Ecdysozoa (ɛkdɪsɵˈzoʊ.ə) is a group of protostome animals (Telford et al. 2008), 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 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 is also supported by morphological characters, and can be considered as including all animals that shed their exoskeleton (see ecdysis). Groups corresponding roughly to the Ecdysozoa had been proposed previously by Perrier in 1897 and Seurat in 1920 based on morphology alone. The group has been contested by a significant minority of biologists. Some have argued for groupings based on more traditional taxonomic techniques (Nielsen 1995), while others have contested the interpretation of the molecular data (Blair et al. 2002, Wägele et al. 1999).
- 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. doi:10.1038/387489a0. PMID 9168109.
- Blair, J. E.; Kazuho Ikeo, Takashi Gojobori and S. Blair Hedges (2002). "The evolutionary position of nematodes". BMC Evolutionary Biology 2: 7. doi:10.1186/1471-2148-2-7. PMC 102755. PMID 11985779.
- Dunn et al., 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. doi:10.1038/nature06614. PMID 18322464.
- Nielsen, Claus (1995). Animal Evolution: Interrelationships of the Living Phyla. Oxford University Press. ISBN 978-0-19-850682-9.
- 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. doi:10.1098/rstb.2007.2243. PMC 2614232. PMID 18192181.
- 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.
The most notable characteristic shared by ecdysozoans is a three-layered cuticle 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.
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