Biology:Eumetazoa

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Eumetazoa (from grc εὖ (eû) 'well', μετά (metá) 'after', and ζῷον (zôion) 'animal'), also known as Epitheliozoa or Histozoa, is a proposed basal animal subkingdom as a sister group of Porifera (sponges).[1][2][3][4][5] The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as an eumetazoan clade in the ParaHoxozoa, based on genetic data. The subkingdom Parazoa comprises the sister taxa, which includes primarily Porifera. The competing hypothesis is the Myriazoa clade, which states Ctenophora diverged earlier than Porifera, who instead would make up a clade with ParaHoxozoa, implying either sponges lost complexity or Ctenophores developed complexity independently, and maybe even a mix of both processes[6][7][8][9]. As far as May 2026, this question remains unresolved and the dispute may became combative, with some scientists deciding to not participate into the debate[10].

Agnothozoa were once considered an intermediary clade between Parazoa and Eumetazoa, being the sister clade to Parazoa which Eumetazoa would late derivate from. The group primarily encompassed the clade Placozoa and the proposed clade Mesozoa who groups Orthonectida, Dicyemida and the dubious Monoblastozoa, but now are largely considered a polyphyletic group of simplified eumetazoans[6][3][11][12][13]. Though some archaic fossil groups such as Chancelloriidae and Petalonamae, may, based on weaker morphological evidence, still be considered members of intermediate clades before the crown group Eumetazoa.[14][15][16]

Several other extinct or obscure life forms, such as Thectardis, may have emerged in the total group.[17] Key characteristics present in most eumetazoans include true tissues organized into germ layers, the presence of neurons and muscles, and an embryo that goes through a gastrula stage.

Some phylogenists once speculated the sponges and eumetazoans evolved separately from different single-celled organisms, which would have meant that the animal kingdom does not form a clade (a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of choanocytes, now unanimously support a common origin.[18]

Traditionally, eumetazoans are a major group of animals in the Five Kingdoms classification of Lynn Margulis and K. V. Schwartz, comprising the Radiata and Bilateria – all animals except the sponges.[19]

Evolutionary origins

It has been suggested that one type of molecular clock and one approach to interpretation of the fossil record both place the evolutionary origins of eumetazoa in the Ediacaran.[20] However, the earliest eumetazoans may not have left a clear impact on the fossil record and other interpretations of molecular clocks suggest the possibility of an earlier origin.[21] The discoverers of Vernanimalcula describe it as the fossil of a bilateral triploblastic animal that appeared at the end of the Marinoan glaciation prior to the Ediacaran period, implying an even earlier origin for eumetazoans.[22] Various ediacaran organisms have been tentatively classified as eumetazoans. But so far, very few Ediacaran organisms have been identified as definite eumetazoans like- Kimberella, Haootia and Dickinsonia. Ediacaran fossils preserve very little details so identifying one as an animal with true tissue is very difficult. Many extinct phyla have been proposed by many researchers that may fall under the clade. These are Proarticulata, Trilobozoa and Petalonamae. The inclusion of these within eumetazoa as well as the position of these within the clade is highly debated and sometimes considered speculative. The proarticulates are considered as stem bilaterians by most authors.[23] Together the three phyla are grouped as the grade Vendobionta.The petalonamids are often considered as early diverging animals before animals with true tissue organisation started to appear.

