About two dozen families of eurypterids “sea scorpions” are known from the fossil record. Although these ancient predators have a superficial similarity, including a defensive needle-like spike or telson at their tail end, they are not true scorpions.
They are an extinct group of arthropods related to spiders, ticks, mites and other extant creepy crawlies.
Eurypterids hunted fish in the muddy bottoms of warm shallow seas before moving on to hunting grounds in fresh and brackish water during the latter part of their reign.
They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event (or sometime shortly before) 251.9 million years ago.
As to the oldest and youngest of the order, we can look to the Stylonurina. Members of the suborder are collectively and informally known as "stylonurine eurypterids" or "stylonurines". They are known from deposits primarily in Europe and North America, but also in Siberia.
Compared to the suborder, Eurypterina, the stylonurines were comparatively rare and retained their posterior prosomal appendages for walking. Despite their rarity, the stylonurines have the longest temporal range of the two suborders. The suborder contains some of the oldest known eurypterids, such as Brachyopterus, from the Middle Ordovician as well as the youngest known eurypterids, from the Late Permian.
They remained rare throughout the Ordovician and Silurian, though the radiation of the mycteropoids (a group of large sweep-feeding forms) in the Late Devonian and Carboniferous is the last major radiation of the eurypterids before their extinction in the Permian.
