The group was initially contested by a significant minority ofbiologists. Some argued for groupings based on more traditional taxonomic techniques,[16] while others contested the interpretation of themolecular data.[17][18]
The most notable characteristic shared by ecdysozoans is a three-layeredcuticle (four inTardigrada[19]) composed of organic material, which is periodically molted as the animal grows. This process of molting is calledecdysis, and gives the group its name. The ecdysozoans lack locomotorycilia and produce mostlyamoeboid sperm, and theirembryos do not undergospiral cleavage as in most other protostomes. Ancestrally, the group exhibited sclerotized teeth within the foregut, and a ring of spines around the mouth opening, though these features have been secondarily lost in certain groups.[20][21] An unpairedventral nerve cord, present in Priapulida and Nematoida, appear to be the ancestral condition, making the paired ventral nerve cord found in Panarthropoda, Kinorhyncha and Loricifera a derived trait.[22] A respiratory and circulatory system is only present inonychophorans and arthropods (often absent in smaller arthropods like mites); in the rest of the groups, both systems are missing. Ecdysozoans rely exclusively on the arginine phosphate/arginine kinase (AP/AK) system, a high-energy phosphate system used to regenerate ATP from ADP. In contrast, vertebrates use only the creatine phosphate/creatine kinase (CP/CK) system, while some other invertebrates may employ both systems.[23][24]
The Ecdysozoa include the following phyla:Arthropoda,Onychophora,Tardigrada,Kinorhyncha,Priapulida,Loricifera,Nematoda, andNematomorpha. A few extinct taxa have been classified asstem group ecdysozoans, such asUncus dzaugisi andAcosmia. Other groups such as thegastrotrichs, have been considered possible members but lack the main characters of the group, and are now placed elsewhere. The Arthropoda, Onychophora, and Tardigrada have been grouped together as thePanarthropoda because they are distinguished by segmented body plans.[25] Dunnet al. in 2008 suggested that the tardigrada could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods.[14] The non-panarthropod members of Ecdysozoa have been grouped asCycloneuralia but they are more usually consideredparaphyletic in representing the primitive condition from which the Panarthropoda evolved.[26]
A modern consensusphylogenetic tree for the protostomes is shown below.[27][28][29][30][31][32] It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya); dashed lines show especially uncertain placements.[33]
The phylogenetic tree is based on Nielsenet al.[34] and Howardet al..[35]
The grouping proposed by Aguinaldoet al. is almost universally accepted, replacing an older hypothesis thatPanarthropoda should be classified withAnnelida in a group called theArticulata, and that Ecdysozoa arepolyphyletic. Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group ofAnnelida,[36] though later considered them unrelated.[37]Inclusion of theroundworms within the Ecdysozoa was initially contested[17][38][39] but since 2003, a broad consensus has formed supporting the Ecdysozoa[40] and in 2011 theDarwin–Wallace Medal was awarded toJames Lake for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, theLophotrochozoa, and the Deuterostomia.[41]
Before Aguinaldo's Ecdysozoa proposal, one of the prevailing theories for the evolution of thebilateral animals was based on the morphology of theirbody cavities. There were three types, orgrades of organization: theAcoelomata (nocoelom), thePseudocoelomata (partial coelom), and theEucoelomata (truecoelom). Adoutte and coworkers were among the first to strongly support the Ecdysozoa.[42] With the introduction of molecular phylogenetics, the coelomate hypothesis was abandoned, although some molecular, phylogenetic support for the Coelomata continued until as late as 2005.[43]
^abLiu, Yunhuan; Qin, Jiachen; Wang, Qi; Maas, Andreas; Duan, Baichuan; Zhang, Yanan; Zhang, Hu; Shao, Tiequan; Zhang, Huaqiao (May 2019). "New armoured scalidophorans (Ecdysozoa, Cycloneuralia) from the Cambrian Fortunian Zhangjiagou Lagerstätte, South China".Papers in Palaeontology.5 (2):241–260.Bibcode:2019PPal....5..241L.doi:10.1002/spp2.1239.
^Zhang, Hua-Qiao (September 2022). "The evolutionary relationships of the earliest known cycloneuralians and a new record from the Cambrian Fortunian of South China".Palaeoworld.31 (3):389–401.doi:10.1016/j.palwor.2021.09.003.
^Shao, T.Q.; Wang, Q.; Liu, Y.H.; Qin, J.C.; Zhang, Y.N.; Liu, M.J.; Shao, Y.; Zhao, J.Y.; Zhang, H.Q. (October 2020). "A new scalidophoran animal from the Cambrian Fortunian Stage of South China and its implications for the origin and early evolution of Kinorhyncha".Precambrian Research.349 105616.Bibcode:2020PreR..34905616S.doi:10.1016/j.precamres.2020.105616.
^Wang, Deng; Han, Jian; Guo, Junfeng; Qiang, Yaqin (March 2025). "Origin and evolution of bodyplans of ecdysozoans during the Cambrian explosion".Chinese Journal of Nature.47 (2):125–133.doi:10.3969/j.issn.0253-9608.2025.02.004.
^Eernisse, D.J.; Albert, J.S.; Anderson, F.E. (1992). "Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralian metazoan morphology".Systematic Biology.41 (3):305–330.doi:10.1093/sysbio/41.3.305.
^Ax, P. (1985). "The position of the Gnathostomulida and Platyhelminthes in the phylogenetic system of the Bilateria". In Conway Morris, S.; George, J.D.; Gibson, R.; Platt, H.M. (eds.).The Origins and Relationships of Lower Invertebrates. Systematics Association Special Volume. Vol. 28. New York, NY: Clarendon / Oxford University Press. pp. 168–180.ISBN0-19-857181-X.OCLC59186778 – via Internet Archive (archive.org).proceedings of an international symposium held in London, September 1983ISBN9780198571810
^Nielsen, Claus (1995).Animal Evolution: Interrelationships of the living phyla. Oxford University Press.ISBN978-0-19-850682-9.
^"Panarthropoda".Paleos (palaeos.com). Invertebrates. Archived fromthe original on 2007-02-07. Retrieved17 February 2007.
^Webster, Bonnie L.; Copley, Richard R.; Jenner, Ronald A.; Mackenzie-Dodds, Jacqueline A.; Bourlat, Sarah J.; Rota-Stabelli, Omar; et al. (November 2006). "Mitogenomics and phylogenomics reveal priapulid worms as extant models of the ancestral Ecdysozoan".Evolution & Development.8 (6):502–510.doi:10.1111/j.1525-142X.2006.00123.x.PMID17073934.S2CID22823313.
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