References

  1. ↑ Feuda, Roberto; Dohrmann, Martin; Pett, Walker; Philippe, Hervé; Rota-Stabelli, Omar; Lartillot, Nicolas; Wörheide, Gert; Pisani, Davide (2017). "Improved Modeling of Compositional Heterogeneity Supports Sponges as Sister to All Other Animals" (in en). Current Biology 27 (24): 3864–3870.e4. doi:10.1016/j.cub.2017.11.008. PMID 29199080. Bibcode: 2017CBio...27E3864F. 
  2. ↑ Pisani, Davide; Pett, Walker; Dohrmann, Martin; Feuda, Roberto; Rota-Stabelli, Omar; Philippe, Hervé; Lartillot, Nicolas; Wörheide, Gert (15 December 2015). "Genomic data do not support comb jellies as the sister group to all other animals". Proceedings of the National Academy of Sciences 112 (50): 15402–15407. doi:10.1073/pnas.1518127112. PMID 26621703. Bibcode: 2015PNAS..11215402P. 
  3. ↑ 3.0 3.1 Simion, Paul; Philippe, Hervé; Baurain, Denis; Jager, Muriel; Richter, Daniel J.; Franco, Arnaud Di; Roure, Béatrice; Satoh, Nori et al. (3 April 2017). "A Large and Consistent Phylogenomic Dataset Supports Sponges as the Sister Group to All Other Animals". Current Biology 27 (7): 958–967. doi:10.1016/j.cub.2017.02.031. PMID 28318975. Bibcode: 2017CBio...27..958S. https://hal.archives-ouvertes.fr/hal-01681528/file/Simion_etal2017_CurrBiol_proofs.pdf. 
  4. ↑ Giribet, Gonzalo (1 October 2016). "Genomics and the animal tree of life: conflicts and future prospects". Zoologica Scripta 45: 14–21. doi:10.1111/zsc.12215. 
  5. ↑ Laumer, Christopher E; Gruber-Vodicka, Harald; Hadfield, Michael G; Pearse, Vicki B; Riesgo, Ana; Marioni, John C; Giribet, Gonzalo (2018-10-30). "Support for a clade of Placozoa and Cnidaria in genes with minimal compositional bias" (in en). eLife 7. doi:10.7554/elife.36278. ISSN 2050-084X. PMID 30373720. 
  6. ↑ 6.0 6.1 Schultz, Darrin T.; Haddock, Steven H. D.; Bredeson, Jessen V.; Green, Richard E.; Simakov, Oleg; Rokhsar, Daniel S. (June 2023). "Ancient gene linkages support ctenophores as sister to other animals" (in en). Nature 618 (7963): 110–117. doi:10.1038/s41586-023-05936-6. ISSN 1476-4687. PMID 37198475. Bibcode: 2023Natur.618..110S. 
  7. ↑ Moroz, Leonid L.; Kocot, Kevin M.; Citarella, Mathew R.; Dosung, Sohn; Norekian, Tigran P.; Povolotskaya, Inna S.; Grigorenko, Anastasia P.; Dailey, Christopher et al. (June 2014). "The ctenophore genome and the evolutionary origins of neural systems" (in en). Nature 510 (7503): 109–114. doi:10.1038/nature13400. ISSN 1476-4687. PMID 24847885. Bibcode: 2014Natur.510..109M. 
  8. ↑ Whelan, Nathan V.; Kocot, Kevin M.; Moroz, Tatiana P.; Mukherjee, Krishanu; Williams, Peter; Paulay, Gustav; Moroz, Leonid L.; Halanych, Kenneth M. (November 2017). "Ctenophore relationships and their placement as the sister group to all other animals". Nature Ecology & Evolution 1 (11): 1737–1746. doi:10.1038/s41559-017-0331-3. ISSN 2397-334X. PMID 28993654. Bibcode: 2017NatEE...1.1737W. 
  9. ↑ Burkhardt, P.; Colgren, J.; Medhus, A.; Digel, L.; Naumann, B.; Soto-Angel, J. J.; Nordmann, E. L.; Sachkova, M. Y. et al. (2023). "Science" (in en). Science 380 (6642): 293–297. doi:10.1126/science.ade5645. PMID 37079688. PMC 7617566. https://www.science.org/action/cookieAbsent. Retrieved 2026-05-20. 
  10. ↑ Callaway, Ewen (2026-01-27). "What were the first animals? The fierce sponge–jelly battle that just won't end" (in en). Nature 649 (8099): 1096–1098. doi:10.1038/d41586-026-00238-z. ISSN 1476-4687. PMID 41593281. Bibcode: 2026Natur.649.1096C. https://www.nature.com/articles/d41586-026-00238-z. 
