Xiphosura

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Xiphosura
Temporal range: Earliest
Ma
Restoration of Lunataspis, the oldest known xiphosuran
The extant Atlantic horseshoe crab (Limulus polyphemus)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Clade: Prosomapoda
Order: Xiphosura
Latreille, 1802
Groups

Xiphosura (

Ancient Greek ξίφος (xíphos) 'sword', and οὐρά (ourá) 'tail', in reference to its sword-like tail) is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Limulidae). They first appeared in the Hirnantian (Late Ordovician
). Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem-genera.

The group has hardly changed in appearance in hundreds of millions of years; the modern horseshoe crabs look almost identical to prehistoric genera and are considered to be living fossils. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.

Xiphosura were historically placed in the class Merostomata, although this term was intended to encompass also the

Sclerophorata together with the arachnids, and therefore, Merostomata is now a synonym of Xiphosura.[5] Several recent phylogenomic studies place Xiphosura within Arachnida, often as the sister group of Ricinulei; included among them are taxonomically comprehensive analyses of both morphology and genomes, which have recovered Merostomata as a derived clade of arachnids.[6][7][8]

Description

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 divided into an anterior

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.[9][10] The exoskeleton consist of a tough cuticle, but do not contain any crystalline biominerals.[11] Like scorpions, xiphosurans have an exocuticular layer of hyaline which exhibits UV fluorescence.[12]

Xiphosurans have up to four eyes, located in the carapace. Two

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.[10]

The first four pairs of legs end in pincers, and have a series of spines, called the

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.[13] These are thought to be vestiges of the limbs of an absorbed first opisthosomal segment.[10]

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.[14] The underside of the opisthosoma carries the genital openings and five pairs of flap-like gills.[10]

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.[10]

Internal anatomy

The mouth opens into a sclerotised

caeca that extend through much of the body, and absorb the nutrients from the food. The intestine terminates in a sclerotised rectum, which opens just in front of the base of the caudal spine.[10]

Xiphosurans have well-developed circulatory systems, with numerous

clotting.[10]

The excretory system consists of two pairs of coxal glands connected to a bladder that opens near the base of the last pair of walking legs. The brain is relatively large, and, as in many arthropods, surrounds the oesophagus. In both sexes, the single gonad lies next to the intestine and opens on the underside of the opisthosoma.[10]

Reproduction

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.[10]

The egg is about 2–3 mm (0.08–0.12 in) across. Inside the egg, the embryo goes through four molts before it hatches into a larva, often called a 'trilobite larva' due to its superficial resemblance to a trilobite. At this stage it has no telson yet, and the larva is lecithotrophic (non-feeding) and planktonic, subsisting on the maternal yolk before settling to the bottom to molt, after which the telson first appears.[15][16] 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.[10]

Evolutionary history

The oldest known stem-Xiphosuran,

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.[18] All extant species are estimated to have diverged from a common ancestor that lived about 135 million years ago in the early Cretaceous.[19] As a group they have never showed much diversity in regard of species. Less than 50 fossil species are known from the Carboniferous period, when they were at their most diverse.[20]

Classification

See also: List of xiphosuran genera
Solnhofen limestone

Xiphosuran classification as of 2018:[21][22]

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
      • Superfamily
        Limuloidea
        • †
          Valloisella
           Racheboeuf, 1992 (Carboniferous)
        • †Austrolimulidae Riek, 1955 (Early Permian-Early Jurassic)
        • Limulidae
          Zittel, 1885 (Carboniferous to recent)

Taxa removed from Xiphosura

Two groups were originally included in the Xiphosura, but since have been assigned to separate classes:

  • Aglaspida
    Walcott, 1911 (Cambrian to Ordovician)
  • Chasmataspidida Caster & Brooks, 1956 (Lower Ordovician)

Cladogram

Cladogram after Lasmdell 2020.[1]

Xiphosura
Limulina

Valloisella lievinensis

Paleolimulidae

Rolfeia fouldenensis

Bellinuroopsis rossicus

Belinurina

Pickettia carteri

Kasibelinurus amoricum

Lunataspis aurora

See also

  • Synziphosurine
  • List of xiphosurans

References

  1. ^
    PMID 33335810
    .
  2. ^ "Xiphosuran". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  3. PMID 25405073
    .
  4. John Wiley & Sons
    . pp. 1–320.
  5. ISSN 0024-4082
    .
  6. PMID 30917194
    .
  7. PMID 31847768
    .
  8. PMID 35137183
    .
  9. ^ Botton, M.I. (1984) Diet and food preferences of the adult horseshoe crab Limulus polyphemus in Delaware Bay, New Jersey, USA, Marine Biology, 81, pp. 199-207
  10. ^
    ISBN 978-0-03-056747-6
    .
  11. ^ Crystallographic Texture of the Arthropod Cuticle Using Synchrotron Wide Angle X-ray Diffraction
  12. ^ Exocuticular hyaline layer of sea scorpions and horseshoe crabs suggests cuticular fluorescence is plesiomorphic in chelicerates
  13. ^ R. C. Brusca & G. J. Brusca (2002). Invertebrates. Massachusetts: Sinauer Associates.
  14. S2CID 55271880
    .
  15. ^ Developmental ecology of the American horseshoe crab Limulus polyphemus
  16. ^ Metamorphosis of Limulus Polyphemus Trilobite Larvae: Role of Chemical and Structural Cues, Competency, and The Cost of Delayed Metamorphosis
  17. doi:10.1111/j.1475-4983.2007.00746.x
    .
  18. ISSN 2296-6463
    .
  19. ^ Horseshoe crab genomes reveal the evolution of genes and microRNAs after three rounds of whole genome duplication
  20. ^ Chromosome-level assembly of the horseshoe crab genome provides insights into its genome evolution
  21. ^ Dunlop, J. A., Penney, D. & Jekel, D. 2018. A summary list of fossil spiders and their relatives. In World Spider Catalog. Natural History Museum Bern
  22. S2CID 85553811
    .

Further reading

External links

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