Horseshoe crabs are sword-tailed cousins of spiders
Order Xiphosura—Greek for sword tail—groups arthropods related to arachnids, the familiar horseshoe crabs among them. Their look barely shifted across hundreds of millions of years, earning a living-fossil label. Adults today reach about 60 cm, while many Paleozoic species measured only 1–3 cm.
The order's name joins Greek xiphos, "sword," and oura, "tail," after the long spike at the back. Only four species survive, all horseshoe crabs of the family Limulidae, but the lineage reaches back to the Early Ordovician: undescribed fossils from Morocco's Fezouata Formation are about 480 million years old, and the oldest named member, Lunataspis, lived in Canada some 445 million years ago. Modern animals look so like their ancestors that they are called living fossils; the main change is that adult abdominal segments are now fused into one piece.
Where they belong has been argued for a long time. They were once lumped with the extinct sea scorpions in a class called Merostomata, a grouping now considered unnatural, and several genome studies place them inside the arachnids, often as the sister group of the order Ricinulei.
Under a semicircular carapace sit five pairs of walking legs and pincer-like chelicerae around a central mouth. The first four leg pairs have spiny bases that shred food before it reaches the mouth, and the last pair pushes mud aside while burrowing. Up to four eyes sense movement but probably cannot form images. Five pairs of flap-like gills line the abdomen, the blood carries blue copper-based haemocyanin along with clotting cells, and the mobile tail spine levers an upturned animal back onto its feet. Like scorpions, they fluoresce under ultraviolet light.
To breed, pairs move into shallow water; the male clings to the female's back while she scoops a hollow in the sand and lays 200 to 300 eggs about 2–3 mm across. The young hatch as tailless, non-feeding "trilobite larvae" and take roughly three years to mature. Today's adults reach 60 cm, yet many Paleozoic species were only 1 to 3 cm long. The group was never species-rich, peaking at fewer than 50 known fossil species in the Carboniferous, when freshwater belinurids flourished.
Source: Xiphosura
