Xiphosura

Xiphosura is an order of chelicerate arthropods, commonly known as horseshoe crabs, whose name comes from Greek words for sword and tail, referring to the animals long, sword-like tail spine. Only four species survive today, all within the single living family Limulidae, while several extinct families, including Belinuridae, Austrolimulidae, and Paleolimulidae, are known from the fossil record; the order first appears in the Early Ordovician, roughly 480 million years ago, and its body plan has changed comparatively little since. Xiphosurans have a semicircular carapace, five pairs of walking legs tipped with pincer-like chelicerae, compound eyes, and a stiff tail spine called a telson. The order was traditionally placed in a separate class, Merostomata, but that grouping has proven paraphyletic, and current phylogenomic evidence instead places Xiphosura within Arachnida, often as the sister group of the small order Ricinulei, though the point remains an area of active taxonomic debate.