:en:megacheira

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| image1 = 20190911 Megacheira Great Appendages.png

| caption1 = Comparison of megacheiran great appendages

| image2 = 20210701 Yohoia tenuis great appendage mobility.gif

| caption2 = Movement range of the great appendage of Yohoia

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File:Tanglangia longicaudata reconstruction.png. en=endopod, ex=exopod]]

Megacheirans are defined by their possession of uniramous "great appendages", which are their first pair of head appendages. The first one or two proximalmost segments/podomeres are spineless (it has been argued that the supposed first of the two proximal podomeres is actually an arthrodial membrane), while the remaining 3–4 more distal podomeres each typically bear a single upward pointing spine attached towards the distal end of the segment, with the spineless proximal segment/s typically being connected to the spined distal segments by an elbow-like joint, which curled upwards.{{Cite journal |last1=Haug |first1=Joachim T. |last2=Waloszek |first2=Dieter |last3=Maas |first3=Andreas |last4=Liu |first4=Yu |last5=Haug |first5=Carolin |date=March 2012 |title=Functional morphology, ontogeny and evolution of mantis shrimp-like predators in the Cambrian: MANTIS SHRIMP-LIKE CAMBRIAN PREDATORS |journal=Palaeontology |language=en |volume=55 |issue=2 |pages=369–399 |doi=10.1111/j.1475-4983.2011.01124.x|doi-access=free }} The great appendages have been interpreted as raptorial limbs involved in predation, with those of some genera such as Yohoia being structurally comparable to the raptorial maxillipeds of mantis shrimp. The spines on the great appendages of leanchoilid megacheirans such as Leanchoilia and Yawunik are elongated into flagella-like structures, suggesting a sensory role alongside predatory function.{{cite journal |last1=Aria |first1=Cédric |last2=Caron |first2=Jean-Bernard |last3=Gaines |first3=Robert |date=2015 |title=A large new leanchoiliid from the Burgess Shale and the influence of inapplicable states on stem arthropod phylogeny |journal=Palaeontology |volume=58 |issue=4 |pages=629–660 |doi=10.1111/pala.12161|s2cid=86443516 |doi-access=free |bibcode=2015Palgy..58..629A }} The body is divided into the head and the trunk. The biramous limbs of megacheirans are homonomous (i.e. having little differentiation from each other), with endopods typically divided into seven segments/podomeres, and paddle-shaped exopods, which are fringed with thin lamellae. The morphology of the terminal telson segment is variable.{{Cite journal |last1=Aria |first1=Cédric |last2=Zhao |first2=Fangchen |last3=Zeng |first3=Han |last4=Guo |first4=Jin |last5=Zhu |first5=Maoyan |date=December 2020 |title=Fossils from South China redefine the ancestral euarthropod body plan |journal=BMC Evolutionary Biology |language=en |volume=20 |issue=1 |pages=4 |doi=10.1186/s12862-019-1560-7 |issn=1471-2148 |pmc=6950928 |pmid=31914921 |doi-access=free |bibcode=2020BMCEE..20....4A }} The biramous limbs of at least some megacheirans have been suggested bear exites.{{Cite journal |last1=Liu |first1=Yu |last2=Edgecombe |first2=Gregory D. |last3=Schmidt |first3=Michel |last4=Bond |first4=Andrew D. |last5=Melzer |first5=Roland R. |last6=Zhai |first6=Dayou |last7=Mai |first7=Huijuan |last8=Zhang |first8=Maoyin |last9=Hou |first9=Xianguang |date=30 July 2021 |title=Exites in Cambrian arthropods and homology of arthropod limb branches |journal=Nature Communications |volume=12 |issue=1 |page=4619 |doi=10.1038/s41467-021-24918-8 |pmid=34330912 |pmc=8324779 |issn=2041-1723|doi-access=free |bibcode=2021NatCo..12.4619L }}

