Editing Merostomata

{{Short description|Class of arthropods}}
{{Automatic_taxobox
| fossil_range = {{fossilrange|Darriwilian|Recent|earliest=Early Ordovician}}
| image = Eurypterus Smithsonian.jpg
| image_caption = Reconstruction of ''[[Eurypterus]]'' (Eurypterida)
| image2 = Limulus polyphemus.jpg
| image2_caption = Five views of ''[[Limulus polyphemus]]'' (Xiphosura)
| taxon = Merostomata
| authority = [[Henry Woodward (geologist)|Woodward]], 1866
| subdivision_ranks = Orders
| subdivision = *†[[Eurypterida]]
*[[Xiphosura]]
}}

'''Merostomata''' is a [[class (biology)|class]] of [[chelicerata|chelicerate]] [[arthropod]]s that contains the extinct [[Eurypterida]] (sea scorpions) and the extant [[Xiphosura]] (horseshoe crabs). The term was originally used by [[James Dwight Dana]] to refer to Xiphosura only, but was emended by [[Henry Woodward (geologist)|Henry Woodward]] to cover both groups.

==Etymology==
The name "Merostomata" derives from the [[Greek language|Greek]] [[Root (linguistics)|roots]] {{lang|el|[[:wikt:μηρός|μηρός]]}} (''{{lang|el|meros}}'', "thigh") and {{Lang|el|[[:wikt:στόμα|στόμα]]}} (''{{lang|el|stoma}}'', "mouth"), in reference to the animals' possession of appendages which are [[arthropod mouthparts|mouthparts]] at their proximal end, but swimming legs at their distal end.<ref>{{cite journal |title=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 |journal=The Intellectual Observer: Review of Natural History, Microscopic Research, and Recreative Science |volume=11 |year=1867 |url=https://books.google.com/books?id=BMgWAQAAIAAJ&pg=PA234 |pages=233–234}}</ref>

==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 [[Arachnid]]a.<ref>{{cite book |editor1=Richard A. Fortey |editor2=Richard H. Thomas |year=1998 |title=Arthropod relationships |series=Volume 55 of Systematics Association Series |publisher=[[Springer Verlag]] |isbn=978-0-412-75420-3 |chapter=The early history and phylogeny of the chelicerates |author1=J. A. Dunlop |author2=P. A. Selden |name-list-style=amp |pages=221–236 |chapter-url=https://books.google.com/books?id=Pj-q9eHyIx0C&pg=PA221 }} Also available as [http://homepage.mac.com/paulselden/Sites/Website/DunlopSelden1997.pdf PDF]{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Some more recent analyses suggest the grouping Merostomata is not [[monophyly|monophyletic]], with Xiphosura being basal to a [[clade]] comprising Eurypterida and Arachnida.<ref name="Tudge">{{cite book |author=Colin Tudge |year=2002 |title=The variety of life: a survey and a celebration of all the creatures that have ever lived |publisher=[[Oxford University Press]] |isbn=978-0-19-860426-6 |chapter=Spiders, scorpions, mites, eurypterids, horseshoe crabs, and sea spiders. Subphylum Chelicerata and Subphylum Pycnogonida |pages=309–326 |chapter-url=https://books.google.com/books?id=YW-2gnuU0L0C&pg=PA312}}</ref> Other recent analyses support the monophyly of this group.<ref name=Garw>{{cite journal|title=Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders|first1=Russell J.|last1=Garwood|first2=Jason A.|last2=Dunlop|year=2014|journal=PeerJ|volume=2|pages=e641|doi=10.7717/peerj.641|pmid=25405073|pmc=4232842 |doi-access=free }}</ref> The Xiphosura are estimated to have diverged from the Arachnida {{Ma|480}}.<ref>{{cite journal |author1=Davide Pisani |author2=Laura L. Poling |author3=Maureen Lyons-Weiler |author4=S. Blair Hedges |year=2004 |title=The colonization of land by animals: molecular phylogeny and divergence times among arthropods |journal=[[BMC Biology]] |volume=2 |page=1 |pmc=333434  |doi=10.1186/1741-7007-2-1 |pmid=14731304 |doi-access=free }}</ref>

The shared features of the two groups traditionally grouped in the Merostomata are now thought to be retentions of primitive conditions ([[symplesiomorphy|symplesiomorphies]]), thus the name Merostomata has been recommended to be abandoned.<ref name="Tudge"/>

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.<ref>{{Cite journal |last1=Ballesteros |first1=Jesús A |last2=Santibáñez-López |first2=Carlos E |last3=Baker |first3=Caitlin M |last4=Benavides |first4=Ligia R |last5=Cunha |first5=Tauana J |last6=Gainett |first6=Guilherme |last7=Ontano |first7=Andrew Z |last8=Setton |first8=Emily V W |last9=Arango |first9=Claudia P |last10=Gavish-Regev |first10=Efrat |last11=Harvey |first11=Mark S |last12=Wheeler |first12=Ward C |last13=Hormiga |first13=Gustavo |last14=Giribet |first14=Gonzalo |last15=Sharma |first15=Prashant P |date=2022-02-03 |editor-last=Teeling |editor-first=Emma |title=Comprehensive Species Sampling and Sophisticated Algorithmic Approaches Refute the Monophyly of Arachnida |url=https://academic.oup.com/mbe/article/doi/10.1093/molbev/msac021/6522129 |journal=Molecular Biology and Evolution |language=en |volume=39 |issue=2 |pages=msac021 |doi=10.1093/molbev/msac021 |issn=0737-4038 |pmc=8845124 |pmid=35137183}}</ref>

== References ==
{{Reflist}}

==External links==
* {{Commons category-inline}}

{{Arthropods}}
{{Taxonbar|from=Q19105}}

[[Category:Arthropod classes]]
[[Category:Late Ordovician first appearances]]
[[Category:Extant Ordovician first appearances]]
[[Category:Chelicerates]]