# Merostomata

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Class of arthropods

Merostomata Temporal range: Darriwilian–Recent PreꞒ Ꞓ O S D C P T J K Pg N Reconstruction of Eurypterus (Eurypterida) Five views of Limulus polyphemus (Xiphosura) Scientific classification Kingdom: Animalia Phylum: Arthropoda Subphylum: Chelicerata Class: Merostomata Woodward, 1866[1] Orders †Eurypterida Xiphosura

**Merostomata** is a [class](/source/Class_(biology)) of [chelicerate](/source/Chelicerata) [arthropods](/source/Arthropod) that contains the extinct [Eurypterida](/source/Eurypterida) (sea scorpions) and the extant [Xiphosura](/source/Xiphosura) (horseshoe crabs). The term was originally used by [James Dwight Dana](/source/James_Dwight_Dana) to refer to Xiphosura only, but was emended by [Henry Woodward](/source/Henry_Woodward_(geologist)) to cover both groups.

## Etymology

The name "Merostomata" derives from the [Greek](/source/Greek_language) [roots](/source/Root_(linguistics)) [μηρός](https://en.wiktionary.org/wiki/%CE%BC%CE%B7%CF%81%CF%8C%CF%82) (**meros**, "thigh") and [στόμα](https://en.wiktionary.org/wiki/%CF%83%CF%84%CF%8C%CE%BC%CE%B1) (**stoma**, "mouth"), in reference to the animals' possession of appendages which are [mouthparts](/source/Arthropod_mouthparts) at their proximal end, but swimming legs at their distal end.[2]

## History

The scientific consensus at the beginning of the 20th century was that these two marine groups were closely related, and only more distantly related to the terrestrial [Arachnida](/source/Arachnid).[3] Some more recent analyses suggest the grouping Merostomata is not [monophyletic](/source/Monophyly), with Xiphosura being basal to a [clade](/source/Clade) comprising Eurypterida and Arachnida.[4] Other recent analyses support the monophyly of this group.[5] The Xiphosura are estimated to have diverged from the Arachnida [480](https://geoltime.github.io/?Ma=480) million years ago.[6]

The shared features of the two groups traditionally grouped in the Merostomata are now thought to be retentions of primitive conditions ([symplesiomorphies](/source/Symplesiomorphy)), thus the name Merostomata has been recommended to be abandoned.[4]

However, a 2022 analysis recovered the monophyly of Merostomata, as opposed to a monophyletic Arachnida, with Xiphosura as the only modern representative in a derived position, indicating the convergence of several characteristics that supposedly united the arachnids.[7]

## References

1. **[^](#cite_ref-CoL_1-0)** ["Merostomata Woodward, 1866"](https://www.catalogueoflife.org/data/taxon/B8VF7). *[Catalogue of Life](/source/Catalogue_of_Life)*. [Species 2000](/source/Species_2000): Leiden, the Netherlands. Retrieved 26 February 2026.

1. **[^](#cite_ref-2)** ["Literary Notices – A monograph of the British Fossil Crustacea, belonging to the Order Merostomata. Part 1. Pterygotus Anglicus, Agassiz. Pages 1–44. Plates i.–ix. By Henry Woodward, F.G.S., F.Z.S., of the British Museum"](https://books.google.com/books?id=BMgWAQAAIAAJ&pg=PA234). *The Intellectual Observer: Review of Natural History, Microscopic Research, and Recreative Science*. **11**: 233–234. 1867.

1. **[^](#cite_ref-3)** J. A. Dunlop & P. A. Selden (1998). ["The early history and phylogeny of the chelicerates"](https://books.google.com/books?id=Pj-q9eHyIx0C&pg=PA221). In Richard A. Fortey & Richard H. Thomas (eds.). *Arthropod relationships*. Volume 55 of Systematics Association Series. [Springer Verlag](/source/Springer_Verlag). pp. 221–236. [ISBN](/source/ISBN_(identifier)) [978-0-412-75420-3](https://en.wikipedia.org/wiki/Special:BookSources/978-0-412-75420-3). Also available as [PDF](http://homepage.mac.com/paulselden/Sites/Website/DunlopSelden1997.pdf)[*[permanent dead link](https://en.wikipedia.org/wiki/Wikipedia:Link_rot)*]

1. ^ [***a***](#cite_ref-Tudge_4-0) [***b***](#cite_ref-Tudge_4-1) Colin Tudge (2002). ["Spiders, scorpions, mites, eurypterids, horseshoe crabs, and sea spiders. Subphylum Chelicerata and Subphylum Pycnogonida"](https://books.google.com/books?id=YW-2gnuU0L0C&pg=PA312). *The variety of life: a survey and a celebration of all the creatures that have ever lived*. [Oxford University Press](/source/Oxford_University_Press). pp. 309–326. [ISBN](/source/ISBN_(identifier)) [978-0-19-860426-6](https://en.wikipedia.org/wiki/Special:BookSources/978-0-19-860426-6).