Several subdivisions within the group are recognised including Jianfengiidae (including Fortiforceps, Jianfengia, Sklerolibyon and possibly Parapeytoia) which are known from the Early Cambrian of China, as well as the Cheiromorpha (containing at least Yohoia, Haikoucaris, and Leanchoiliidae), known with certainty from the Early-Mid Cambrian of North America, China and Australia, which is distinguished from Jianfengiidae by having a fewer number of body segments (20+ in Jianfengiidae, as compared to typically only 11 to 13 in Cheiromorpha). The monophyly of Megacheira is uncertain, with some studies recovering the group as paraphyletic. The latest unambiguous megacheiran is the leanchoiliid Lomankus from the Upper Ordovician of North America.{{Cite journal |last=Parry |first=Luke A. |last2=Briggs |first2=Derek E.G. |last3=Ran |first3=Ruixin |last4=O’Flynn |first4=Robert J. |last5=Mai |first5=Huijuan |last6=Clark |first6=Elizabeth G. |last7=Liu |first7=Yu |date=October 2024 |title=A pyritized Ordovician leanchoiliid arthropod |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982224013678 |journal=Current Biology |language=en |doi=10.1016/j.cub.2024.10.013|doi-access=free }}

Parapeytoia from the Cambrian of China which formerly misinterpreted as a radiodont was later suggested to be a member of this group.{{Cite journal |last=Stein |first=Martin |date=1 March 2010 |title=A new arthropod from the Early Cambrian of North Greenland, with a 'great appendage'-like antennula |url=https://academic.oup.com/zoolinnean/article/158/3/477/3798454 |journal=Zoological Journal of the Linnean Society |language=en |volume=158 |issue=3 |pages=477–500 |doi=10.1111/j.1096-3642.2009.00562.x |issn=0024-4082 |url-access=subscription |access-date=29 October 2019 |archive-date=3 November 2019 |archive-url=https://web.archive.org/web/20191103021239/https://academic.oup.com/zoolinnean/article/158/3/477/3798454 |url-status=live }}{{Cite book |last1=Xian-Guang |first1=Hou |url=https://books.google.com/books?id=2YWhDgAAQBAJ&q=Tanglangia&pg=PA184 |title=The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life |last2=Siveter |first2=David J. |last3=Siveter |first3=Derek J. |last4=Aldridge |first4=Richard J. |last5=Pei-Yun |first5=Cong |last6=Gabbott |first6=Sarah E. |last7=Xiao-Ya |first7=Ma |last8=Purnell |first8=Mark A. |last9=Williams |first9=Mark |date=24 April 2017 |publisher=John Wiley & Sons |isbn=9781118896389 |language=en |access-date=10 May 2021 |archive-date=8 May 2024 |archive-url=https://web.archive.org/web/20240508131607/https://books.google.com/books?id=2YWhDgAAQBAJ&q=Tanglangia&pg=PA184#v=snippet&q=Tanglangia&f=false |url-status=live }}{{Cite journal |last1=Daley |first1=Allison C. |last2=Budd |first2=Graham E. |last3=Caron |first3=Jean-Bernard |last4=Edgecombe |first4=Gregory D. |last5=Collins |first5=Desmond |date=20 March 2009 |title=The Burgess Shale Anomalocaridid Hurdia and Its Significance for Early Euarthropod Evolution |journal=Science |language=en |volume=323 |issue=5921 |pages=1597–1600 |doi=10.1126/science.1169514 |issn=0036-8075 |pmid=19299617 |bibcode=2009Sci...323.1597D |s2cid=206517995}} Possible megacheirans include Enalikter described from the Silurian of the United Kingdom, and Bundenbachiellus from the Early Devonian of Germany;{{Cite journal |last1=Siveter |first1=Derek J. |last2=Briggs |first2=Derek E. G. |last3=Siveter |first3=David J. |last4=Sutton |first4=Mark D. |last5=Legg |first5=David |last6=Joomun |first6=Sarah |date=7 March 2014 |title=A Silurian short-great-appendage arthropod |journal=Proceedings of the Royal Society B |volume=281 |issue=1778 |pages=20132986 |doi=10.1098/rspb.2013.2986 |pmc=3906945 |pmid=24452026}}{{Cite journal |author1=Derek J. Siveter |author2=Derek E. G. Briggs |author3=David J. Siveter |author4=Mark D. Sutton |author5=David Legg |author6=Sarah Joomun |year=2015 |title=Enalikter aphson is an arthropod: a reply to Struck et al. (2014) |journal=Proceedings of the Royal Society B |volume=282 |issue=1804 |pages=20142663 |doi=10.1098/rspb.2014.2663 |pmc=4375861}} due to their possession of great appendage-like cephalic appendages. However, their relationship to megacheirans has been questioned, due to the uncertain homology of their appendages.{{Cite journal |last=Aria |first=Cédric |date=October 2022 |title=The origin and early evolution of arthropods |url=http://paleorxiv.org/4zmey/download |journal=Biological Reviews |language=en |volume=97 |issue=5 |pages=1786–1809 |doi=10.1111/brv.12864 |pmid=35475316 |s2cid=243269510 |issn=1464-7931 |access-date=8 May 2024 |archive-date=8 May 2024 |archive-url=https://web.archive.org/web/20240508131708/http://paleorxiv.org/4zmey/download |url-status=live }} Kootenichela has been suggested to be a chimera of various arthropod taxa. Previous inclusion of some "bivalved" genera such as Forfexicaris, Ovalicephalus, and Occacaris to Megacheira was questioned by later investigations.{{Cite journal |last1=Ortega-Hernández |first1=Javier |last2=Janssen |first2=Ralf |last3=Budd |first3=Graham E. |date=1 May 2017 |title=Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective |journal=Arthropod Structure & Development |series=Evolution of Segmentation |volume=46 |issue=3 |pages=354–379 |doi=10.1016/j.asd.2016.10.011 |issn=1467-8039 |pmid=27989966|doi-access=free |bibcode=2017ArtSD..46..354O }} The Late Cambrian Orsten taxon Oelandocaris typically considered to be a crustacean relative, has also been suggested in some studies to be a megacheiran.{{Cite journal |last1=Aria |first1=Cédric |last2=Caron |first2=Jean-Bernard |date=May 2017 |title=Burgess Shale fossils illustrate the origin of the mandibulate body plan |url=http://www.nature.com/articles/nature22080 |journal=Nature |language=en |volume=545 |issue=7652 |pages=89–92 |doi=10.1038/nature22080 |pmid=28445464 |bibcode=2017Natur.545...89A |s2cid=4454526 |issn=0028-0836 |url-access=subscription |access-date=12 January 2023 |archive-date=23 September 2022 |archive-url=https://web.archive.org/web/20220923174326/https://www.nature.com/articles/nature22080 |url-status=live }}

