Merostomata

Merostomata is a traditional subdivision of the arthropods that historically included the eurypterids (sea scorpions) and the xiphosurans (horseshoe crabs). In many contemporary classifications, the group is no longer treated as a natural clade; eurypterids are regarded as an extinct order within Chelicerata, and the living xiphosurans are placed within Xiphosura, a lineage of chelicerates closely related to arachnids. As such, Merostomata is often described as paraphyletic or obsolete in modern taxonomy.

Members share chelicerate features such as a prosoma (cephalothorax) with paired chelicerae and pedipalps, and an abdomen (opisthosoma); they lack antennae and jaws. Eurypterids possessed a large, often armored body and varied in size; many had a metastoma and a posterior telson or long spike. Xiphosurans, exemplified by Limulus, have a horseshoe-shaped carapace, six pairs of appendages, book gills, and a long spiked telson. They have simple eyes and brood eggs on beaches.

Eurypterids thrived from the Ordovician to the Permian, occupying freshwater to coastal marine environments and ranging from small swimmers to apex predators. Xiphosurans are marine to estuarine and have persisted from the Ordovician to the present, with the modern Limulus polyphemus living along the Atlantic coast of North America. While the two groups differ markedly in ecology, they share a chelicerate limb plan and a dorsal carapace.

Today, horseshoe crabs are of medical and evolutionary interest due to their blood's sensitivity to endotoxins and their status as living fossils. The Merostomata concept illustrates how traditional classifications can conflict with cladistic understandings of arthropod evolution.