Eumetazoa (from Ancient Greekεὖ (eû)'well'μετά (metá)'after' and ζῷον (zôion)'animal'), also known asEpitheliozoa orHistozoa, is a proposedbasalanimalsubkingdom as asister group ofPorifera (sponges).[7][8][9][10][11] The basal eumetazoan clades are theCtenophora and theParaHoxozoa.Placozoa is now also seen as a eumetazoan in the ParaHoxozoa. The competing hypothesis is theMyriazoa clade.[12] The subkingdomParazoa andAgnotozoa are the other taxa, and agnotozoa may be fake or even nonexistent at studies. Parazoa or Agnotozoa are a main sister group to eumetazoans, forming clade Blastozoa/Diploblastozoa. Alternatively,Parazoa was considered as a sister group to Agnotozoa (now considered polyphyletic).Several other extinct or obscure life forms, such asIotuba andThectardis, appear to have emerged in the group.[13] Characteristics of eumetazoans include truetissues organized intogerm layers, the presence ofneurons andmuscles, and an embryo that goes through agastrula stage.
Somephylogenists once speculated the sponges and eumetazoansevolved separately from different single-celled organisms, which would have meant that the animal kingdom does not form aclade (a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence ofchoanocytes, now unanimously support a common origin.[14]
Traditionally, eumetazoans are a major group ofanimals in the Five Kingdoms classification ofLynn Margulis and K. V. Schwartz, comprising theRadiata andBilateria – all animals except thesponges.[15]
It has been suggested that one type ofmolecular clock and one approach to interpretation of the fossil record both place the evolutionary origins of eumetazoa in theEdiacaran.[16] 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.[17] The discoverers ofVernanimalcula describe it as the fossil of abilateraltriploblastic animal that appeared at the end of theMarinoan glaciation prior to theEdiacaran period, implying an even earlier origin for eumetazoans.[18] Various ediacaran organisms have been tentatively classified aseumetazoans. But so far, very fewEdiacaran organisms have been identified as definite eumetazoans like-Kimberella,Haootia andDickinsonia. 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 areProarticulata,Trilobozoa andPetalonamae. The inclusion of these within eumetazoa as well as the position of these within the clade is highly debated and sometimes considered speculative. Theproarticulates are considered as stembilaterians by most authors.[19] Together the three phyla are grouped as the gradeVendobionta.The petalonamids are often considered as early diverging animals before animals with true tissue organisation started to appear.
^Fedonkin, M. A. (1990). Systematic description of Vendian Metazoa. In: Sokolov, B. S. & Iwanowski, A. B. (eds), *The Vendian System: Historical–Geological and Paleontological Foundation*, Vol. 1. Springer, pp. 71–120. Fedonkin describes trilobozoans as coelenterate-like organisms, implying eumetazoan affinities.
^Lankester, Ray (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".Quarterly Journal of Microscopical Science (N.S.), No. 68: 399–454.
^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,[1]. Akademia Nauk, Moscow, Leningrad.
^Ulrich, W. (1950). "Begriff und Einteilung der Protozoen". In Grüneberg, H. (ed.).Moderne Biologie. Festschrift zum 60. Geburtstag von Hans Nachtsheim (in German). Berlin: Peters. pp. 241–250.
^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.
