Cheloniellida

File:20200615 Cheloniellon calmani anterior ventral.png calmani, showing differentiation between appendages.]]

The flattened, ovoid body of cheloniellid comprises an eye-bearing cephalon (head) and segmented trunk region, dorsally divided by a series of tergites (dorsal exoskeleton). The cephalon could be divided into procephalon and gnathocephalon. Compared to other members of Artiopoda, the head shield (dorsal exoskeleton of cephalon) of cheloniellid is relatively reduced. The trunk is wider than the cephalon and is made up of 8-13 tergites. The pleural (lateral) tips of first few tergites are directed anterolaterally, becoming increasingly posterolaterally directed rearward, giving the segmental boundaries between tergites a radiated appearance. The last trunk segment, also known as postabdomen, is tiny and laterally encompassed by the pleural regions of previous tergite.

Based on available materials, the cephalon comprises a pair of antennae and 5 pairs of uniramus appendages, with the posterior 4 pairs bore gnathobases.{{Cite journal|last1=Stürmer|first1=Wilhelm|last2=Bergström|first2=Jan|date=1978|title=The arthropod Cheloniellon from the devonian hunsrück shale|journal=Paläontologische Zeitschrift|language=en|volume=52|issue=1|pages=57–81|doi=10.1007/BF03006730|s2cid=87725308}} There are evidences that the non-gnathobasic second cephalic appendages are specialized or even raptorial in some species. Each of the trunk segments (except the last one) has a pair of biramous appendages each consisting of a leg-like endopod and a shorter exopod. The last trunk segment has a pair of spine/whip-like appendages referred as furcae or cerci, some species bore a medial spine between it which may or may not be a telson.

File:Diagram of Tardisia broedeae and various Cheloniellids.png, (D) Cheloniellon calmani, (E) Triopus draboviensis, (F) Paraduslia talimaae.]]

File:20200615 Cheloniellon calmani anterior ventral abbreviation.png had been compared to chelicerate prosomal appendages by some studies.]]

