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Temporal range: Early Cambrian –Recent
A centipede (Arthropoda)
Scientific classification e
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Superphylum: Ecdysozoa
Aguinaldo et al., 1997

Ecdysozoa (/ˌɛkdɪsˈzə/) is a group of protostome animals,[1] 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.[2] 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.[3]

The group is also supported by morphological characters, and includes all animals that grow by ecdysis, moulting their exoskeleton.

The group was initially contested by a significant minority of biologists. Some argued for groupings based on more traditional taxonomic techniques,[4] while others contested the interpretation of the molecular data.[5][6]


From Ancient Greek ἔκδυσις (ékdysis 'shedding') + ζῷον (zōon 'animal').


The most notable characteristic shared by ecdysozoans is a three-layered cuticle (four in Tardigrada[7]) 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 and produce mostly amoeboid sperm, and their embryos do not undergo spiral 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.[8] A respiratory and circulatory system is only present in onychophorans and arthropods (often absent in smaller arthropods like mites), in the rest of the groups both systems are missing.


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.[9] 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.[3] The non-panarthropod members of Ecdysozoa have been grouped as Cycloneuralia but they are more usually considered paraphyletic in as representing the primitive condition from which the Panarthropoda evolved.[10]

A modern consensus phylogenetic tree for the protostomes is shown below.[11][12][13][14][15][16] It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya); dashed lines show especially uncertain placements.[17]

The phylogenic tree is based on Nielsen[18] with provisional placement of Loricifera from Hiroshi et al.[15]


Xenacoelomorpha Proporus sp.png


Priapulida Priapulus caudatus 20150625.jpg

Kinorhyncha Pycnophyes zelinkaei.jpg


Nematoda CelegansGoldsteinLabUNC.jpg

Nematomorpha Paragordius tricuspidatus.jpeg

Loricifera Pliciloricus enigmatus.jpg


Onychophora Velvet worm.jpg


Tardigrada Echiniscus L.png

Arthropoda Long nosed weevil edit.jpg

>529 mya

Spiralia Grapevinesnail 01.jpg Polychaeta (no).JPG

Deuterostomia Cyprinus carpio3.jpg Portugal 20140812-DSC01434 (21371237591).jpg

610 mya

Older alternative groupings[edit]

Articulata hypothesis[edit]

The grouping proposed by Aguinaldo et al. is almost universally accepted, replacing an older hypothesis 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,[19] though later considered them unrelated.[20] Inclusion of the roundworms within the Ecdysozoa was initially contested[5][21] but since 2003, a broad consensus has formed supporting the Ecdysozoa [22] and in 2011 the Darwin–Wallace Medal was awarded to James Lake for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia.[23]

Coelomata hypothesis[edit]

Before Aguinaldo's Ecdysozoa proposal, 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.[24] 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.[25]


