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Deuterostomia (; in Greek) 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. 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 An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
(e.g.
starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
), 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,
Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...
,
Chaetognatha The Chaetognatha or chaetognaths (meaning ''bristle-jaws'') are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and ca ...
, and Phoronida based on morphological and embryological characteristics. However, ''Superphylum Deuterostomia'' was redefined in 1995 based on DNA molecular sequence analyses when the
lophophorate The Lophophorata are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa. They have a lophophore. Molecular phylogenetic analyses suggest that lophophorates are protostomes Protostomia () is the clade of animals once though ...
s were removed from it and combined with other protostome animals to form superphylum
Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, bra ...
. The phylum
Chaetognatha The Chaetognatha or chaetognaths (meaning ''bristle-jaws'') are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and ca ...
(arrow worms) may belong here, but molecular studies have placed them in the protostomes more often. 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.


Classification

These are the following phyla/subgroups of the deuterostomes: *Superphylum Deuterostomia **Phylum Chordata ( vertebrates,
tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s, and lancelets) ***Subphylum Cephalochordata – 1 class ( lancelets) ***Subphylum Tunicata (
Urochordata A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time cal ...
) – 4 classes (
tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s) ***Subphylum Vertebrata (
Craniata A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...
) – 9 classes ( vertebrates – mammals, reptiles, amphibians,
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s, and
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
) ****Superclass Agnatha ( Cyclostomata or incertae sedis) – 2 classes ( jawless fishhagfish and lampreys) ****Infraphylum
Gnathostomata Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, livi ...
– 7 classes ( jawed vertebratesmammals, reptiles, amphibians,
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s, bony fish, and cartilaginous fish) *****Superclass incertae sedis – 1 class ( cartilaginous fish
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s,
skate Skate or Skates may refer to: Fish * Skate (fish), several genera of fish belonging to the family Rajidae * Pygmy skates, several genera of fish belonging to the family Gurgesiellidae * Smooth skates or leg skates, several genera of fish belongi ...
s,
ray Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gr ...
s, and chimaeras) *****Superclass Osteichthyes – 2 classes ( bony fish, 98.8 percent of all
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
ray-finned fish Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species. The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...
and lobe-finned fish) *****Superclass Tetrapoda – 4 classes ( four-limbed vertebrates – mammals, reptiles, amphibians, and
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s) **Phylum Hemichordata – 3 classes ( hemichordates, known as acorn worms) **Phylum
Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
(
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
s – sea stars, brittle stars,
sea lilies Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which ar ...
, sea urchins, and sea cucumbers) ***Subphylum
Asterozoa The Asterozoa are a subphylum in the phylum Echinodermata. Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the class Asteroidea (the starfish), the class Ophiuroidea (the brittle stars ...
– 2 classes ( sea stars and brittle stars) ***Subphylum
Crinozoa Crinozoa is a subphylum of mostly sessile echinoderms, of which the crinoids, or sea lilies, are the only extant members. Crinozoans have an extremely extensive fossil history, which may or may not extend into the Precambrian (provided the enigma ...
– 1 class (
sea lilies Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which ar ...
) ***Subphylum Echinozoa – 2 classes ( sea urchins and sea cucumbers) Echinodermata and Hemichordata form the clade
Ambulacraria Ambulacraria , or Coelomopora , is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 ...
. Moreover, there is a possibility that Ambulacraria can be the sister clade to Xenacoelomorpha, and form the Xenambulacraria group.


Notable characteristics

In both deuterostomes and protostomes, a
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...
first develops into a hollow ball of cells, called a
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fro ...
. 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
lophophorate The Lophophorata are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa. They have a lophophore. Molecular phylogenetic analyses suggest that lophophorates are protostomes Protostomia () is the clade of animals once though ...
s. 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
evagination Endodermic evagination relates to the inner germ layers of cells of the very early embryo, from which is formed the lining of the digestive tract, of other internal organs, and of certain glands, implies the extension of a layer of body tissue to ...
s 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 slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In co ...
s into the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
, which is also found in some primitive fossil
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
s ( mitrates). A hollow nerve cord is found in all chordates, including
tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s (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 An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
, both the hemichordates and the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
s have a thickening of the aorta, homologous to the chordate
heart The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
, 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.


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. 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, during the fourth week of development, and the anus forms four weeks later, temporarily forming a cloaca.


Origins and evolution

The majority of animals more complex than
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...
and other Cnidarians are split into two groups, the
protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
s and deuterostomes. Chordates (which include all the vertebrates) are deuterostomes. It seems likely that the '' Kimberella'' was a member of the protostomes. That implies that the protostome and deuterostome lineages split some time before ''Kimberella'' appeared — at least , and hence well before the start of the Cambrian , ''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. Fossils of one major deuterostome group, the
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
s (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 The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
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 ''Haikouella'' is an agnathan chordate from the Lower Cambrian Maotianshan Shales of Chengjiang County in Yunnan Province, China. An analysis in 2015 placed ''Haikouella'' as a junior synonym of ''Yunnanozoon,'' another Maotianshan shale Cambrian ...
'', 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 ''Haikouichthys'' is an extinct genus of craniate (animals with notochords and distinct heads) that lived 518 million years ago, during the Cambrian explosion of multicellular life. ''Haikouichthys'' had a defined skull and other characteristi ...
'' and ''
Myllokunmingia ''Myllokunmingia'' is a genus of basal chordate from the Lower Cambrian Maotianshan shales of China 518 to 490 mya and is thought to be a vertebrate, although this is not conclusively proven. The species M. fengjiaoa is 28 mm long and 6&nbs ...
'', also from the Chengjiang fauna, are regarded as
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
. '' 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 The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...
-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 Ptychoderidae is a family of acorn worms. Genera and species The World Register of Marine Species lists the following:Introduction to the Deuterostomia
UCMP
''Deuterostomia''
at ''
Encyclopædia Britannica The (Latin for "British Encyclopædia") is a general knowledge English-language encyclopaedia. It is published by Encyclopædia Britannica, Inc.; the company has existed since the 18th century, although it has changed ownership various t ...
'' {{Taxonbar, from=Q150866 Ediacaran first appearances Superphyla Taxa named by Karl Grobben