Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "mouth second") are any members of a superphylum of animals. Deuterostomia is a subtaxon of the Bilateria branch of the subkingdom Eumetazoa, within Animalia, and are opposed to the protostomes. Deuterostomes are distinguished from protostomes by their embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth. Deuterostomes are also known as enterocoelomates because their coelom develops through enterocoely.
There are four extant phyla of deuterostomes:
- Phylum Chordata (vertebrates and their kin)
- Phylum Echinodermata (sea stars, sea urchins, sea cucumbers, etc.)
- Phylum Hemichordata (acorn worms and possibly graptolites)
- Phylum Xenacoelomorpha (2 species of worm-like animals)
Superphylum Deuterostomia was redefined in 1995 based on molecular sequence analyses when the lophophorates were removed from it and combined with other protostome animals to form superphylum Lophotrochozoa. The phylum Chaetognatha (arrow worms) may also belong here. Extinct groups may include the phylum Vetulicolia. Echinodermata, Hemichordata and Xenoturbellida form the clade Ambulacraria. The present phylum Priapulida also has a deuterostomic development, despite being otherwise placed within 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 are not determined by the identity of the parent cell. Thus if the first four cells are separated, each cell is capable of forming a complete small larva, and if a cell is removed from the blastula the other cells will compensate.
Both the Hemichordata and Chordata have gill slits, and primitive fossil echinoderms also show signs of gill slits. 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. Because of the degenerated nervous system of echinoderms, it is not possible to discern much about their ancestors in this matter, but based on different facts it is quite possible that all the present deuterostomes evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord, circular and longitudinal muscles and a segmented body. It could have resembled the small group of Cambrian urochordate deuterostomes named Vetulicolia.
Formation of 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.
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. It seems very likely that the Kimberella was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least , and hence well before the start of the Cambrian . The Ediacaran fossil Ernietta, from about , may represent a deuterostome animal.
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, . The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. Another Chengjiang fossil, Haikouella lanceolata, also from the Chengjiang fauna, 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. Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish. Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate. 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.
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- Deciphering deuterostome phylogeny: molecular, morphological and palaeontological perspectives
- Deuterostomia at Encyclopædia Britannica