Selkirkia columbia, from Smith et al. (2015).

Belongs within: Bilateria.
Contains: Priapulida, Nematoda, Nematomorpha, Kinorhyncha, Onychophora, Tardigrada, Cephalata.

The Ecdysozoa is a clade of animals primarily identified by molecular analyses though members of the clade are also characterised by the possession of a thick cuticle that is periodically moulted, together with a ventral nerve cord (Cavalier-Smith 1998). Ecdysozoans include arthropods and related taxa, together with a number of worm-like groups including nematodes and priapulids. Other ecdysozoans include the Loricifera, minute animals found in marine sediment in which the body is contained within a protective shell or lorica. Ecdysozoans with functional legs form a clade Panarthropoda; in members of the Onychophora and Tardigrada, as well as various early fossil members of the clade, the legs are fleshy and un-segmented.

The fossil record includes a number of worm-like taxa from the early Palaeozoic such as Selkirkia and Palaeoscolex that are believed to represent relatives of the priapulids, many of which resembled priapulids in the possession of a more or less invertable, spinose proboscis. Members of the Middle Cambrian genus Selkirkia lived enclosed in an elongate, probably chitinous tube.

<==Ecdysozoa (see below for synonymy)
    |--+--Ancalagon DD04
    |  `--Fieldia DD04
    `--+--Markuelia Val’kov 1983 DD04
       |    |--*M. secunda DD04
       |    `--M. hunanensis Dong, Donoghue et al. 2004 DD04
       `--+--Priapozoa C-S98
          |    |  i. s.: Milaculum SHC01
          |    |--Loricifera DD04
          |    |    |--Sirilorica EL11
          |    |    `--Nanaloricus Kristensen 1983 [Nanaloricidae] K86
          |    |         `--*N. mysticus Kristensen 1983 K86
          |    `--+--Priapulida HO09
          |       `--+--Selkirkia Walcott 1911 DD04, H62
          |          |    |--*S. major (Walcott 1908) [=Orthotheca major] H62
          |          |    |--S. columbia CM98
          |          |    `--S. willoughbyi Conway Morris & Robison 1986 BL05
          |          `--+--+--Maotianshania DD04
          |             |  `--+--Cricocosmia DD04
          |             |     `--Palaeoscolex Whittard 1953 DD04, H62 [Palaeoscolecidae]
          |             |          |--*P. piscatorum Whittard 1953 H62
          |             |          `--P. ratcliffei Robison 1969 H75
          |             `--+--Ottoia Walcott 1911 DD04, H62 [Ottoidae, Ottoiidae]
          |                |    `--*O. prolifica Walcott 1911 H62
          |                `--+--Scolecofurca DD04
          |                   `--Louisella Walcott 1911 DD04, H62
          |                        `--*L. pedunculata Walcott 1911 H62
          `--+--+--Nematoda HO09
             |  `--+--Nematomorpha HO09
             |     `--Kinorhyncha HO09
             `--Panarthropoda SO-H14
                  |--Aysheaia Walcott 1911 SO-H14, CM98
                  |    `--A. pedunculata G02
                  `--+--Onychophora SO-H14
                     `--Tactopoda SO-H14
                          |--+--Onychodictyon ferox SO-H14
                          |  `--Tardigrada SO-H14
                          `--+--+--Siberion SO-H14
                             |  `--+--Megadictyon SO-H14
                             |     `--Jianshanopodia SO-H14
                             `--+--Hadranax SO-H14
                                `--+--Kerygmachela Budd 1993 KBR09, W96
                                   |    `--K. kierkegaardi KBR09
                                   `--+--Cephalata KBR09
                                      `--Pambdelurion whittingtoni SO-H14, KBR09

Ecdysozoa [Cephalorhyncha, Cycloneuralia, Dicephalosomata, Haemopoda, Introverta, Lobopoda, Mandibulopoda, Nematoida, Nematoidea, Nematozoa, Opabiniida, Opabiniidacea, Podophora, Probosciferidea, Scalidophora, Scalidorhyncha, Tritocerebra, Uniantennata, Uniramia]

*Type species of generic name indicated


[BL05] Briggs, D. E. G., B. S. Lieberman, S. L. Halgedahl & R. D. Jarrard. 2005. A new metazoan from the Middle Cambrian of Utah and the nature of the Vetulicolia. Palaeontology 48 (4): 681–686.

[C-S98] Cavalier-Smith, T. 1998. A revised six-kingdom system of life. Biological Reviews 73: 203–266.

[CM98] Conway Morris, S. 1998. The Crucible of Creation. Oxford University Press: Oxford.

[DD04] Dong, X-P., P. C. J. Donoghue, H. Cheng & J.-B. Liu. 2004. Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. Nature 427: 237–240.

[EL11] Erwin, D. H., M. Laflamme, S. M. Tweedt, E. A. Sperling, D. Pisani & K. J. Peterson. 2011. The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334: 1091–1097.

[G02] Giribet, G. 2002. Current advances in the phylogenetic reconstruction of metazoan evolution. A new paradigm for the Cambrian explosion? Molecular Phylogenetics and Evolution 24: 345–357.

[H75] Häntzschel, W. 1975. Treatise on Invertebrate Paleontology pt W. Miscellanea Suppl. 1. Trace Fossils and Problematica 2nd ed. The Geological Society of America: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[HO09] Hejnol, A., M. Obst, A. Stamatakis, M. Ott, G. W. Rouse, G. D. Edgecombe, P. Martinez, J. Baguña, X. Bailly, U. Jondelius, M. Wiens, W. E. G. Müller, E. Seaver, W. C. Wheeler, M. Q. Martindale, G. Giribet & C. W. Dunn. 2009. Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society of London Series B—Biological Sciences 276 (1677): 4261–4270.

[H62] Howell, B. F. 1962. Worms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W144–W177. Geological Society of America, and University of Kansas Press.

[K86] Kristensen, R. M. 1986. Loricifera. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 119–121. E. J. Brill/Dr W. Backhuys: Leiden.

[KBR09] Kühl, G., D. E. G. Briggs & J. Rust. 2009. A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsrück Slate, Germany. Science 323: 771–773.

[SO-H14] Smith, M. R., & J. Ortega-Hernández. 2014. Hallucigenia's onychophoran-like claws and the case for Tactopoda. Nature 514: 363–366.

[SHC01] Sutton, M. D., L E. Holmer & L Cherns. 2001. Small problematic phosphatic sclerites from the Ordovician of Iapetus. Journal of Paleontology 75 (1): 1–8.

[W96] Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45 (2): 190–222.

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