Xiphosura

Modern xiphosurans reach up to 60 cm (24 in) in adult length, but the Paleozoic species were often far smaller, some as small as 1 to 3 cm (0.39 to 1.18 in) long.

Their bodies are covered with a tough cuticle, but do not contain any crystalline biominerals,[7] and are divided into an anterior prosoma and a posterior opisthosoma, or abdomen. The upper surface of the prosoma is covered by a semicircular carapace, while the underside bears five pairs of walking legs and a pair of pincer-like chelicerae. The mouth is located on underside of the center of the prosoma, between the bases of the walking legs, and lies behind a lip-like structure called the labrum.[8][9]

Xiphosurans have up to four eyes, located in the carapace. Two compound eyes are on the side of the prosoma, with one or two median ocelli towards the front. The compound eyes are simpler in structure than those of other arthropods, with the individual ommatidia not being arranged in a compact pattern. They can probably detect movement, but are unlikely to be able to form a true image. In front of the ocelli is an additional organ that probably functions as a chemoreceptor.[9]

The first four pairs of legs end in pincers, and have a series of spines, called the gnathobase, on the inner surface. The spines are used to masticate the food, tearing it up before passing it to the mouth. The fifth and final pair of legs, however, has no pincers or spines, instead having structures for cleaning the gills and pushing mud out of the way while burrowing. Behind the walking legs is a sixth set of appendages, the chilaria, which are greatly reduced in size and covered in hairs and spines.[10] These are thought to be vestiges of the limbs of an absorbed first opisthosomal segment.[9]

The opisthosoma is divided into a forward mesosoma, with flattened appendages, and a metasoma at the rear, which has no appendages. In modern forms, the whole of the opisthosoma is fused into a single unsegmented structure.[11] The underside of the opisthosoma carries the genital openings and five pairs of flap-like gills.[9]

The opisthosoma terminates in a long caudal spine, commonly referred to as a telson (though this same term is also used for a different structure in crustaceans). The spine is highly mobile, and is used to push the animal upright if it is accidentally turned over.[9]

Xiphosurans move to shallow water to mate. The male climbs onto the back of the female, gripping her with his first pair of walking legs. The female digs out a depression in the sand, and lays from 200 to 300 eggs, which the male covers with sperm. The pair then separates, and the female buries the eggs.[9]

The egg is about 2–3 mm (0.08–0.12 in) across, and hatches into a larva that superficially resembles a trilobite. Indeed, it is often referred to as the 'trilobite larva'. Through a series of successive moults, the larva develops additional gills, increases the length of its caudal spine, and gradually assumes the adult form. Modern xiphosurans reach sexual maturity after about three years of growth.[9]

Lunataspis, the oldest known stem-Xiphosuran

The oldest known stem-Xiphosuran, Lunataspis, is known from the late Ordovician of Canada, around 445 million years ago.[12] No xiphosurans are known from the following Silurian. Xiphosurida first appears during the late Devonian. A major radiation of freshwater xiphosurids, the Belinuridae is known from the Carboniferous, with the oldest representatives of the modern family Limulidae also possibly appearing during this time, though they only appear in abundance during the Triassic. Another major radiation of freshwater xiphosurans, the Austrolimulidae, is known from the Permian and Triassic.[13]

Mesolimulus from the Solnhofen limestone

Xiphosuran classification as of 2018:[14][15]

Order Xiphosura Latreille, 1802

• †Maldybulakia Tesakov & Alekseev, 1998 (Devonian)
• †Willwerathia Størmer, 1969 (Devonian)
• †Kasibelinuridae Pickett, 1993 (Middle Devonian to Late Devonian)
• Suborder Xiphosurida
  • †Infraorder Belinurina
    • †Belinuridae Zittel & Eastman, 1913 (Middle Devonian to Upper Carboniferous)
  • Infraorder Limulina
    • †Bellinuroopsis Chernyshev, 1933 (Carboniferous)
    • †Rolfeiidae Selden & Siveter, 1987 (Early Carboniferous to Early Permian)
    • Superfamily †Paleolimuloidea Anderson & Selden, 1997
      • †Paleolimulidae Raymond, 1944 (Carboniferous to Permian)
    • Superfamily Limuloidea
      • †Valloisella Racheboeuf, 1992 (Carboniferous)
      • †Austrolimulidae Riek, 1955 (Early Permian-Early Jurassic)
      • Limulidae Zittel, 1885 (Carboniferous to recent)
        • Limulinae Zittel, 1885 (Late Jurassic-Present)
        • Tachypleinae Pocock, 1902 (Late Cretaceous-Recent)

• Synziphosurine
• List of xiphosurans

• Jason A. Dunlop (1997). "Palaeozoic arachnids and their significance for arachnid phylogeny" (PDF). Proceedings of the 16th European Colloquium of Arachnology: 65–82. Archived from the original (PDF) on 2011-07-27. Retrieved 2011-03-27.
• J. A. Dunlop and P. A. Selden (1997). "The early history and phylogeny of the chelicerates" (PDF). In R. A. Fortey & R. H. Thomas (ed.). Arthropod Relationships. Systematics Association Special Volume Series 55. Chapman & Hall. pp. 221–235. ISBN 978-0-412-75420-3.
• B. B. Rohdendorf (ed.) Fundamentals of Paleontology, vol. 9, Arthropoda-Tracheata and Chelicerata: 894 pp. [1991 English translation of Russian original, Smithsonian Institution Libraries and National Science Foundation].
• R. E. Snodgrass. 1952. A Textbook of Arthropod Anatomy. Hafner Publishing Company, New York.

Wikimedia Commons has media related to Xiphosura.

• Peripatus - an overview of arthropod relationships.
• Paleos - a site with a synoptic account of the Xiphosura, focused on fossils.
• Xiphosura - the article from the UCMP Web Taxa project.
• Xiphosura Educational Worksheet for Kids on EasyScienceforKids.