Previously,spiral cleavage was thought to be unique to the Spiralia in the strictest sense—animals such as molluscs and annelids which exhibit classical spiral cleavage. The presence of spiral cleavage in animals such as platyhelminths could be difficult to correlate with some phylogenies.[6]
Evidence of a close relationship betweenmolluscs,annelids and lophophorates was found in 1995 and Lophotrochozoa was defined as the group containing these taxa and all the descendants of their last common ancestor.[7] More recent research has established theLophotrochozoa as a superphylum within the Metazoa.[8]With this understanding, the presence of spiral cleavage in polyclad platyhelminths, as well as the more traditional Spiralia, has led to the hypothesis that spiral cleavage was present ancestrally across the Lophotrochozoa as a whole.[5]With the introduction ofPlatytrochozoa andRouphozoa, the cladogram is as follows, with an indication approximately how many million years ago (Mya) the clades radiated into newer clades.[9][10][11][12][13][14][15]
An alternative phylogeny was given in 2019, with a basal grouping Mollusca with Entoprocta grouping namedTetraneuralia, and a second grouping of Nemertea with Platyhelminthes namedParenchymia as sister of Annelida. In their proposal and according to the original definition, Lophotrochozoa may become a senior synonym for Platytrochozoa.[1][16][17][18]
A 2022 study supported theTrochozoa andPlatyzoa hypotheses, as shown below.[20] The same year another study placed bryozoans, entoproctans and cycliophorans in the group Polyzoa as one of the earliest branches among Lophotrochozoa.[21]
^Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y. (2007), "New data onKimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications", in Vickers-Rich, Patricia; Komarower, Patricia (eds.),The Rise and Fall of the Ediacaran Biota, Special publications, vol. 286, London: Geological Society, pp. 157–179,doi:10.1144/SP286.12,ISBN978-1-86239-233-5,OCLC156823511
^Dunn, C.W.; Hejnol, A.; Matus, D. Q.; Pang, K.; Browne, W. E.; Smith, S.A.; Seaver, E.; Rouse, G.W.; Obst, M.; Sørensen, M. V.;Haddock, S. H. D.; Schmidt-Rhaesa, A.; Okusu, A.; Kristensen, R.M.; Wheeler, W. C.; Martindale, M. Q.; Giribet, G. (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.PMID18322464.S2CID4397099.
^Giribet, G; Dunn, CW; Edgecombe, GD; Hejnol, A; Martindale, MQ; Rouse, GW (2009)."Assembling the spiralian tree of life". In Telford, MJ; Littlewood, DT (eds.).Animal Evolution — Genomes, Fossils, and Trees. pp. 52–64.
^Hankeln, Thomas; Wey-Fabrizius, Alexandra; Herlyn, Holger; Witek, Alexander; Weber, Mathias; Nesnidal, Maximilian; Struck, Torsten (2014)."Phylogeny of platyzoan taxa based on molecular data". In Wägele, J. Wolfgang; Bartolomaeus, Thomas (eds.).Deep Metazoan Phylogeny: The Backbone of the Tree of Life. Walter de Gruyter GmbH. pp. 105–125.
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