Taxonavigation
Superregnum: Eukaryota
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladi (2): Acoelomorpha - Nephrozoa
[source: Hejnol et al. (2009)]
References
Phylogeny
Balavoine, G. & Adoutte, A. 1998. One or three Cambrian radiations? Science 280: 397–398. doi: 10.1126/science.280.5362.397
Hejnol, A. et al. 2009 (online). Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society (B) doi: 10.1098/rspb.2009.0896
Jondelius, U., Ruiz-Trillo, I., Baguñà, J. & Riutort, M. 2002. The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes. Zoologica scripta 31: 201–215. doi: 10.1046/j.1463-6409.2002.00090.x
Lartillot, N. & Philippe, H. 2008. Improvement of molecular phylogenetic inference and the phylogeny of Bilateria. Philosophical transactions of the Royal Society (B) 363: 1463–1472. doi: 10.1098/rstb.2007.2236
Paps, J., Baguñà, J. & Riutort, M. 2009. Bilaterian phylogeny: a broad sampling of 13 nuclear genes provides a new Lophotrochozoa phylogeny and supports a paraphyletic basal Acoelomorpha. Molecular biology and evolution 26(10): 2397–2406. doi: 10.1093/molbev/msp150
Rieger, R. & Ladurner, P. 2001. Searching for the stem species of the Bilateria. Belgian journal of zoology 131(supplement 1): 27–34. ISSN: 0777-6276 PDF
Wallberg, A., Curini-Galletti, M., Ahmadzadeh, A. & Jondelius, U. 2007. Dismissal of Acoelomorpha: Acoela and Nemertodermatida are separate early bilaterian clades. Zoologica scripta 36: 509–523. doi: 10.1111/j.1463-6409.2007.00295.x
Zrzavý, J. 2002. Gastrotricha and metazoan phylogeny. Zoologica scripta 32: 61–81. doi: 10.1046/j.1463-6409.2003.00104.x
Vernacular names
български: Двустранносиметрични
català: Bilateri
Deutsch: Bilateria oder Zweiseitentiere
eesti: Kahekülgsed
English: Bilaterians
français: Bilatériens, Bilatéraux
한국어: 좌우대칭동물(左右對稱動物)
magyar: Kétoldali szimmetriájú állatok
македонски: Животни со билатерална симетрија
日本語: 左右相称動物上門 (さゆうそうしょうどうぶつじょうもん)
norsk: Bilaterier
occitan: Bilaterians
polski: Zwierzęta dwubocznie symetryczne
português: Bilaterados
русский: Двусторонне-симметричные
svenska: Tvåsidiga djur
Türkçe: Bilateral simetrililer , Bakışımlı simetrililer , Çift taraflı simetrililer
中文: 左右对称动物总纲
The Bilateria /ˌbaɪləˈtɪəriə/ or bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head ("anterior") and a tail ("posterior") as well as a back ("dorsal") and a belly ("ventral"); therefore they also have a left side and a right side. In contrast, radially symmetrical animals like jellyfish have a topside and a downside, but no identifiable front or back.[2]
The bilateria are a major group of animals, including the majority of phyla but not sponges, cnidarians, placozoans and ctenophores. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.
Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary body cavities (deriving from the blastocoel, as pseudocoel) or secondary cavities (that appear de novo, for example the coelom).[3][4]
Evolution
Illustration of the different types of symmetry in lifeforms (Field Museum, Chicago). Bilateral forms can have heads. Lifeforms with other types of symmetry have corresponding organs, if not a head.
The hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian".[5][6] The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid-aceloid hypothesis by Graff, Metchnikoff, Hyman, or Salvini Plawen), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla (flatworms and gastrotrichs) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as the Gastrea by Haeckel or Sedgwick, the Bilaterosgastrea by Gösta Jägersten (sv.), or the Trochaea by Nielsen).
The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.[7] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[8] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[9] Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and are believed to be at least 585 million years old.[10]
Phylogeny
See also: List of bilateral animal orders
There are two main superphyla (main lineages) of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and a few smaller phyla. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are a number of differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among the protostomes, Ecdysozoa[11] (molting animals) and Lophotrochozoa (also referred to as Spiralia).[11] Within the latter, some researchers also recognize another superphylum, Platyzoa,[12] while others reject the Platyzoa monophyly.[13][14][15] The arrow worms (Chaetognatha) have proven particularly difficult to classify, with some taxonomists placing them among the deuterostomes and others placing them among the protostomes. The two most recent studies to address the question of chaetognath origins support protostome/gnathifera affinities.[16][17][18]
A modern (2011) consensus phylogeny for Bilateria is shown below, although the position of certain clades are still controversial and the tree has changed considerably between 2000 and 2010.[19][20][21][22] Nodes marked with * have received broad consensus. A prominent alternative tree is championed by Nielsen (2001).[23][24][25] It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya).[26]
Evolutionary origin
The original bilateria are hypothesized to be a bottom dwelling worm with a single body opening (the blastopore).[27]
See also
Embryological origins of the mouth and anus
References
Martin, M. W.; Grazhdankin, D. V; Bowring, S. A; Evans, D. A; Fedonkin, M. A; Kirschvink, J. L. (5 May 2000). "Age of Neoproterozoic bilatarian [sic] body and trace fossils, White Sea, Russia: implications for metazoan evolution". Science. 288: 841–5. PMID 10797002. doi:10.1126/science.288.5467.841.