While Boudreaux (1979) regarded Cheloniellida as a class,Boudreaux H. B., 1979. Significance of intersegmental tendon system in arthropod phylogeny and monophyletic classification of Arthropoda. pp. 551-586. In.: Gupta A.P. [Ed.]. Arthropod Phylogeny. Van Nostrand Reinhold Company, N.Y., 1-762 further studies usually treat Cheloniellida as an order. Cheloniellida has a controversial phylogenetic position within arthropod higher classifications, with studies mainly around 20th century suggested it as a relative/member of either Crustacea, Trilobita, Chelicerata or Aglaspidida. Some species even had been misidentified as polyplacophoran mollusks (chiton) when being first described.Chlupác I. The enigmatic arthropod Duslia from the Ordovician of Czechoslovakia. Palaeontol. 1988; 31:611–620.[https://www.palass.org/sites/default/files/media/publications/palaeontology/volume_31/vol31_part3_pp611-620.pdf] Originally, the iconic cheloniellid Cheloniellon was believed to be a crustacean similar to trilobites.BROILI, F. (1932): Ein neuer Crustacee aus dem rheinischen Unterdevon. -- Sitzungsber. bayer. Akad. Wiss.: 27--38; Miinchen.[https://www.zobodat.at/pdf/Sitz-Ber-Akad-Muenchen-math-Kl_1932_0027-0038.pdf]BROILI, F. (1933): Ein zweites Exemplar von Cheloniellon. -- Sitzungsber. bayer. Akad. Wiss.: 11--32; Miinchen.[https://www.zobodat.at/pdf/Sitz-Ber-Akad-Muenchen-math-Kl_1933_0011-0032.pdf] Subsequent authors suggests that it occupied a position intermediate between trilobitomorphs and chelicerates,STfJRMER, W. t~ BERGSTROM, J. 1981. Weinbergina, a xiphosuran arthropod from the Devonian Hunsriick Slate. - Palaontologische Zeitschrift 55: 237-255.[http://www.museunacional.ufrj.br/mndi/Aracnologia/Xiphosura/St%FCrmer_&_Bergstr%F6m%201981_Weinbergina.pdf] {{Webarchive|url=https://web.archive.org/web/20200620211959/http://www.museunacional.ufrj.br/mndi/Aracnologia/Xiphosura/St%FCrmer_%26_Bergstr%F6m%201981_Weinbergina.pdf|date=2020-06-20}} while some also interpreted it as a sister group of crustaceansDelle Cave, L. and A.M. Simonetta (1991) Early Palaeozoic arthropods and problems of arthropod phylogeny; with some notes on taxa of doubtful affinities. In The Early Evolution of Metazoa and the Significance of Problematic Taxa, eds. A. M. Simonetta and S. Conway Morris. Cambridge University Press, Cambridge, England. Vol. 189–244. or cheliceratesWills, M. A., Briggs, D. E. G., Fortey, R. A. & Wilkinson, M. 1995. The significance of fossils in understanding arthropod evolution. Verhandlungen der deutschen zoologischen Gesselschaft 88, 203–15.{{Citation|last1=Dunlop|first1=J. A.|title=The early history and phylogeny of the chelicerates|date=1998|work=Arthropod Relationships|pages=221–235|editor-last=Fortey|editor-first=R. A.|series=The Systematics Association Special Volume Series|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-011-4904-4_17|isbn=978-94-011-4904-4|last2=Selden|first2=P. A.|editor2-last=Thomas|editor2-first=R. H.}} as well. The suggested close relationship between cheloniellids and chelicerates was inferred by the gnathobasic appendages similar to those of merostomes (e.g. Xiphosurans and Eurypterids), and the hypothesis that the chelicerates arose from trilobitomorphs through the loss of deutocerebral antennae (i.e. first antennae) and specialization of tritocerebral appendages into chelicerae (comparable to the cheloniellid antennae and specialized 2nd appendages), a scenario which is not supported by gene expression,{{Cite journal|last1=Telford|first1=Maximilian J.|last2=Thomas|first2=Richard H.|date=1998-09-01|title=Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment|journal=Proceedings of the National Academy of Sciences|language=en|volume=95|issue=18|pages=10671–10675|doi=10.1073/pnas.95.18.10671|issn=0027-8424|pmid=9724762|pmc=27953|bibcode=1998PNAS...9510671T |doi-access=free}}{{Cite journal|last=Damen|first=Wim G. M.|date=2002-03-01|title=Parasegmental organization of the spider embryo implies that the parasegment is an evolutionary conserved entity in arthropod embryogenesis|url=https://dev.biologists.org/content/129/5/1239|journal=Development|language=en|volume=129|issue=5|pages=1239–1250|doi=10.1242/dev.129.5.1239|issn=0950-1991|pmid=11874919}}{{Cite journal|last1=Jager|first1=Muriel|last2=Murienne|first2=Jérôme|last3=Clabaut|first3=Céline|last4=Deutsch|first4=Jean|last5=Guyader|first5=Hervé Le|last6=Manuel|first6=Michaël|date=2006|title=Homology of arthropod anterior appendages revealed by Hox gene expression in a sea spider|url=https://www.nature.com/articles/nature04591|journal=Nature|language=en|volume=441|issue=7092|pages=506–508|doi=10.1038/nature04591|pmid=16724066|bibcode=2006Natur.441..506J |s2cid=4307398|issn=1476-4687}}{{Cite journal|last1=Manuel|first1=Michaël|last2=Jager|first2=Muriel|last3=Murienne|first3=Jérôme|last4=Clabaut|first4=Céline|last5=Guyader|first5=Hervé Le|date=2006-07-01|title=Hox genes in sea spiders (Pycnogonida) and the homology of arthropod head segments|journal=Development Genes and Evolution|language=en|volume=216|issue=7|pages=481–491|doi=10.1007/s00427-006-0095-2|pmid=16820954|s2cid=833103|issn=1432-041X}}{{Cite journal|last1=Brenneis|first1=Georg|last2=Ungerer|first2=Petra|last3=Scholtz|first3=Gerhard|date=2008|title=The chelifores of sea spiders (Arthropoda, Pycnogonida) are the appendages of the deutocerebral segment|journal=Evolution & Development|volume=10|issue=6|pages=717–724|doi=10.1111/j.1525-142X.2008.00285.x|issn=1525-142X|pmid=19021742|s2cid=6048195}} neuroanatomical{{Cite journal|last1=Mittmann|first1=Beate|last2=Scholtz|first2=Gerhard|date=2003-02-01|title=Development of the nervous system in the "head" of Limulus polyphemus (Chelicerata: Xiphosura): morphological evidence for a correspondence between the segments of the chelicerae and of the (first) antennae of Mandibulata|url=https://www.researchgate.net/publication/10897784|journal=Development Genes and Evolution|language=en|volume=213|issue=1|pages=9–17|doi=10.1007/s00427-002-0285-5|pmid=12590348|s2cid=13101102|issn=1432-041X}}{{Cite journal|last1=Harzsch|first1=Steffen|last2=Wildt|first2=Miriam|last3=Battelle|first3=Barbara|last4=Waloszek|first4=Dieter|date=2005-07-01|title=Immunohistochemical localization of neurotransmitters in the nervous system of larval Limulus polyphemus (Chelicerata, Xiphosura): evidence for a conserved protocerebral architecture in Euarthropoda|url=http://www.sciencedirect.com/science/article/pii/S1467803905000186|journal=Arthropod Structure & Development|series=Arthropod Brain Morphology and Evolution|language=en|volume=34|issue=3|pages=327–342|doi=10.1016/j.asd.2005.01.006|issn=1467-8039}} and developmental{{Cite journal|last1=Mittmann|first1=Beate|last2=Scholtz|first2=Gerhard|date=2003-02-01|title=Development of the nervous system in the "head" of Limulus polyphemus (Chelicerata: Xiphosura): morphological evidence for a correspondence between the segments of the chelicerae and of the (first) antennae of Mandibulata|journal=Development Genes and Evolution|language=en|volume=213|issue=1|pages=9–17|doi=10.1007/s00427-002-0285-5|pmid=12590348|s2cid=13101102|issn=1432-041X}} evidences (suggests that chelicerae are in fact deutocerebral).

