Xiphosura

Xiphosura is an order of marine chelicerate arthropods that includes living horseshoe crabs and extinct relatives. Its members have a broad prosoma, a segmented opisthosoma, and a long telson.

Chelicerata: A subphylum of arthropods whose members have chelicerae as their first pair of appendages. Xiphosurans are chelicerates, sharing this defining appendage arrangement with spiders and scorpions.
Atlantic horseshoe crab: Limulus polyphemus, the horseshoe crab species native to the Atlantic coast of North America. It is the best-known living xiphosuran and the sole extant species in its genus.
Paleozoic: The geological era lasting from about 539 to 252 million years ago. Many of the oldest known xiphosuran fossils come from Paleozoic rocks.
Limulus amebocyte lysate: A clotting reagent made from horseshoe crab blood cells and used to detect bacterial endotoxins. It is a major biomedical use of blood from living xiphosurans.
Arthropod phylogeny: The evolutionary relationships among arthropod groups, inferred from anatomical and genetic evidence. Xiphosuran placement within chelicerates and the status of fossil relatives remain debated.
Arthropod: An invertebrate animal with a segmented body, jointed appendages, and an external skeleton. Xiphosura belongs to this larger animal group and inherits its basic segmented construction.
Tachypleus tridentatus: An Asian horseshoe crab species found in coastal waters of East and Southeast Asia. This living species shows that xiphosurans persist beyond the Atlantic horseshoe crab.
Cambrian explosion: A period of rapid animal diversification recorded in fossils from roughly 540 million years ago. Cambrian arthropod fossils provide context for debates about the origins of chelicerates.
Endotoxin: A toxic lipopolysaccharide component of the outer membrane of many Gram-negative bacteria. LAL testing detects endotoxin through the horseshoe crab’s sensitive clotting response.
Molecular phylogenetics: The study of evolutionary relationships using DNA, RNA, or protein sequence data. Genetic evidence has challenged classifications based mainly on horseshoe crab body form.

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Horseshoe crabNarrower topic: This is the larger taxonomic group to which horseshoe crabs belong.