"Bilateria" (PDF).
"Probable ancestor of cephalochordates" (PDF). PENICHEFOSSIL. Retrieved 9 January 2017.
"Ediacaran fauna worms". Walter Jahn. Retrieved 9 January 2017. – Suny Orange
Knoll, Andrew H.; Carroll, Sean B. (25 June 1999). "Early Animal Evolution: Emerging Views from Comparative Biology and Geology". Science. 284 (5423): 2129–2137. PMID 10381872. doi:10.1126/science.284.5423.2129.
Balavoine, G.; Adoutte, Andre (2003). "The segmented Urbilateria: A testable scenario". Integrative and Comparative Biology. 43 (1): 137–147. doi:10.1093/icb/43.1.137.
For references see Ediacara biota
For references see Vernanimalcula
For references see Fossil embryos
Pecoits, E.; Konhauser, K. O.; Aubet, N. R.; Heaman, L. M.; Veroslavsky, G.; Stern, R. A.; Gingras, M. K. (June 29, 2012). "Bilaterian burrows and grazing behavior at >585 million years ago". Science. 336 (6089): 1693–1696. doi:10.1126/science.1216295.
Halanych, K.; Bacheller, J.; Aguinaldo, A.; Liva, S.; Hillis, D.; Lake, J. (17 March 1995). "Evidence from 18S ribosomal DNA that the lophophorates are protostome animals". Science. 267 (5204): 1641–1643. PMID 7886451. doi:10.1126/science.7886451.
Giribet, Gonzalo; at al (September 2000). "Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology". Systematic Biology. 49 (3): 539–62. PMID 12116426. doi:10.1080/10635159950127385.
Paps, J.; Baguna, J.; Riutort, M. (14 July 2009). "Bilaterian phylogeny: a broad sampling of 13 nuclear genes provides a new Lophotrochozoa phylogeny and supports a paraphyletic basal Acoelomorpha". Molecular Biology and Evolution. 26 (10): 2397–2406. PMID 19602542. doi:10.1093/molbev/msp150.
Telford, Maximilian J. (15 April 2008). "Resolving animal phylogeny: a sledgehammer for a tough nut?". Developmental Cell. 14 (4): 457–459. PMID 18410719. doi:10.1016/j.devcel.2008.03.016.
The Invertebrate Animals
Helfenbein, Kevin G.; Fourcade, H. Matthew; Vanjani, Rohit G.; Boore, Jeffrey L. (20 July 2004). "The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes". Proceedings of the National Academy of Sciences of the United States of America. 101 (29): 10639–10643. PMC 489987 Freely accessible. PMID 15249679. doi:10.1073/pnas.0400941101.
Papillon, Daniel; Perez, Yvan; Caubit, Xavier; Yannick Le, Parco (November 2004). "Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome". Molecular Biology and Evolution. 21 (11): 2122–2129. PMID 15306659. doi:10.1093/molbev/msh229.
Fröbius, Andreas C.; Funch, Peter (2017-04-04). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". Nature Communications. 8 (1). ISSN 2041-1723. doi:10.1038/s41467-017-00020-w.
Edgecombe, Gregory D.; Giribet, Gonzalo; Dunn, Casey W.; Hejnol, Andreas; Kristensen, Reinhardt M.; Neves, Ricardo C.; Rouse, Greg W.; Worsaae, Katrine; Sørensen, Martin V. (June 2011). "Higher-level metazoan relationships: recent progress and remaining questions". Organisms, Diversity & Evolution. 11 (2): 151–172. doi:10.1007/s13127-011-0044-4.
Fröbius, Andreas C.; Funch, Peter (2017-04-04). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". Nature Communications. 8 (1). ISSN 2041-1723. doi:10.1038/s41467-017-00020-w.
Smith, Martin R.; Ortega-Hernández, Javier. "Hallucigenia’s onychophoran-like claws and the case for Tactopoda". Nature. 514 (7522): 363–366. doi:10.1038/nature13576.
"Palaeos Metazoa: Ecdysozoa". palaeos.com. Retrieved 2017-09-02.
Nielsen, C. (2002). Animal Evolution: Interrelationships of the Living Phyla (2nd ed.). England: Oxford University Press. ISBN 0-19-850682-1.
"Bilateria". Tree of Life Web Project. 2001. Retrieved August 11, 2014.
"Palaeos Metazoa: Ecdysozoa". palaeos.com. Retrieved 2017-09-02.
Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (2008-04-27). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 363 (1496): 1435–1443. ISSN 0962-8436. PMID 18192191. doi:10.1098/rstb.2007.2233.
Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas. "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. doi:10.1038/nature16520.
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