Eumetazoa
Encyclopedia
Eumetazoa is a clade
comprising all major animal
groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues
organized into germ layer
s, and an embryo that goes through a gastrula stage. The clade is usually held to contain at least Ctenophora
, Cnidaria
, and Bilateria
. Whether mesozoans and placozoans belong is in dispute.
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 organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of choanocyte
s, support a common origin.
Eumetazoans are a major group of animal
s in the Five Kingdoms classification of Lynn Margulis
and K. V. Schwartz, comprising the Radiata
and Bilateria
— all animals except the sponge
s, placozoa
ns and mesozoa
ns. 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.
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 groups: 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
.
and one approach to interpretation of the fossil record both place the evolutionary origins of eumetazoa in the Ediacaran
. 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. 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.
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
comprising all major animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
organized into germ layer
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...
s, and an embryo that goes through a gastrula stage. The clade is usually held to contain at least Ctenophora
Ctenophore
The Ctenophora are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size...
, Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, and Bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
. Whether mesozoans and placozoans belong is in dispute.
Some phylogenists
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
have speculated the sponges and eumetazoans evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
separately from single-celled organisms, which would mean that the animal kingdom does not form a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
(a complete grouping of organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of choanocyte
Choanocyte
Choanocytes are cells that line the interior of Asconoid, syconoid and leuconoid body type sponges that contain a central flagellum surrounded by a collar of microvilli which are connected by a thin membrane. It is the closest family member to the free-living ancestor called choanoflagellate...
s, support a common origin.
Eumetazoans are a major group of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s in the Five Kingdoms classification of Lynn Margulis
Lynn Margulis
Lynn Margulis was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted...
and K. V. Schwartz, comprising the Radiata
Radiata
The Radiata are the radially symmetric animals of the Eumetazoa subkingdom. The term Radiata has had various meanings in the history of classification...
and Bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
— all animals except the sponge
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...
s, placozoa
Trichoplax
The Placozoa are a basal form of invertebrate. They are the simplest in structure of all non-parasitic multicellular animals . They are generally classified as a single species, Trichoplax adhaerens, although there is enough genetic diversity that it is likely that there are multiple,...
ns and mesozoa
Mesozoa
The Mesozoa are enigmatic, minuscule, worm-like parasites of marine invertebrates. It is still unclear as to whether they are degenerate platyhelminthes or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm of ciliated cells surrounding one or...
ns. 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 biologistsDevelopmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
and molecular phylogeneticists
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
spawned new ideas about bilaterian relationships resulting in a paradigm shift
Paradigm shift
A Paradigm shift is, according to Thomas Kuhn in his influential book The Structure of Scientific Revolutions , a change in the basic assumptions, or paradigms, within the ruling theory of science...
.
The current widely accepted hypothesis, based on molecular data (mostly 18S rRNA
Non-coding RNA
A non-coding RNA is a functional RNA molecule that is not translated into a protein. Less-frequently used synonyms are non-protein-coding RNA , non-messenger RNA and functional RNA . The term small RNA is often used for short bacterial ncRNAs...
sequences), divides bilateria into the following four groups: Deuterostomia
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
, Ecdysozoa
Ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
, Lophotrochozoa
Lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
, and Platyzoa
Platyzoa
The Platyzoa are a group of protostome animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the Phylum Platyhelminthes or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals...
(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
University of Copenhagen Faculty of Science
The Faculty of Science at the University of Copenhagen consists of both mathematical and natural sciences, and is divided into 11 institutes including the Natural History Museum of Denmark...
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
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
and Deuterostomia
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
.
Evolutionary origins
It has been suggested that one type of molecular clockMolecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
and one approach to interpretation of the fossil record both place the evolutionary origins of eumetazoa in the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
. 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. The discoverers of Vernanimalcula
Vernanimalcula
Vernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria . It is known from deposits dating to . The fossils are between 0.1 and 0.2 mm across...
describe it as the fossil of a bilateral
Bilateralism
Bilateralism consists of the political, economic, or cultural relations between two sovereign states. For example, free trade agreements signed by two states are examples of bilateral treaties. It is in contrast to unilateralism or multilateralism, which refers to the conduct of diplomacy by a...
triploblastic animal that appeared at the end of the Marinoan glaciation prior to the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
Period, implying an even earlier origin for eumetazoans.
External links
- Bilateria. Tree of Life web project, US National Science Foundation. 2002. 6 January 2006.
- Invertebrates and the Origin of Animal Diversity
- Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.
- Metazoa: the Animals
- Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla, 2nd edition, 563 pp. Oxford Univ. Press, Oxford. ISBN 0-19-850681-3
- Borchiellini, C. Manuel, M., Alivon, E., Boury-Esnault N., Vacelet, J., Le-Parco, Y. 2001. Journal of Evolutionary Biology 14 (1): 171–179.
- Peterson, Kevin J., McPeek, Mark A., & Evans, David A.D. 2005. Tempo & mode of early animal evolution: inferences from rocks, Hox, & molecular clocks. Paleobiology 31(2, Supplement): 36–55.