{{cladogram

|title=

|align= right

|caption= Summarized phylogeny of Artiopoda with focus on Cheloniellida, based on Lerosey-Aubril et al. (2017).

|cladogram=

{{clade| style=width:32em;font-size:100%;line-height:100%

|label1=Artiopoda|1={{clade

|1=Squamacula

|2={{clade

|1=Retifacies

|2=Kwanyinaspis

|label3=|3=Conciliterga

|4={{clade

|1=Petalopleura

|2={{clade

|1=Nektaspidida

|2={{clade

|1=Phytophylaspis

|2=Trilobita

}} }} }}

|label5=Vicissicaudata|5={{clade

|1=Sidneyia

|2={{clade

|label1=Cheloniellida|1={{clade

|1=Neostrabops

|2={{clade

|1=Cheloniellon

|2={{clade

|1=Triopus

|2=Duslia

}} }} }}

|2={{clade

|1=Emeraldella

|2={{clade

|1=Kodymirus

|2={{clade

|1=Eozetetes

|2=Aglaspidida

}} }} }} }} }} }} }} }} }}

As of 21st century, Cheloniellida was mostly found to form a clade with Aglaspidida and Xenopoda (e.g. Sidneyia and Emeraldella).{{Cite journal|last1=Edgecombe|first1=Gregory D.|last2=García-Bellido|first2=Diego C.|last3=Paterson|first3=John R.|date=2011|title=A New Leanchoiliid Megacheiran Arthropod from the Lower Cambrian Emu Bay Shale, South Australia|url=http://www.app.pan.pl/article/item/app20100080.html|journal=Acta Palaeontologica Polonica|language=en|volume=56|issue=2|pages=385–400|doi=10.4202/app.2010.0080|issn=0567-7920|doi-access=free|hdl=10261/61352|hdl-access=free}}{{Cite journal|last=Legg|first=David A.|date=2014|title=Sanctacaris uncata: the oldest chelicerate (Arthropoda)|url=https://www.researchgate.net/publication/266683582|journal=Naturwissenschaften|language=en|volume=101|issue=12|pages=1065–1073|doi=10.1007/s00114-014-1245-4|pmid=25296691|bibcode=2014NW....101.1065L |s2cid=15290784|issn=0028-1042}}{{Cite journal|last1=Edgecombe|first1=Gregory D.|last2=Paterson|first2=John R.|last3=García-Bellido|first3=Diego C.|date=2017|title=A new aglaspidid-like euarthropod from the lower Cambrian Emu Bay Shale of South Australia|url=https://www.cambridge.org/core/journals/geological-magazine/article/new-aglaspididlike-euarthropod-from-the-lower-cambrian-emu-bay-shale-of-south-australia/6E5897CE0D2D6C7C95274518304D1453|journal=Geological Magazine|language=en|volume=154|issue=1|pages=87–95|doi=10.1017/S0016756815001053|bibcode=2017GeoM..154...87E |s2cid=133058010|issn=0016-7568}} The clade was formally named Vicissicaudata in 2013, united by a differentiated terminal trunk area (postabdomen) that bears a pair of non-leg-like appendages. Numerous phylogenetic analyses also retrieved Vicissicaudata within Artiopoda, a diverse arthropod taxon comprising trilobites and similar fossil taxa that may{{Cite journal|last1=Legg|first1=David A.|last2=Sutton|first2=Mark D.|last3=Edgecombe|first3=Gregory D.|date=2013|title=Arthropod fossil data increase congruence of morphological and molecular phylogenies|url=https://www.researchgate.net/publication/257205419|journal=Nature Communications|volume=4|pages=2485|doi=10.1038/ncomms3485|issn=2041-1723|pmid=24077329|bibcode=2013NatCo...4.2485L |doi-access=free}} or may not be closely related to chelicerates.

