Deuterostomia

Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Superphylum: Deuterostomia
Phyla: Chordata - Echinodermata - Hemichordata - Xenoturbellida - †Vetulicolia

Name

Deuterostomia
References

Gordon, D.P. (ed.) 2009: New Zealand inventory of biodiversity. Volume 1. Kingdom Animalia. Radiata, Lophotrochozoa, Deuterostomia. Canterbury University Press, Christchurch, New Zealand.
Grobben K. (1908). Die systematische Einteilung des Tierreiches. - Verhandlungen der k.k. zoologisch-botanischen Gesellschaft in Wien.
Olga I. Raikova, M. Reuter, Ulf Jondelius, Margaretha K. S. Gustafsson, (2000). An immunocytochemical and ultrastructural study of the nervous and muscular systems of Xenoturbella westbladi (Bilateria inc. sed.), Zoomorphology 120(2): 107–118

Vernacular names
العربية: ثانويات الفم
беларуская: Другаснаротыя
català: Deuteròstoms
čeština: Druhoústí
Deutsch: Neumünder
Ελληνικά: Δευτεροστόμια
English: Deuterostome
Esperanto: Novbuŝulo
español: Deuteróstomos, Deuterostomados
فارسی: پسین‌دهانیان (pasin-dahanian)
suomi: Jälkisuiset
Nordfriisk: Neimüsdiarten
français: Deutérostomes, Deutérostomiens
galego: Deuteróstomos
עברית: בעלי פה משני
magyar: Újszájúak
italiano: Deuterostomi
日本語: 新口動物
한국어: 후구동물상문(後口動物上門)
македонски: Второусни животни
Nederlands: Nieuwmondigen
norsk: Deuterostomier
polski: wtórouste, wtórnogębowce
português: Deuterostómios/Deuterostômios/Deuterostomados
русский: Вторичноротые
slovenčina: druhoústovce
svenska: Deuterostomia
Türkçe: İkincil ağızlılar
українська: Вториннороті
中文: 後口動物

Deuterostomia (/ˌdjuːtərəˈstoʊmi.ə/; lit. 'second mouth' in Greek)[2][3] are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some examples of deuterostomes include vertebrates (and thus humans), sea stars, and crinoids.

In deuterostomy, the developing embryo's first opening (the blastopore) becomes the anus, while the mouth is formed at a different site later on. This was initially the group's distinguishing characteristic, but deuterostomy has since been discovered among protostomes as well.[4] This group is also known as enterocoelomates, because their coelom develops through enterocoely.

The three major clades of deuterostomes are Chordata (e.g. vertebrates), Echinodermata (e.g. starfish), and Hemichordata (e.g. acorn worms). Together with Protostomia and their out-group Xenacoelomorpha, these compose the Bilateria, animals with bilateral symmetry and three germ layers.
Systematics
History

Initially, Deuterostomia included the phyla Brachiopoda,[5] Bryozoa,[6] Chaetognatha,[7] and Phoronida[5] based on morphological and embryological characteristics. However, Superphylum Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses when the lophophorates were removed from it and combined with other protostome animals to form superphylum Lophotrochozoa.[8] The phylum Chaetognatha (arrow worms) may belong here,[7] but molecular studies have placed them in the protostomes more often.[9][10]

While protostomes as a monophyletic group has strong support, research have shown that deuterostomes may be paraphyletic, and what was once considered traits of deuterostomes could instead be traits of the last common bilaterian ancestor. This suggests the deuterostome branch is very short or non-existent. The Xenambulacraria's sister group could be both the chordates or the protostomes, or be equally distantly related to them both.[11]
Classification
See also: List of bilateral animal orders

These are the following phyla/subgroups of the deuterostomes:

Superphylum Deuterostomia
Phylum Chordata (vertebrates, tunicates, and lancelets)
Subphylum Cephalochordata – 1 class (lancelets)
Subphylum Tunicata (Urochordata) – 4 classes (tunicates)
Subphylum Vertebrata (Craniata) – 9 classes (vertebrates – mammals, reptiles, amphibians, birds, and fish)
Superclass Agnatha (Cyclostomata or incertae sedis) – 2 classes (jawless fish – hagfish and lampreys)
Infraphylum Gnathostomata – 7 classes (jawed vertebrates – mammals, reptiles, amphibians, birds, bony fish, and cartilaginous fish)
Superclass incertae sedis – 1 class (cartilaginous fish – sharks, skates, rays, and chimaeras)
Superclass Osteichthyes – 2 classes (bony fish, 98.8 percent of all fish – ray-finned fish and lobe-finned fish)
Superclass Tetrapoda – 4 classes (four-limbed vertebrates – mammals, reptiles, amphibians, and birds)
Phylum Hemichordata – 3 classes (hemichordates, known as acorn worms)
Phylum Echinodermata (echinoderms – sea stars, brittle stars, sea lilies, sea urchins, and sea cucumbers)
Subphylum Asterozoa – 2 classes (sea stars and brittle stars)
Subphylum Crinozoa – 1 class (sea lilies)
Subphylum Echinozoa – 2 classes (sea urchins and sea cucumbers)

Echinodermata and Hemichordata form the clade Ambulacraria. Moreover, there is a possibility that Ambulacraria can be the sister clade to Xenacoelomorpha, and form the Xenambulacraria group.[12][13][14]
Notable characteristics
Early development differences between deuterostomes versus protostomes. In deuterostomes, blastula divisions occur as radial cleavage because they occur parallel or perpendicular to the major polar axis. In protostomes the cleavage is spiral because division planes are oriented obliquely to the polar major axis. During gastrulation, deuterostome embryos' anus is given first by the blastopore while the mouth is formed secondarily, and vice versa for the protostomes

In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates.

