Ecdysozoa // is a group of protostome animals, including Arthropoda (insects, chelicerata, crustaceans, and myriapods), nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes. A large study in 2008 by Dunn et al. strongly supported the Ecdysozoa as a clade, that is, a group consisting of a common ancestor and all its descendants.
The group was initially contested by a significant minority of biologists. Some argued for groupings based on more traditional taxonomic techniques, while others contested the interpretation of the molecular data.
|A phylogenetic tree of the Ecdysozoa hypothesis as suggested by Dunn et al. (2008)|
The most notable characteristic shared by ecdysozoans is a three-layered cuticle (four in Tardigrada) composed of organic material, which is periodically molted as the animal grows. This process of molting is called ecdysis, and gives the group its name. The Ecdysozoans lack locomotory cilia, produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other protostomes. Various other features are found in the group, for instance, tardigrades, pycnogonids, and roundworms have a triradiate pharynx.
The Ecdysozoa include the following phyla: Arthropoda, Onychophora, Tardigrada, Kinorhyncha, Priapulida, Loricifera, Nematoda, and Nematomorpha. A few other groups, such as the gastrotrichs, 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 the Panarthropoda because they are distinguished by segmented body plans. Dunn et al. in 2008 suggested that the tardigrada could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods.
The grouping proposed by Aguinaldo et al. is widely accepted, although some zoologists still hold to the original view that Panarthropoda should be classified with Annelida in a group called the Articulata, and that Ecdysozoa are polyphyletic. Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group of Annelida., though later considered them unrelated. Inclusion of the roundworms within the Ecdysozoa was initially contested but since 2003, a broad consensus has formed supporting the Ecdysozoa  and in 2011 the Darwin–Wallace Medal was awarded for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia.
Before Ecdysozoa, one of the prevailing theories for the evolution of the bilateral animals was based on the morphology of their body cavities. There were three types, or grades of organization: the Acoelomata (no coelom), the Pseudocoelomata (partial coelom), and the Eucoelomata (true coelom). Adoutte and coworkers were among the first to strongly support the Ecdysozoa. 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.
- Telford MJ, Bourlat SJ, Economou A, Papillon D, Rota-Stabelli O (April 2008). "The evolution of the Ecdysozoa". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 363 (1496): 1529–37. PMC 2614232. PMID 18192181. doi:10.1098/rstb.2007.2243.
- Aguinaldo, A. M. A.; J. M. Turbeville; L. S. Linford; M. C. Rivera; J. R. Garey; R. A. Raff; J. A. Lake (1997). "Evidence for a clade of nematodes, arthropods, and other moulting animals". Nature 387 (6632): 489–493. PMID 9168109. doi:10.1038/387489a0.
- Dunn, CW; Hejnol, A; Matus, DQ; Pang, K; Browne, WE; Smith, SA; Seaver, E; Rouse, GW et al. (2008). "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature 452 (7188): 745–749. PMID 18322464. doi:10.1038/nature06614.
- Nielsen, Claus (1995). Animal Evolution: Interrelationships of the Living Phyla. Oxford University Press. ISBN 978-0-19-850682-9.
- Blair, J. E.; Kazuho Ikeo; Takashi Gojobori; S. Blair Hedges (2002). "The evolutionary position of nematodes". BMC Evolutionary Biology 2: 7. PMC 102755. PMID 11985779. doi:10.1186/1471-2148-2-7.
- Wägele, J. W.; T. Erikson; P. Lockhart; B. Misof (1999). "The Ecdysozoa: Artifact or monophylum?". Journal of Zoological Systematics and Evolutionary Research 37: 211–223. doi:10.1111/j.1439-0469.1999.tb00985.x.
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 877–880. ISBN 0-03-056747-5.
- Paleos Invertebrates: Panarthropoda - URL retrieved February 17, 2007
- Nielsen, C. (2003) Proposing a solution to the Articulata–Ecdysozoa controversy. Zoologica Scripta 32:5, 475-482
- Nielsen, Claus (2012). Animal Evolution: Interrelationships of the Living Phyla 3rd ed. Oxford University Press. ISBN 978-0199606030.
- Wägele, J. W.; B. Misof (2001). "On quality of evidence in phylogeny reconstruction: a reply to Zrzavý's defence of the 'Ecdysozoa' hypothesis". J. Zool. Syst. Evol. Research 39 (3): 165–176. doi:10.1046/j.1439-0469.2001.00177.x.
- Maximilian J Telford and D. Timothy J Littlewood (2008). "The evolution of the animals: introduction to a Linnean tercentenary celebration". Phil. Trans. R. Soc. B 363: 1421–1424. doi:10.1098/rstb.2007.2231.
- Adoutte, A.; Balavoine, G.; Lartillot, N.; Lespinet, O.; Prud'homme, B.; de Rosa, R. (25 April 2000). "Special Feature: The new animal phylogeny: Reliability and implications". Proceedings of the National Academy of Sciences 97 (9): 4453–4456. PMC 34321. PMID 10781043. doi:10.1073/pnas.97.9.4453. Retrieved 5 April 2013.
- Philip, G.K.; C.J. Creevey; J.O. McInerney (9 February 2005). "The Opisthokonta and the Ecdysozoa May Not Be Clades: Stronger Support for the Grouping of Plant and Animal than for Animal and Fungi and Stronger Support for the Coelomata than Ecdysozoa" (PDF). Molecular Biology and Evolution 22 (5): 1175–1184. doi:10.1093/molbev/msi102. Retrieved 5 April 2013.
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