{{gallery|Artistic reconstructions of Fortiforceps foliosa and Sklerolibyon maomima gen. et sp. nov. in early Cambrian Gondwanian seas.png|Life reconstruction of Fortiforceps (left) and Sklerolibyon (right)|Jianfengia reconstruction.png|Life restoration of Jianfengia|2012 Leanchoilia superlata 06.png|Closeup of great appendages of Leanchoilia superlata|Y._kootenayi.png|Life restoration of Yawunik|File:Parry et al. 2024 f03 (reconstruction).png|Life restoration of Lomankus|File:Oestokerkus megacholix reconstruction.png|Life restoration of Oestokerkus|||width=200|height=180|align=center}}

• †Tanglangia
• †Oelandocaris?
• †Jianfengiidae
• Sklerolibyon
• Jianfengia
• Parapeytoia?
• Fortiforceps
• †Kootenichelidae?
• Kootenichela?
• Worthenella?
• †Cheiromorpha
• Yohoia
• Haikoucaris
• Leanchoiliida
• Enaliktidae?
• Enalikter?
• Bundenbachiellus?
• Leanchoiliidae
• Actaeus
• Alalcomenaeus
• Leanchoilia
• Oestokerkus
• Yawunik
• Kanoshoia
• Lomankus

{{Main|Arthropod head problem}}

Megacheirans are either suggested to be stem-group chelicerates or stem-group arthropods, with the former hypothesis based on the chelicerae-like morphology of the great appendages{{Cite journal|last1=Chen|first1=Junyuan|last2=Waloszek|first2=Dieter|last3=Maas|first3=Andreas|date=2004|title=A new 'great-appendage' arthropod from the Lower Cambrian of China and homology of chelicerate chelicerae and raptorial antero-ventral appendages|journal=Lethaia|language=en|volume=37|issue=1|pages=3–20|doi=10.1080/00241160410004764|bibcode=2004Letha..37....3C |issn=1502-3931}}{{Cite journal |last=Chen |first=Jun-Yuan |date=2009 |title=The sudden appearance of diverse animal body plansduring the Cambrian explosion |journal=The International Journal of Developmental Biology |volume=53 |issue=5–6 |pages=733–751 |doi=10.1387/ijdb.072513cj |issn=1696-3547 |pmid=19557680 |doi-access=free}}{{Cite journal |last1=Haug |first1=Joachim T. |last2=Waloszek |first2=Dieter |last3=Maas |first3=Andreas |last4=Liu |first4=Yu |last5=Haug |first5=Carolin |date=March 2012 |title=Functional morphology, ontogeny and evolution of mantis shrimp-like predators in the Cambrian: MANTIS SHRIMP-LIKE CAMBRIAN PREDATORS |journal=Palaeontology |language=en |volume=55 |issue=2 |pages=369–399 |doi=10.1111/j.1475-4983.2011.01124.x |s2cid=82841481|doi-access=free }} alongside neuroanatomy{{Cite journal |last1=Tanaka |first1=Gengo |last2=Hou |first2=Xianguang |last3=Ma |first3=Xiaoya |last4=Edgecombe |first4=Gregory D. |last5=Strausfeld |first5=Nicholas J. |date=17 October 2013 |title=Chelicerate neural ground pattern in a Cambrian great appendage arthropod |journal=Nature |volume=502 |issue=7471 |pages=364–367 |doi=10.1038/nature12520 |pmid=24132294 |bibcode=2013Natur.502..364T |s2cid=4456458}} and the presence of a reduced labrum{{Cite journal|last1=Liu|first1=Yu|last2=Ortega-Hernández|first2=Javier|last3=Zhai|first3=Dayou|last4=Hou|first4=Xianguang|date=25 June 2020|title=A Reduced Labrum in a Cambrian Great-Appendage Euarthropod|journal=Current Biology|volume=30|issue=15|pages=3057–3061.e2|language=en|doi=10.1016/j.cub.2020.05.085|issn=0960-9822|pmid=32589912|s2cid=220057956|doi-access=free|bibcode=2020CBio...30E3057L }} resembling those of modern chelicerates, it being argued that chelicerae and the great appendages are homologous structures.{{Cite journal |last1=Tanaka |first1=Gengo |last2=Hou |first2=Xianguang |last3=Ma |first3=Xiaoya |last4=Edgecombe |first4=Gregory D. |last5=Strausfeld |first5=Nicholas J. |date=17 October 2013 |title=Chelicerate neural ground pattern in a Cambrian great appendage arthropod |url=http://www.nature.com/articles/nature12520 |journal=Nature |language=en |volume=502 |issue=7471 |pages=364–367 |doi=10.1038/nature12520 |pmid=24132294 |bibcode=2013Natur.502..364T |s2cid=4456458 |issn=0028-0836 |url-access=subscription |access-date=12 January 2023 |archive-date=16 October 2022 |archive-url=https://web.archive.org/web/20221016211810/https://www.nature.com/articles/nature12520 |url-status=live }} Other studies suggest that the megacheirans are stem-group arthropods based on the argument that the great appendages are homologous to the frontal appendages of stem-group arthropods like Isoxys and radiodonts. This identity is disputed, with other authors suggesting that the frontal appendages of radiodonts are homologous to the labrum of modern arthropods.

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{{Megacheira}}

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Category:Arthropod classes

Category:Prehistoric protostome classes