Most deuterostomes display indeterminate cleavage, in which the developmental fate of the cells in the developing embryo is not determined by the identity of the parent cell. Thus, if the first four cells are separated, each can develop into a complete small larva; and if a cell is removed from the blastula, the other cells will compensate.

In deuterostomes the mesoderm forms as evaginations of the developed gut that pinch off to form the coelom. This process is called enterocoely.

Another feature present in both the Hemichordata and Chordata is pharyngotremy; the presence of spiracles or gill slits into the pharynx, which is also found in some primitive fossil echinoderms (mitrates).[15][16] A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord. In the early embryonic stage, it looks like the hollow nerve cord of chordates.

Except for the echinoderms, both the hemichordates and the chordates have a thickening of the aorta, homologous to the chordate heart, which contracts to pump blood. This suggests a presence in the deuterostome ancestor of the three groups, with the echinoderms having secondarily lost it.[citation needed]

The highly modified nervous system of echinoderms obscures much about their ancestry, but several facts suggest that all present deuterostomes evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord, circular and longitudinal muscles and a segmented body.[17]
Formation of mouth and anus
Main article: Embryological origins of the mouth and anus

The defining characteristic of the deuterostome is the fact that the blastopore (the opening at the bottom of the forming gastrula) becomes the anus, whereas in protostomes the blastopore becomes the mouth. The deuterostome mouth develops at the opposite end of the embryo, from the blastopore, and a digestive tract develops in the middle, connecting the two.

In many animals these early development stages later evolved in ways that no longer reflect these original patterns. For instance, humans have already formed a gut tube at the time of formation of the mouth and anus. Then the mouth forms first[citation needed], during the fourth week of development, and the anus forms four weeks later, temporarily forming a cloaca.
Origins and evolution
EarlyDeuterostome NT.jpg
Saccorhytus coronarius, (540 mya) may be one of the earliest deuterostomes, but also possibly an ecdysozoan.

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes. Chordates (which include all the vertebrates) are deuterostomes.[18] It seems likely that the 555 million year old Kimberella was a member of the protostomes.[19][20] That implies that the protostome and deuterostome lineages split some time before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 538.8 million years ago,[18] i.e. during the later part of the Ediacaran Period (circa 635-539 Mya, around the end of global Marinoan glaciation in the late Neoproterozoic). The oldest proposed deuterostome is Saccorhytus coronarius, which lived approximately 540 million years ago, but this later challenged and an ecdysozoan connection is proposed.[21][3]

Fossils of one major deuterostome group, the echinoderms (whose modern members include sea stars, sea urchins and crinoids), are quite common from the start of Series 2 of the Cambrian, 521 million years ago.[22] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[23] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[24][25] Another Chengjiang fossil, Haikouella lanceolata, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes — although it also had short tentacles round its mouth.[25] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[26][27] Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate.[28]

On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no bone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from the Permian-aged Paleobranchiostoma, trace fossils of the Ordovician colonial tunicate Catellocaula, and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians.
Phylogeny

Below is a phylogenetic tree showing consensus relationships among deuterostome taxa. Phylogenomic evidence suggests the enteropneust family, Torquaratoridae, fall within the Ptychoderidae. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.[29][11] The approximate dates for each radiation into a new clade are given in millions of years ago (Mya). Not all dates are consistent, as of date ranges only the center is given.[30]

Support for the clade Deuterostomia is not unequivocal. In particular, the Ambulacraria appear to be related to the Xenacoelomorpha. If upheld, this raises two possibilities: either the Ambulacraria are taken out of the deuterostome-protostome dichotomy (in which case the grouping Deuterostomia dissolves, with Chordata and Protostomia grouped together as Centroneuralia), or the Xenacoelomorpha are re-positioned next to Ambulacraria within the Deuterostomia as in the above diagram.[11][31][32][33][34][35][36][37]
See also

iconEvolutionary biology portal

Timeline of the evolutionary history of life – Major events during the development of life

Urbilaterian, a hypothethical common ancestor to Protostomes and Deuterostomes

References

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Further reading
Swalla, B. J.; Smith, A. B. (2008). "Deciphering deuterostome phylogeny: Molecular, morphological and palaeontological perspectives". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 363 (1496): 1557–1568. doi:10.1098/rstb.2007.2246. PMC 2615822. PMID 18192178.