Eumetazoa

Eumetazoans
Temporal range: Ediacaran - Present, 635–0 Ma
A lancelet
Scientific classification
Kingdom: Animalia
Subkingdom: Eumetazoa
Buetschli, 1910
Phyla

Radiata (unranked)

Bilateria (unranked)

Synonyms

Enterozoa Lankester, 1877, em. Beklemishev[1][2]

Eumetazoa (Greek: εὖ [eu], well + μετά [metá], after + ζῷον [zóon], animal) is a clade comprising all major animal groups except sponges, placozoa, and several other extinct or obscure life forms, such as Iotuba and Thectardis. Characteristics of eumetazoans include true tissues organized into germ layers, the presence of neurons, and an embryo that goes through a gastrula stage. The clade is usually held to contain at least Ctenophora, Cnidaria, and Bilateria. Whether mesozoans belong is in dispute. Ctenophora now appear basal eumetazoa, and placozoa also appear to have emerged in eumetazoa. Eumetazoa would then be a basal Metazoan clade as sister of Porifera.[3]

Some phylogenists have speculated the sponges and eumetazoans evolved separately from single-celled organisms, which would mean 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, support a common origin.[4]

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, placozoans and mesozoans.[5] When treated as a formal taxon Eumetazoa is typically ranked as a subkingdom. The name Metazoa has also been used to refer to this group, but more often refers to the Animalia as a whole. Many classification schemes do not include a subkingdom Eumetazoa.

Taxonomy

Over the last decade, the work of developmental biologists and molecular phylogeneticists spawned new ideas about bilaterian relationships resulting in a paradigm shift.

The current widely accepted hypothesis, based on molecular data (mostly 18S rRNA sequences), divides Bilateria into the following four superphylums: Deuterostomia, Ecdysozoa, Lophotrochozoa, and Platyzoa (sometimes included in Lophotrochozoa). The last three groups are also collectively known as Protostomia.

However, many skeptics emphasize the pitfalls and inconsistencies associated with the new data. Claus Nielsen, a professor of evolutionary invertebrate embryology at the Zoological Museum University of Copenhagen champions one of the most prominent alternative views based on morphological evidence. In his 2001 book Animal Evolution: Interrelationships of the Living Phyla, he maintains the traditional divisions of Protostomia and Deuterostomia.

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.[6] 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.[7] 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.[8]

References

  1. ↑ Lankester, R. (1877). Notes on the Embryology and classification of the Animal kingdom: comprising a revision of speculations relative to the origin and significance of the germ-layers. Quartely Journal of Microscopical Science (N.S.), No. 68: 399–454.
  2. ↑ Beklemishev, V.L. The basis of the comparative anatomy of the invertebrates [Основы сравнительной анатомии беспозвоночных]. 1st ed., 1944; 2nd ed., 1950; 3rd ed. (2 vols.), 1964. English translation, 1969, . Akademia Nauk, Moscow, Leningrad.
  3. ↑ Ryan, J. F.; Pang, K.; Schnitzler, C. E.; Nguyen, A.-D.; Moreland, R. T.; Simmons, D. K.; Koch, B. J.; Francis, W. R.; Havlak, P.; Smith, S. A.; Putnam, N. H.; Haddock, S. H. D.; Dunn, C. W.; Wolfsberg, T. G.; Mullikin, J. C.; Martindale, M. Q.; Baxevanis, A. D. (13 December 2013). "The Genome of the Ctenophore Mnemiopsis leidyi and its Implications for Cell Type Evolution". Science. 342 (6164, number 6164): 1242592. PMC 3920664 Freely accessible. PMID 24337300. doi:10.1126/science.1242592.
  4. ↑ Philippe H, Derelle R, Lopez P, et al. (April 2009). "Phylogenomics revives traditional views on deep animal relationships". Curr. Biol. 19 (8): 706–12. PMID 19345102. doi:10.1016/j.cub.2009.02.052.
  5. ↑ Systema Naturae 2000 Taxon: Subkingdom Eumetazoa — retrieved February 2, 2006
  6. ↑ Peterson KJ, Butterfield NJ (July 2005). "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. Bibcode:2005PNAS..102.9547P. PMC 1172262 Freely accessible. PMID 15983372. doi:10.1073/pnas.0503660102.
  7. ↑ Blair, J. E.; Hedges, S. B. (March 2005). "Molecular clocks do not support the Cambrian explosion". Molecular Biology and Evolution. 22 (3): 387–390. PMID 15537810. doi:10.1093/molbev/msi039.
  8. ↑ Chen, J.-Y.; Bottjer, D.J.; Oliveri, P.; Dornbos, S.Q.; Gao, F.; Ruffins, S.; Chi, H.; Li, C.-W.; Davidson, E.H.; et al. (9 July 2004). "Small bilaterian fossils from 40 to 55 million years before the Cambrian". Science. 305 (5681): 218–222. Bibcode:2004Sci...305..218C. PMID 15178752. doi:10.1126/science.1099213.
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