Deuterostomia is a subtaxon of the Bilateria branch of the subkingdom Eumetazoa, within Animalia, and are distinguished from protostomes by their deuterostomic embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes, it becomes the mouth. (There are some occurrences of deuterostomy among protostomes.)
There are three major clades of deuterostomes:
|A sea cucumber|
Previously, Deuterostomia also included the phyla Brachiopoda, Bryozoa, Chaetognatha, and Phoronida based on 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. The phylum Chaetognatha (arrow worms) may belong here, but molecular studies have placed them in the protostomes more often.
Extinct deuterostome groups may include the phylum Vetulicolia.
Echinodermata and Hemichordata form the clade Ambulacraria.
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.
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). 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 highly modified 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.
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.
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 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 Era (circa 635-542 Mya, around the end of global Marinoan glaciation in the late Neoproterozoic). The oldest discovered proposed deuterostome is Saccorhytus coronarius, which lived approximately 540 million years ago. The researchers that made the discovery believe that the Saccorhytus is a common ancestor to all previously-known deuterostomes.
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.
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. 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.
These are the following phyla/subgroups of the deuterostomes:
The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. Their closest relatives are the echinoderms. There are 111 known species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii. Two families - Harrimaniidae and Ptychoderidae - separated at least 370 million years ago.Until recently, it was thought that all species lived in the sediment on the seabed, subsisting as deposit feeders or suspension feeders. However, the last decade has seen the description of a new family, the Torquaratoridae, evidently limited to the deep sea, in which most of the species crawl on the surface of the ocean bottom and alternatively rise into the water column, evidently to drift to new foraging sites. It is assumed that the ancestors of acorn worms used to live in tubes like their relatives Pterobranchia, but that they eventually started to live a safer and more sheltered existence in sediment burrows instead. Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (8 ft 2 in), although most acorn worms are much smaller. Due to secretions containing elements like iodine, the animals have an iodoform-like smell.Ambulacraria
Ambulacraria or Coelomopora is a Superphylum of invertebrate phyla which includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggest the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a larger clade that also includes the Chordata, Vetulicolia and Saccorhytus.
The two living clades with representative organisms are:
Echinodermata (sea stars, sea urchins, brittle stars, sea cucumbers, feather stars, sea lilies, etc.)
Hemichordata (acorn worms, Pterobranchia, and possibly graptolites)The group Xenoturbellida (two species of worm-like animals) has previously been considered to be in this clade, but is now considered to be placed more basally among Metazoans.Fossil taxa that may lie on the stem lineage:
unranked clade Cambroernida
† unranked clade = Eldoniida
† Stellostomites Caron, Conway Morris & Shu, 2010
† Herpetogaster Caron, Conway Morris & Shu, 2010 - with one species: † Herpetogaster collinsi Caron, Conway Morris & Shu, 2010
† Seputus? Murray, J & MacGabhannArticulata (Crinoidea)
Articulata are the only extant subclass of the class Crinoidea. The group includes "sea lilies" and "feather stars". The Articulata are differentiated from the extinct subclasses by their lack of an anal plate in the adult stage and the presence of an entoneural system.Banffia
Banffia is a genus of animals described from Middle Cambrian fossils. The genus commemorates Banff, Alberta, near where the first fossil specimens were discovered. Its placement in higher taxa is controversial.Bourgueticrinida
Bourgueticrinida is an order of crinoids that typically live deep in the ocean. Members of this order are attached to the seabed by a slender stalk and are known as sea lilies. While other groups of crinoids flourished during the Permian, bourgueticrinids along with other extant orders did not appear until the Triassic, following a mass extinction event in which nearly all crinoids died out.Callograptus
Callograptus is a genus of graptolites.Cladida
Cladida is an extinct order of crinoids from the Middle to Late Devonian.Cornuta
Cornuta is an extinct order of echinoderms.Denticle (tooth feature)
Denticles, also called serrations, are small bumps on a tooth that serve to give the tooth a serrated edge. In paleontology, denticle characteristics such as size and density (denticles per unit distance) are used to describe and classify fossilized teeth, especially those of dinosaurs. Denticles are also present on the teeth of varanoid lizards, sharks, and mammals.Enterogona
Enterogona is an order of tunicates in the class Ascidiacea. It describes a group of marine animals.Graptolithina
Graptolithina is a subclass of the class Pterobranchia, the members of which are known as graptolites. These organisms are colonial animals known chiefly as fossils from the Middle Cambrian (Miaolingian, Wuliuan) through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. One analysis suggests that the pterobranch Rhabdopleura represents extant graptolites. Studies on the tubarium of fossil and living graptolites showed similarities in the basic fusellar construction and it is considered that the group most probably evolved from a Rhabdopleura-like ancestor.The name graptolite comes from the Greek graptos meaning "written", and lithos meaning "rock", as many graptolite fossils resemble hieroglyphs written on the rock. Linnaeus originally regarded them as 'pictures resembling fossils' rather than true fossils, though later workers supposed them to be related to the hydrozoans; now they are widely recognized as hemichordates.Haikouella
Haikouella is an agnathan chordate from the Lower Cambrian Maotianshan shales of Chengjiang County in Yunnan Province, China.
