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Deuterostomes (from Ancient Greek: ) are bilaterian of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryogenesis. Deuterostomia comprises three Phylum: chordate, , hemichordate, and the extinct clade .
In deuterostomes, the developing embryo's first opening (the blastopore) becomes the anus and cloaca, 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 as well. The deuterostomes are also known as enterocoelomates, because their coelom develops through pouching of the gut, enterocoely.
Deuterostomia's sister clade is Protostomia, animals that develop mouth first and whose digestive tract development is more varied. Protostomia includes the and , as well as the extinct Kimberella. Together with the Xenacoelomorpha, these constitute the large clade Bilateria, i.e. animals with bilateral symmetry and three .
There is a possibility that Ambulacraria is the sister clade to Xenacoelomorpha, and could form the Xenambulacraria group.
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. This is the source of identical twins.
The mesoderm forms as 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 into the pharynx, which is also found in some primitive fossil ().
A hollow nerve cord is found in all chordates, including (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.
Both the and the 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.
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.
The last common ancestor of the deuterostomes had lost all innexin diversity. Connexins evolved after early chordates lost innexin diversity
Fossils of one major deuterostome group, the (whose modern members include , and ), are quite common from the start of Stage 3 of the Cambrian, starting with forms such as Helicoplacus. Two other Cambrian Stage 3 (521-514 mya) species, Haikouichthys and Myllokunmingia from the Chengjiang biota, are the earliest body fossils of fish, whereas Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is now regarded as a primitive chordate. The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate, whereas Spartobranchus is an acorn-worm from the Burgess Shale, providing proof that all main lineages were already well established 508 mya.
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.. Fossils of Echinodermata are very common after the Cambrian. Fossils of Hemichordata are less common, except for graptolites until the Lower Carbonoferous.
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