the Ordovician Period into the Permian. Their bodies had a front section with eyes and jointed appendages, plus a segmented abdomen. Many species possessed broad swimming paddles, while others were adapted for walking or bottom-dwelling lifestyles.

The nickname “sea scorpions” reflects their general body shape and their relationship to modern chelicerates such as scorpions and horseshoe crabs. They were not true scorpions, however, and the name can be misleading: although the earliest eurypterids were marine, many later species inhabited brackish or freshwater environments.

Eurypterids ranged from tiny animals only a few centimetres long to enormous predators. Jaekelopterus is often cited as the largest known arthropod, with estimates reaching about 2.5 metres. Fossils have been found on every continent, although many important discoveries come from ancient deposits in North America and Europe. They disappeared by the end of the Permian or possibly earlier, depending on the classification of the youngest fossils.