  1. ^ Telford, Maximilian J.; Bourlat, Sarah J.; Economou, Andrew; Papillon, Daniel; Rota-Stabelli, Omar (2008). "The evolution of the Ecdysozoa". Philosophical Transactions of the Royal Society B: Biological Sciences. 363 (1496): 1529–1537. doi:10.1098/rstb.2007.2243. ISSN 0962-8436. PMC 2614232.
  2. ^ Aguinaldo, A. M. A.; J. M. Turbeville; L. S. Linford; M. C. Rivera; J. R. Garey; R. A. Raff; J. A. Lake (29 May 1997). "Evidence for a clade of nematodes, arthropods, and other moulting animals". Nature. 387 (6632): 489–493. Bibcode:1997Natur.387R.489A. doi:10.1038/387489a0. PMID 9168109.
  3. ^ a b Dunn, C. W.; Hejnol, A.; Matus, D.Q.; et al. (10 April 2008). "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature. 452 (7188): 745–749. Bibcode:2008Natur.452..745D. doi:10.1038/nature06614. PMID 18322464.
  4. ^ Nielsen, Claus (1995). Animal Evolution: Interrelationships of the Living Phyla. Oxford University Press. ISBN 978-0-19-850682-9.
  5. ^ a b Blair, J. E.; Kazuho Ikeo; Takashi Gojobori; S. Blair Hedges (8 April 2002). "The evolutionary position of nematodes". BMC Evolutionary Biology. 2: 7. doi:10.1186/1471-2148-2-7. PMC 102755. PMID 11985779.
  6. ^ Wägele, J. W.; T. Erikson; P. Lockhart; B. Misof (December 1999). "The Ecdysozoa: Artifact or monophylum?". Journal of Zoological Systematics and Evolutionary Research. 37 (4): 211–223. doi:10.1111/j.1439-0469.1999.tb00985.x.
  7. ^ Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 877–880. ISBN 0-03-056747-5.
  8. ^ Smith, Martin R.; Caron, Jean-Bernard (2 July 2015). "Hallucigenia's head and the pharyngeal armature of early ecdysozoans" (PDF). Nature. 523 (7558): 75–8. Bibcode:2015Natur.523...75S. doi:10.1038/nature14573. PMID 26106857.
  9. ^ Paleos Invertebrates: Panarthropoda Archived 2007-02-07 at the Wayback Machine, retrieved February 17, 2007
  10. ^ Webster, Bonnie L.; Copley, Richard R.; Jenner, Ronald A.; Mackenzie-Dodds, Jacqueline A.; Bourlat, Sarah J.; Rota-Stabelli, Omar; Littlewood, D. T. J.; Telford, Maximilian J. (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. PMID 17073934.
  11. ^ Edgecombe, Gregory D.; Giribet, Gonzalo; Dunn, Casey W.; Hejnol, Andreas; Kristensen, Reinhardt M.; Neves, Ricardo C.; Rouse, Greg W.; Worsaae, Katrine; Sørensen, Martin V. (June 2011). "Higher-level metazoan relationships: recent progress and remaining questions". Organisms, Diversity & Evolution. 11 (2): 151–172. doi:10.1007/s13127-011-0044-4.
  12. ^ Fröbius, Andreas C.; Funch, Peter (2017-04-04). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". Nature Communications. 8 (1): 9. Bibcode:2017NatCo...8....9F. doi:10.1038/s41467-017-00020-w. PMC 5431905. PMID 28377584.
  13. ^ Smith, Martin R.; Ortega-Hernández, Javier (2014). "Hallucigenia's onychophoran-like claws and the case for Tactopoda" (PDF). Nature. 514 (7522): 363–366. Bibcode:2014Natur.514..363S. doi:10.1038/nature13576. PMID 25132546.
  14. ^ "Palaeos Metazoa: Ecdysozoa". Retrieved 2017-09-02.
  15. ^ a b Yamasaki, Hiroshi; Fujimoto, Shinta; Miyazaki, Katsumi (June 2015). "Phylogenetic position of Loricifera inferred from nearly complete 18S and 28S rRNA gene sequences". Zoological Letters. 1: 18. doi:10.1186/s40851-015-0017-0. PMC 4657359. PMID 26605063.
  16. ^ Nielsen, C. (2002). Animal Evolution: Interrelationships of the Living Phyla (2nd ed.). Oxford University Press. ISBN 0-19-850682-1.
  17. ^ Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (2008-04-27). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 363 (1496): 1435–1443. doi:10.1098/rstb.2007.2233. PMC 2614224. PMID 18192191.
  18. ^ Nielsen, Claus; Brunet, Thibaut; Arendt, Detlev (2018-08-22). "Evolution of the bilaterian mouth and anus". Nature Ecology & Evolution. 2 (9): 1358–1376. doi:10.1038/s41559-018-0641-0. ISSN 2397-334X. PMID 30135501.
  19. ^ Nielsen, C (September 2003). "Proposing a solution to the Articulata–Ecdysozoa controversy". Zoologica Scripta. 32 (5): 475–482. doi:10.1046/j.1463-6409.2003.00122.x. S2CID 1416582.
  20. ^ Nielsen, Claus (2012). Animal Evolution: Interrelationships of the Living Phyla 3rd ed. Oxford University Press. ISBN 978-0-19-960603-0.
  21. ^ Wägele, J. W.; B. Misof (September 2001). "On quality of evidence in phylogeny reconstruction: a reply to Zrzavý's defence of the 'Ecdysozoa' hypothesis". Journal of Zoological Systematics and Evolutionary Research. 39 (3): 165–176. doi:10.1046/j.1439-0469.2001.00177.x.
  22. ^ Maximilian J Telford; D Timothy J Littlewood (27 April 2008). "The evolution of the animals: introduction to a Linnean tercentenary celebration". Philosophical Transactions of the Royal Society B: Biological Sciences. 363 (1496): 1421–1424. doi:10.1098/rstb.2007.2231. PMC 2394567. PMID 18192193.
  23. ^ "The Darwin-Wallace Medal". The Linnean Society of London. Retrieved 28 February 2018.
  24. ^ 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. Bibcode:2000PNAS...97.4453A. doi:10.1073/pnas.97.9.4453. PMC 34321. PMID 10781043.
  25. ^ 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". Molecular Biology and Evolution. 22 (5): 1175–1184. doi:10.1093/molbev/msi102. PMID 15703245.

External links[edit]