In deuterostomes, the developing embryo's first opening (theblastopore) becomes the anus andcloaca, 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 amongprotostomes as well.[5] The deuterostomes are also known asenterocoelomates, because theircoelom develops through pouching of the gut,enterocoely.
Initially, Deuterostomia included the phylaBrachiopoda,[6]Bryozoa,[7]Chaetognatha,[8] andPhoronida[6] based on morphological and embryological characteristics. However, Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses, leading to the removal of thelophophorates which was later combined with other protostome animals to form the superphylumLophotrochozoa.[9] Thearrow worms may also be deuterostomes,[8] but molecular studies have placed them in the protostomes more often.[10][11] Genetic studies have also revealed that deuterostomes have more than 30 genes not found in any other animal groups, but which yet are present in some marine algae and prokaryotes. This could mean that these are ancient genes that were lost in other organisms, or that a common ancestor acquired them throughhorizontal gene transfer.[12]
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 deuterostomes, the developing embryo's first opening, theblastopore,becomes the anus, while the gut eventually tunnels through the embryo until it reaches the other side, forming an opening that becomes the mouth. This distinguishes them from protostomes, which have a variety of patterns of development.[16]
In both deuterostomes and protostomes, azygote first develops into a hollow ball of cells, called ablastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is calledradial cleavage, and also occurs in certain protostomes, such as thelophophorates.
Most deuterostomes displayindeterminate 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 ofidentical twins.
Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles orgill slits into thepharynx, which is also found in some primitive fossilechinoderms (mitrates).[17][18]
A hollow nerve cord is found in all chordates, includingtunicates (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 thehemichordates and thechordates have a thickening of theaorta, homologous to the chordateheart, which contracts to pump blood. This suggests a presence in the deuterostome ancestor of the three groups, with theechinoderms 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.[19]
Bilateria, one of the five major lineages of animals, is split into two groups; theprotostomes and deuterostomes. Deuterostomes consist of chordates (which include the vertebrates) and ambulacrarians.[20] It seems likely that the555 million year oldKimberella was a member of the protostomes.[21][22] That implies that the protostome and deuterostome lineages split long beforeKimberella appeared, and hence well before the start of the Cambrian538.8 million years ago,[20]i.e. during the earlier part of theEdiacaran Period (circa 635-539 Mya, around the end of globalMarinoan glaciation in the lateNeoproterozoic). It has been proposed that the ancestral deuterostome, before the chordate/ambulacrarian split, could have been a chordate-like animal with a terminal anus and pharyngeal openings but no gill slits, with active suspension feeding strategy.[23]
The last common ancestor of the deuterostomes had lost allinnexin diversity.[24]
Deuterostomes have a rich fossil record with thousands of fossil species being found throughout thePhanerozoic. There are also a few earlier fossils that may represent deuterostomes, but these remain debated. The earliest of these disputed fossils are thetunicate-like organismsBurykhia andAusia from theEdiacaran period. While these may in fact be tunicates, others have interpreted them ascnidarians[25] orsponges,[26] and as such their true affinity remains uncertain. Another Ediacaran fossil,Arkarua, may represent the earliest echinoderm, whileYanjiahella from the early Cambrian (Fortunian) period is another notable stem group echinoderm.[27][28]
Fossils of one major deuterostome group, theechinoderms (whose modern members includesea stars,sea urchins andcrinoids), are quite common from the start of Stage 3 of the Cambrian,521 million years ago[29] starting with forms such asHelicoplacus. Two other Cambrian Stage 3 (521-514 mya) species,Haikouichthys andMyllokunmingia from the Chengjiang biota, are the earliest body fossils of fish,[30][31] whereasPikaia, discovered much earlier but from the Mid CambrianBurgess Shale, is now regarded as a primitive chordate.[32] The MidCambrian fossilRhabdotubus johanssoni has been interpreted as apterobranch hemichordate,[33] whereasSpartobranchus 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 nobone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from thePermian-agedPaleobranchiostoma, trace fossils of theOrdovician colonial tunicateCatellocaula, and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians.[citation needed]. Fossils of Echinodermata are very common after the Cambrian. Fossils of Hemichordata are less common, except for graptolites until the Lower Carbonoferous.
As of 2024[update], the deuterostomes are considered to be monophyletic. The ancestral deuterostome was most likely abenthic worm that possessed a cartilaginous skeleton, a central nervous system, and gill slits.[34] Approximate dates for clades are given in millions of years ago (mya).[35]
^Bourlat, Sarah J.; Juliusdottir, Thorhildur; Lowe, Christopher J.; Freeman, Robert; Aronowicz, Jochanan; et al. (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida".Nature.444 (7115):85–88.Bibcode:2006Natur.444...85B.doi:10.1038/nature05241.PMID17051155.S2CID4366885.
^Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y. (2007), "New data onKimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications", in Vickers-Rich, Patricia; Komarower, Patricia (eds.),The Rise and Fall of the Ediacaran Biota, Special publications, vol. 286, London: Geological Society, pp. 157–179,doi:10.1144/SP286.12,ISBN978-1-86239-233-5,OCLC156823511
^Butterfield, N.J. (December 2006). "Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale".BioEssays.28 (12):1161–1166.doi:10.1002/bies.20507.PMID17120226.S2CID29130876.
^M. A. Fedonkin (1996). "Ausia as an ancestor of archeocyathans, and other sponge-like organisms". In:Enigmatic Organisms in Phylogeny and Evolution. Abstracts. Moscow, Paleontological Institute, Russian Academy of Sciences, p. 90-91.
^Bengtson, S.; Urbanek, A. (October 2007). "Rhabdotubus, a Middle Cambrian rhabdopleurid hemichordate".Lethaia.19 (4):293–308.doi:10.1111/j.1502-3931.1986.tb00743.x.
^Swalla, Billie J (21 November 2024). "Deuterostome Ancestors and Chordate Origins".Integrative and Comparative Biology.64 (5):1175–1181.doi:10.1093/icb/icae134.PMID39104213.
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