The unambiguous members of cheloniellids shown as follows:

• Cheloniellon Broili, 1932
• Cheloniellon calmani Broili, 1932—Lower Devonian, Germany
• Duslia Jahn, 1893
• Duslia insignis Jahn, 1893—Upper Ordovician, Czech Republic, Morocco
• Eoduslia Van Roy, 2006 (unpublished?)
• Eoduslia brahimtahiri Van Roy, 2006—Upper Ordovician, Morocco (unpublished?)
• Neostrabops Caster & Macke, 1952
• Neostrabops martini Caster & Macke, 1952—Upper Ordovician, United States
• Paraduslia Dunlop, 2002
• Paraduslia talimaae Dunlop, 2002—Lower Devonian, Russia
• Pseudarthron Selden & White, 1984
• Pseudarthron whittingtoni Selden & White, 1984—Upper Silurian
• Triopus Barrande, 1872
• Triopus drabowiensis Barrande, 1872—Upper Ordovician, Czech Republic, Morocco

In 2018, a new species (informally named "Latromirus" in an unpublished thesis from 2016{{Cite thesis |title=Paleobiology and Taphonomy of Exceptionally Preserved Organisms from the Brandon Bridge Formation (Silurian), Wisconsin, USA |url=https://etd.ohiolink.edu/apexprod/rws_olink/r/1501/10?clear=10&p10_accession_num=osu1468844814#abstract-files |publisher=The Ohio State University |date=2016 |language=en |first=Andrew J. |last=Wendruff}}) was described in a preprint by Wendruff et al. According to Braddy & Dunlop 2021, the enigmatic Parioscorpio may have cheloniellid affinities,{{cite journal |last1=Braddy |first1=S.J. |last2=Dunlop |first2=J.A. |year=2021 |title=A sting in the tale of Parioscorpio venator from the Silurian of Wisconsin: is it a cheloniellid arthropod? |journal=Lethaia |volume=54 |issue=1 |pages=1–7 |doi=10.1111/let.12457|s2cid=245285654 }} however that interpretation is denied by Van Roy et al. 2022, only remaining specimen UWGM 2439 (holotype specimen of "Latromirus") as possible cheloniellid.{{Cite journal |last1=Roy |first1=Peter Van |last2=Rak |first2=Štěpán |last3=Budil |first3=Petr |last4=Fatka |first4=Oldřich |date=2022-06-13 |title=Redescription of the cheloniellid euarthropod Triopus draboviensis from the Upper Ordovician of Bohemia, with comments on the affinities of Parioscorpio venator |url=https://www.cambridge.org/core/journals/geological-magazine/article/abs/redescription-of-the-cheloniellid-euarthropod-triopus-draboviensis-from-the-upper-ordovician-of-bohemia-with-comments-on-the-affinities-of-parioscorpio-venator/60A567EA1D28C2A7311BB68CAD427F22 |journal=Geological Magazine |volume=159 |issue=9 |language=en |pages=1471–1489 |doi=10.1017/S0016756822000292 |hdl=1854/LU-8756253 |s2cid=249652930 |issn=0016-7568}}

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{{Reflist}}

{{refbegin|32em}}

• {{cite book |last=Boudreaux |first=H. Bruce |title=Arthropod phylogeny with special reference to insects |year=1979 |publisher=John Wiley & Sons |isbn=978-0471042907 |url-access=registration |url=https://archive.org/details/arthropodphyloge0000boud }}
• {{cite book |last1=Budd |first1=Graham E. |author-link1=Graham Budd |editor1-first=Richard A. |editor1-last=Fortey |editor1-link=Richard Fortey |others=Richard H. Thomas |title=Arthropod Relationships. Systematics Association Special Volume Series 55. |year=1997 |publisher=Springer |isbn=978-0412754203 |chapter=Stem Group Arthropods from the Lower Cambrian Sirius Passet Fauna of North Greenland}}
• {{cite book |last1=Dunlop |first1=Jason A. |last2=Selden |first2=Paul A. |editor1-first=Richard A. |editor1-last=Fortey |editor1-link=Richard Fortey |others=Richard H. Thomas |title=Arthropod Relationships. Systematics Association Special Volume Series 55. |year=1997 |publisher=Springer |isbn=978-0412754203 |chapter=The early history and phylogeny of the chelicerates |pages=237–245}}
• {{cite book |author1=Hou Xianguang |last2=Bergström |first2=Jan |year=2006 |title=Arthropods of the Lower Cambrian Chengliang fauna, southwest China |series=Fossils & Strata Monograph Series|issue=45 |publisher=Wiley-Blackwell |isbn=978-8200376934}}

{{refend}}

{{Artiopoda}}

{{Taxonbar|from=Q5090001}}

Category:Paleozoic arthropods

Category:Prehistoric arthropod orders

Category:Artiopoda