It is similar to the form Yunnanozoon, which is possibly a hemichordate. Still, there are anatomical differences from Yunnanozoon, including a larger stomach and smaller (0.1 mm) pharyngeal teeth. Haikouella does not have bones or a movable jaw, but it otherwise resembles vertebrates. Almost certain fish Haikouichthys and Myllokunmingia have been found in the same beds. Suspected hemichordates are also known from these deposits as well as from the Middle Cambrian Burgess Shale of British Columbia. Other than possible fish scales/plates from the Upper Cambrian of Wyoming, these Chinese fish-like chordates are one of the only known pre-Ordovician craniates.
Haikouella is known from 305 specimens mostly from a single bed in the Maotianshan shales of Yunnan province. The animal is 20 to 30 mm (40 mm max) in length and has a head, gills, brain, notochord, well developed musculature, heart and circulatory system. It has a bent caudal projection of the notochord that might be a primitive tail fin. It might have a pair of lateral eyes. Very small (0.1 mm) structures that are probably pharyngeal teeth are present in the body cavity. A few specimens display dorsal and ventral fins.
There are two known species, H. lanceolata (Chen, Huang, Li), the type species, and H. jianshanensis.Nephrozoa
Nephrozoa is a major clade of bilaterians, divided into the protostomes and the deuterostomes, containing almost all animal phyla and over a million extant species. Its sister clade is the Xenacoelomorpha. The coelom, the excretory organs, and nerve cords developed in the Nephrozoa.Chordates (which include all the vertebrates) are deuterostomes. It seems very likely that the 555 million year old 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 558 million years ago, and hence well before the start of the Cambrian 541 million years ago.Pleurogona
Pleurogona is an order of tunicates which is no longer in use as it contains the same families as Stolidobranchia.One of the more invasive species of this order is the "stalked (or leathery or solitary) sea squirt" Styela clava . Another invasive genus include the "chain sea squirts or chain tunicates" of the genus Botrylloides and the "golden star tunicate" Botryllus schlosseri .Stylophora
The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornutes and mitrates. It is synonymous with the subphylum Calcichordata.Xenoturbella
Xenoturbella is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.Yuknessia
Yuknessia is an early pterobranch, known from the Burgess shale, the Chengjiang and the Wheeler shale. Long, unbranched fronds emerge from a central holdfast-like body covered in small conical plates. 23 specimens of Yuknessia are known from the Greater Phyllopod bed, where they comprise < 0.1% of the community. The genus contains two species: the type species Y. simplex and Y. stephenensis. It was originally interpreted as a green alga, and has since been reinterpreted it as a colonial pterobranch.Yunnanozoon
Yunnanozoon lividum (Yunnan + Greek ζῷον zôion, lividum; "livid animal of Yunnan") is an extinct species from the Lower Cambrian, Chengjiang biota of Yunnan province, China. It is thought of as a deuterostome suspected of being either a hemichordate or chordate,Yunnanozoon is similar to the form Haikouella, which is almost certainly a chordate. Still, there are anatomical differences from Haikouella, including a smaller stomach and much larger (1 mm) pharyngeal teeth. It is by no means certain whether Yunannozoon possessed features such as a heart, gills, etc., which are seen in well-preserved specimens of Haikouella. Yunnanozoon somewhat resembles the Middle Cambrian Pikaia from the Burgess shale of British Columbia in Canada. Thirteen pairs of symmetrically arranged gonads have been identified, as have possible gill slits. However, some authors think that Yunnanozoon is closely related to the chordate Haikouella and that Yunnanozoon is probably a chordate rather than a hemichordate. A close relationship between Yunnanozoon and the taxon Vetulicolia has also been proposed.Yuyuanozoon
Yuyuanozoon magnificissimi is the largest known vetulicolian, with the holotype (and only known specimen) measuring about 202 millimeters in length. In life, it would have been an egg-shaped animal with a long cylindrical tail. The body is divided into 6 segments, with a gill opening (that has gill filaments extending out from each opening) placed symmetrically on each side of every body segment starting at the second segment. The cylindrical tail is divided into 7 segments, and differs from the tails of all other known vetulicolians (not being flattened like vetulicolids like Vetulicola, and not oar-like or leaf-like like those of the didazoonids like Didazoon, and is unlike the tails of banffozoans like Banffia and Skeemella).
The details of the tail anatomy, and of the gill openings with gill filaments currently leave Y. magnificissimi as incertae sedis.