Cheloniellida is a taxon (usually referred to as an order[1][2]) of extinctPaleozoicarthropods. As of 2018,[2] 7monotypic genera ofcheloniellids had been formally described, whose fossils are found in marine strata ranging fromOrdovician toDevonian in age. Cheloniellida has a controversialphylogenetic position, with previous studies associated it as either a member or relative of various fossil and extant arthropod taxa.[2] It was later accepted as a member ofVicissicaudata withinArtiopoda.[3][4][5]
Ventral structures of the anterior region ofCheloniellon calmani, showing differentiation between appendages.
The flattened, ovoid body of cheloniellid comprises aneye-bearingcephalon (head) and segmented trunk region, dorsally divided by a series oftergites (dorsalexoskeleton). The cephalon could be divided into procephalon and gnathocephalon.[2] Compared to other members ofArtiopoda, the head shield (dorsal exoskeleton of cephalon) of cheloniellid is relatively reduced.[2][3] The trunk is wider than the cephalon and is made up of 8-13 tergites.[2] 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.[3][2] The last trunk segment, also known as postabdomen,[4] is tiny and laterally encompassed by the pleural regions of previous tergite.[2]
Based on available materials, the cephalon comprises a pair ofantennae and 5 pairs of uniramus appendages, with the posterior 4 pairs boregnathobases.[6] There are evidences that the non-gnathobasic second cephalic appendages are specialized or evenraptorial in some species.[6][2] 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.[2] The last trunk segment has a pair of spine/whip-like appendages referred asfurcae[3] orcerci,[6] some species bore a medial spine between it which may or may not be atelson.[2]
The specialized second appendages (Pap, green) and gnathobasic appendages (Gap, cyan) ofCheloniellon had been compared tochelicerate prosomal appendages by some studies.[6][7]
While Boudreaux (1979) regarded Cheloniellida as a class,[8] further studies usually treat Cheloniellida as an order.[1][2] Cheloniellida has a controversialphylogenetic position within arthropod higher classifications, with studies mainly around 20th century suggested it as a relative/member of eitherCrustacea,Trilobita,Chelicerata orAglaspidida.[2] Some species even had been misidentified aspolyplacophoranmollusks (chiton) when being first described.[9] Originally, the iconic cheloniellidCheloniellon was believed to be a crustacean similar to trilobites.[10][11] Subsequent authors suggests that it occupied a position intermediate between trilobitomorphs and chelicerates,[6][7] while some also interpreted it as asister group of crustaceans[12] or chelicerates[13][14] as well. The suggested close relationship between cheloniellids and chelicerates was inferred by the gnathobasic appendages similar to those of merostomes (e.g.Xiphosurans andEurypterids), and the hypothesis that the chelicerates arose from trilobitomorphs through the loss of deutocerebralantennae (i.e. first antennae) and specialization of tritocerebral appendages intochelicerae (comparable to the cheloniellid antennae and specialized 2nd appendages),[6][7] a scenario which is not supported bygene expression,[15][16][17][18][19]neuroanatomical[20][21] anddevelopmental[22] evidences (suggests that chelicerae are in fact deutocerebral).
Summarized phylogeny ofArtiopoda with focus on Cheloniellida, based on Lerosey-Aubril et al. (2017).[4]
As of 21st century, Cheloniellida was mostly found to form a clade withAglaspidida andXenopoda (e.g.Sidneyia andEmeraldella).[23][3][24][25][4] The clade was formally namedVicissicaudata in 2013,[3] united by a differentiated terminal trunk area (postabdomen) that bears a pair of non-leg-like appendages.[3][4] Numerous phylogenetic analyses also retrieved Vicissicaudata withinArtiopoda,[4] a diverse arthropod taxon comprisingtrilobites and similar fossil taxa that may[26][24] or may not[3][25][4] be closely related tochelicerates.
In 2018, a new species (informally named "Latromirus" in an unpublished thesis from 2016[27]) was described in a preprint by Wendruff et al.[2] According to Braddy & Dunlop 2021, the enigmaticParioscorpio may have cheloniellid affinities,[28] however that interpretation is denied by Van Roy et al. 2022, only remaining specimen UWGM 2439 (holotype specimen of "Latromirus") as possible cheloniellid.[29]
^abcdefghijklmnWendruff, Andrew James, et al. "New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA." BioRxiv (2018): 407379.[1]
^abcdefStürmer, Wilhelm; Bergström, Jan (1978). "The arthropodCheloniellon from the devonian hunsrück shale".Paläontologische Zeitschrift.52 (1):57–81.doi:10.1007/BF03006730.S2CID87725308.
^abcSTfJRMER, W. t~ BERGSTROM, J. 1981. Weinbergina, a xiphosuran arthropod from the Devonian Hunsriick Slate. - Palaontologische Zeitschrift 55: 237-255.[2]Archived 2020-06-20 at theWayback Machine
^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
^Chlupác I. The enigmatic arthropod Duslia from the Ordovician of Czechoslovakia. Palaeontol. 1988; 31:611–620.[3]
^BROILI, F. (1932): Ein neuer Crustacee aus dem rheinischen Unterdevon. -- Sitzungsber. bayer. Akad. Wiss.: 27--38; Miinchen.[4]
^BROILI, F. (1933): Ein zweites Exemplar von Cheloniellon. -- Sitzungsber. bayer. Akad. Wiss.: 11--32; Miinchen.[5]
^Delle 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.
^Wills, 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.
^Dunlop, J. A.; Selden, P. A. (1998), Fortey, R. A.; Thomas, R. H. (eds.), "The early history and phylogeny of the chelicerates",Arthropod Relationships, The Systematics Association Special Volume Series, Dordrecht: Springer Netherlands, pp. 221–235,doi:10.1007/978-94-011-4904-4_17,ISBN978-94-011-4904-4
^Manuel, Michaël; Jager, Muriel; Murienne, Jérôme; Clabaut, Céline; Guyader, Hervé Le (2006-07-01). "Hox genes in sea spiders (Pycnogonida) and the homology of arthropod head segments".Development Genes and Evolution.216 (7):481–491.doi:10.1007/s00427-006-0095-2.ISSN1432-041X.PMID16820954.S2CID833103.
^Mittmann, Beate; Scholtz, Gerhard (2003-02-01). "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".Development Genes and Evolution.213 (1):9–17.doi:10.1007/s00427-002-0285-5.ISSN1432-041X.PMID12590348.S2CID13101102.
^Braddy, S.J.; Dunlop, J.A. (2021). "A sting in the tale of Parioscorpio venator from the Silurian of Wisconsin: is it a cheloniellid arthropod?".Lethaia.54 (1):1–7.doi:10.1111/let.12457.S2CID245285654.
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Dunlop, Jason A.; Selden, Paul A. (1997). "The early history and phylogeny of the chelicerates". InFortey, Richard A. (ed.).Arthropod Relationships. Systematics Association Special Volume Series 55. Richard H. Thomas. Springer. pp. 237–245.ISBN978-0412754203.
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