  11. ↑ Zverkov, Oleg A.; Mikhailov, Kirill V.; Isaev, Sergey V.; Rusin, Leonid Y.; Popova, Olga V.; Logacheva, Maria D.; Penin, Alexey A.; Moroz, Leonid L. et al. (2019-05-24). "Dicyemida and Orthonectida: Two Stories of Body Plan Simplification" (in English). Frontiers in Genetics 10. doi:10.3389/fgene.2019.00443. ISSN 1664-8021. PMID 31178892. 
  12. ↑ Nikolaeva, Olga V.; Mikhailov, Kirill V.; Muntyan, Maria S.; Zverkov, Oleg A.; Spirin, Sergey A.; Lyubetsky, Vassily A.; Slyusarev, Georgy S.; Aleoshin, Vladimir V. (2025-06-21). "Rare Evolutionary Events Support the Phylogenetic Placement of Orthonectida Within Annelida". International Journal of Molecular Sciences 26 (13): 5983. doi:10.3390/ijms26135983. ISSN 1422-0067. PMID 40649762. 
  13. ↑ Mendivil Ramos, Olivia; Barker, Daniel; Ferrier, David E. K. (2012-10-23). "Ghost loci imply Hox and ParaHox existence in the last common ancestor of animals". Current Biology: CB 22 (20): 1951–1956. doi:10.1016/j.cub.2012.08.023. ISSN 1879-0445. PMID 23022064. Bibcode: 2012CBio...22.1951M. 
  14. ↑ Hoyal Cuthill, Jennifer F.; Han, Jian (November 2018). Álvaro, Javier. ed. "Cambrian petalonamid Stromatoveris phylogenetically links Ediacaran biota to later animals" (in en). Palaeontology 61 (6): 813–823. doi:10.1111/pala.12393. ISSN 0031-0239. Bibcode: 2018Palgy..61..813H. 
  15. ↑ Dunn, F. S.; Liu, A. G.; Grazhdankin, D. V.; Vixseboxse, P.; Flannery-Sutherland, J.; Green, E.; Harris, S.; Wilby, P. R. et al. (2021). "Science" (in en). Science Advances 7 (30). doi:10.1126/sciadv.abe0291. PMID 34301594. PMC 8302126. Bibcode: 2021SciA....7..291D. https://www.science.org/action/cookieAbsent. Retrieved 2026-04-28. 
  16. ↑ Yun, Hao; Zhang, Xingliang; Brock, Glenn A.; Li, Luoyang; Li, Guoxiang (June 2021). "Biomineralization of the Cambrian chancelloriids". Geology 49 (6): 623–628. doi:10.1130/G48428.1. ISSN 0091-7613. Bibcode: 2021Geo....49..623Y. https://www.scopus.com/pages/publications/85108142702. 
  17. ↑ Martindale, Mark Q.; Kourakis, Matthew J. (1999). "Hox clusters: Size doesn't matter". Nature 399 (6738): 730–731. doi:10.1038/21530. PMID 10391234. Bibcode: 1999Natur.399..730M. 
  18. ↑ H., Philippe et al. (April 2009). "Phylogenomics revives traditional views on deep animal relationships". Current Biology 19 (8): 706–712. doi:10.1016/j.cub.2009.02.052. PMID 19345102. Bibcode: 2009CBio...19..706P. 
  19. ↑ "Systema Naturae 2000 Taxon: Subkingdom Eumetazoa". . Retrieved February 2, 2006
  20. ↑ "Origin of the Eumetazoa: testing ecological predictions of molecular clocks against the Proterozoic fossil record". Proc. Natl. Acad. Sci. U.S.A. 102 (27): 9547–52. July 2005. doi:10.1073/pnas.0503660102. PMID 15983372. Bibcode: 2005PNAS..102.9547P. 
  21. ↑ Blair, J. E.; Hedges, S. B. (March 2005). "Molecular clocks do not support the Cambrian explosion". Molecular Biology and Evolution 22 (3): 387–390. doi:10.1093/molbev/msi039. PMID 15537810. 
  22. ↑ Chen, J.-Y. et al. (9 July 2004). "Small bilaterian fossils from 40 to 55 million years before the Cambrian". Science 305 (5681): 218–222. doi:10.1126/science.1099213. PMID 15178752. Bibcode: 2004Sci...305..218C. 
  23. ↑ Ivantsov, A. Yu. (2021). "Proarticulates—an extinct phylum of soft-bodied metazoans, or a group of vendobionts par excellence?" Symmetry. 13(2): 160. Full text. doi:10.3390/sym13020160.
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