Eurypterida

Eurypterida is an extinct order of aquatic chelicerate arthropods, commonly known as sea scorpions. They first appeared in the Ordovician and survived until the late Permian, roughly 485 to 250 million years ago, with a fossil record that spans much of the Paleozoic. Eurypterids occupied a variety of marine, brackish, and some freshwater environments, and they ranged from small, shrimp-sized forms to enormous predators.

Anatomy and appearance vary across genera, but eurypterids typically had a dorsoventrally flattened body with a large, shield-like head (prosoma) and segmented trunk (opisthosoma). They possessed chelicerate mouthparts, including large chelicerae, and paired pedipalps that in many groups resembled pincers. Walking legs were adapted for swimming or walking, and in several lineages one or more pairs were modified into paddles for aquatic locomotion. The tail (telson) varied in shape; some forms had a slender telson, while others developed a broader tail spine or paddle-like structures. Large species, such as members of the family Pterygotidae, bore especially enlarged chelae and robust limbs.

Eurypterids were diverse ecologically, with many acting as top predators of their ecosystems, preying on fish, invertebrates, and possibly other eurypterids. Some smaller species likely fed on detritus or small invertebrates. They are among the most important Paleozoic arthropods for understanding the early evolution of chelicerates, and their fossils help illuminate ancient marine and freshwater ecosystems.

Taxonomically, Eurypterida is divided into suborders such as Eurypterina and Pterygotina, containing families like Eurypteridae and Pterygotidae. The group declined after the Devonian and became extinct by the end of the Permian.