Xiphosura

Lore & Background

Xiphosurans have a body divided into an anterior prosoma and a posterior opisthosoma. The prosoma is covered by a semicircular carapace and bears five pairs of walking legs and a pair of chelicerae. They have up to four eyes, including two compound eyes and one or two median ocelli. The opisthosoma terminates in a long, mobile caudal spine (telson) used to right the animal if overturned. Internally, they possess a sclerotised oesophagus, crop, gizzard, and a stomach that secretes digestive enzymes. Their circulatory system includes a tubular heart and blood containing haemocyanin, a blue copper-based pigment. Excretion is via two pairs of coxal glands. The larvae, called 'trilobite larvae', are lecithotrophic and planktonic before settling to the bottom.

Reader's Guide

Xiphosura are significant as living fossils, having changed little since the Paleozoic. Their modern representatives, horseshoe crabs, are crucial for biomedical research due to their blood's clotting properties, though the source article does not mention this. The group's evolutionary history shows low species diversity, with less than 50 fossil species known from the Carboniferous, their most diverse period. Phylogenetic studies have placed Xiphosura within Arachnida, often as the sister group of Ricinulei, and have recovered Merostomata as a derived clade of arachnids. Their classification has been debated, with Merostomata now considered a synonym of Xiphosura. The oldest known xiphosurans are undescribed specimens from the Early Ordovician Fezouata Formation, while the oldest named stem-xiphosuran is Lunataspis from the late Ordovician of Canada.

Did You Know?

• The name Xiphosura comes from Ancient Greek for 'sword' and 'tail', referring to its sword-like telson.
• Modern xiphosurans can reach up to 60 cm (24 in) in adult length, but Paleozoic species were often as small as 1 to 3 cm.
• The blood of xiphosurans contains haemocyanin, a blue copper-based pigment.