1. **[^](#cite_ref-Garw_5-0)** Garwood, Russell J.; Dunlop, Jason A. (2014). ["Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4232842). *PeerJ*. **2** e641. [doi](/source/Doi_(identifier)):[10.7717/peerj.641](https://doi.org/10.7717%2Fpeerj.641). [PMC](/source/PMC_(identifier)) [4232842](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4232842). [PMID](/source/PMID_(identifier)) [25405073](https://pubmed.ncbi.nlm.nih.gov/25405073).

1. **[^](#cite_ref-6)** Davide Pisani; Laura L. Poling; Maureen Lyons-Weiler; S. Blair Hedges (2004). ["The colonization of land by animals: molecular phylogeny and divergence times among arthropods"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC333434). *[BMC Biology](/source/BMC_Biology)*. **2**: 1. [doi](/source/Doi_(identifier)):[10.1186/1741-7007-2-1](https://doi.org/10.1186%2F1741-7007-2-1). [PMC](/source/PMC_(identifier)) [333434](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC333434). [PMID](/source/PMID_(identifier)) [14731304](https://pubmed.ncbi.nlm.nih.gov/14731304).

1. **[^](#cite_ref-7)** Ballesteros, Jesús A; Santibáñez-López, Carlos E; Baker, Caitlin M; Benavides, Ligia R; Cunha, Tauana J; Gainett, Guilherme; Ontano, Andrew Z; Setton, Emily V W; Arango, Claudia P; Gavish-Regev, Efrat; Harvey, Mark S; Wheeler, Ward C; Hormiga, Gustavo; Giribet, Gonzalo; Sharma, Prashant P (2022-02-03). Teeling, Emma (ed.). ["Comprehensive Species Sampling and Sophisticated Algorithmic Approaches Refute the Monophyly of Arachnida"](https://academic.oup.com/mbe/article/doi/10.1093/molbev/msac021/6522129). *Molecular Biology and Evolution*. **39** (2) msac021. [doi](/source/Doi_(identifier)):[10.1093/molbev/msac021](https://doi.org/10.1093%2Fmolbev%2Fmsac021). [ISSN](/source/ISSN_(identifier)) [0737-4038](https://search.worldcat.org/issn/0737-4038). [PMC](/source/PMC_(identifier)) [8845124](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8845124). [PMID](/source/PMID_(identifier)) [35137183](https://pubmed.ncbi.nlm.nih.gov/35137183).

## External links

- Media related to [Merostomata](https://commons.wikimedia.org/wiki/Category:Merostomata) at Wikimedia Commons

v t e Extant Arthropoda classes by subphylum Kingdom Animalia Subkingdom Eumetazoa Clade Bilateria Clade Protostomia Superphylum Ecdysozoa Chelicerata Pycnogonida (sea spiders) Euchelicerata Xiphosura (horseshoe crabs) Arachnida (spiders, scorpions, ticks, mites) Mandibulata Myriapoda Chilopoda (centipedes) Symphyla Pauropoda Diplopoda (millipedes) Pancrustacea Crustacea Ostracoda (seed shrimps) Mystacocarida Pentastomida (tongue worms) Branchiura (fish lice) Malacostraca (woodlice, shrimps, crayfish, lobsters, crabs) Thecostraca (barnacles and relatives) Copepoda Tantulocarida Cephalocarida (horseshoe shrimps) Branchiopoda (fairy, tadpole, clam shrimps, water fleas) Remipedia Hexapoda Entognatha Protura (coneheads) Collembola (springtails) Diplura (two-pronged bristletails) Insecta (insects) italic are paraphyletic groups

Taxon identifiers Merostomata Wikidata: Q19105 Wikispecies: Merostomata BOLD: 74 BugGuide: 352889 CoL: B8VF7 EoL: 52223786 EPPO: 1MEROC GBIF: 351 iNaturalist: 48309 IRMNG: 1048 ITIS: 82698 NCBI: 6844 Open Tree of Life: 1041483 WoRMS: 150507

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Adapted from the Wikipedia article [Merostomata](https://en.wikipedia.org/wiki/Merostomata) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Merostomata?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.