any aquatic arthropod of the extinct order Eurypterida, from the Paleozoic Era, closely related to trilobites and scorpions.

What is a Eurypterid exactly?

: any of an order (Eurypterida) of usually large aquatic Paleozoic arthropods resembling scorpions and related to the horseshoe crabs. — called also sea scorpion.

Is Eurypterid extinct?

Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago.