Bilateria#Phylogeny

Animals with a bilaterally symmetric body plan that mainly move in one direction have a head end (anterior) and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.{{cite book |chapter-url=http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |url-status=dead |archive-url=https://web.archive.org/web/20200605145237/http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |archive-date=5 June 2020 |chapter=Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |title=Invertebrates |last=Brusca |first=Richard C. |date=2016 |publisher=Sinauer Associates |pages=345–372 |isbn=978-1-60535-375-3 }} Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth.{{cite journal |last=Finnerty |first=John R. |title=Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals? |journal=BioEssays |date=November 2005 |volume=27 |issue=11 |pages=1174–1180 |doi=10.1002/bies.20299 |pmid=16237677 |url=http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf |access-date=2018-03-07 |archive-url=https://web.archive.org/web/20190702004055/http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf |archive-date=2019-07-02 |url-status=dead }} Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus (sometimes called a "through gut"{{Cite journal |last1=Nielsen |first1=Claus |last2=Brunet |first2=Thibaut |last3=Arendt |first3=Detlev |date=2018-08-22 |title=Evolution of the bilaterian mouth and anus |url=https://www.nature.com/articles/s41559-018-0641-0 |journal=Nature Ecology & Evolution |language=en |volume=2 |issue=9 |pages=1358–1376 |doi=10.1038/s41559-018-0641-0 |pmid=30135501 |bibcode=2018NatEE...2.1358N |issn=2397-334X|url-access=subscription }}), and sometimes a wormlike body plan with a hydrostatic skeleton. Xenacoelomorphs, on the other hand, have a bag gut with one opening. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells.{{cite book |last=Minelli |first=Alessandro |title=Perspectives in Animal Phylogeny and Evolution |url=https://books.google.com/books?id=jIASDAAAQBAJ&pg=PA53 |year=2009 |publisher=Oxford University Press |isbn=978-0-19-856620-5 |page=53}}

Some bilaterians have only weakly condensed nerve nets (similar to those in cnidarians), while others have either a ventral nerve cord, a dorsal nerve cord, or both (e.g. in Hemichordate).{{cite journal | pmc=5756474 | date=2017 | last1=Martín-Durán | first1=J. M. | last2=Pang | first2=K. | last3=Børve | first3=A. | last4=Semmler Lê | first4=H. | last5=Furu | first5=A. | last6=Cannon | first6=J. T. | last7=Jondelius | first7=U. | last8=Hejnol | first8=A. | title=Convergent evolution of bilaterian nerve cords | journal=Nature | volume=553 | issue=7686 | pages=45–50 | doi=10.1038/nature25030 | pmid=29236686 }}

{{main|Urbilaterian}}

The hypothetical most recent common ancestor of all Bilateria is termed the 'urbilaterian'. The nature of this first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff, Elie Metchnikoff, Libbie Hyman, or {{ill|Luitfried von Salvini-Plawen|nl}}). This means that the urbilaterian had a solid body, and all body cavities therefore secondarily arose later in different groups. The other side poses that the urbilaterian had a coelom, meaning that the main acoelomate phyla (flatworms and gastrotrichs) have secondarily lost their body cavities.{{cite journal |last1=Knoll |first1=Andrew H. |last2=Carroll |first2=Sean B. |author2-link=Sean B. Carroll |s2cid=8908451 |date=25 June 1999 |title=Early Animal Evolution: Emerging Views from Comparative Biology and Geology |journal=Science |volume=284 |issue=5423 |pages=2129–2137 |doi=10.1126/science.284.5423.2129 |pmid=10381872}}{{cite journal |last1=Balavoine |first1=G. |last2=Adoutte |first2=Andre |title=The segmented Urbilateria: A testable scenario |journal=Integrative and Comparative Biology |date=2003 |volume=43 |issue=1 |pages=137–147 |doi=10.1093/icb/43.1.137 |pmid=21680418 |citeseerx=10.1.1.560.8727 |s2cid=80975506 }}

This is the Archicoelomata hypothesis first proposed by A. T. Masterman in 1899.{{cite journal |last=Masterman |first=A. T. |title=On the Theory of Archimeric Segmentation and its bearing upon the Phyletic Classification of the Cœlomata |journal=Proceedings of the Royal Society of Edinburgh |volume=22 |date=1899 |doi=10.1017/S0370164600051245 |pages=270–310}} Variations of the Archicoelomata hypothesis are the Gastraea by Ernst Haeckel in 1872{{cite journal |last1=Levit |first1=Georgy S. |last2=Hoßfeld |first2=Uwe |last3=Naumann |first3=Benjamin |last4=Lukas |first4=Paul |last5=Olsson |first5=Lennart |title=The biogenetic law and the Gastraea theory: From Ernst Haeckel's discoveries to contemporary views |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=338 |issue=1–2 |date=2022 |doi=10.1002/jez.b.23039 |doi-access=free |pages=13–27|pmid=33724681 |bibcode=2022JEZB..338...13L }} or Adam Sedgwick, and more recently the Bilaterogastrea by {{Interlanguage link|Gösta Jägersten|sv}},{{cite journal |last=Olsson |first=Lennart |title=A clash of traditions: the history of comparative and experimental embryology in Sweden as exemplified by the research of Gösta Jägersten and Sven Hörstadius |journal=Theory in Biosciences |volume=126 |issue=4 |date=2007 |doi=10.1007/s12064-007-0008-6 |pages=117–129|pmid=18008099 }} and the Trochaea by Claus Nielsen.{{cite journal |last=Nielsen |first=Claus |title=Animal phylogeny in the light of the trochaea theory |journal=Biological Journal of the Linnean Society |volume=25 |issue=3 |date=1985 |doi=10.1111/j.1095-8312.1985.tb00396.x |pages=243–299}}

File:Xenoturbella japonica.jpg.]]

One proposal, by Johanna Taylor Cannon and colleagues, is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella.{{Cite journal |last1=Cannon |first1=Johanna Taylor |last2=Vellutini |first2=Bruno Cossermelli |last3=Smith |first3=Julian |last4=Ronquist |first4=Fredrik |last5=Jondelius |first5=Ulf |last6=Hejnol |first6=Andreas |title=Xenacoelomorpha is the sister group to Nephrozoa |journal=Nature |volume=530 |issue=7588 |pages=89–93 |doi=10.1038/nature16520 |pmid=26842059 |year=2016 |bibcode=2016Natur.530...89C |s2cid=205247296 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 }} An alternative proposal, by Jaume Baguñà and colleagues, is that it may have resembled the planula larvae of some cnidarians, which unlike the radially-symmetric adults have some bilateral symmetry.{{Cite journal |last1=Baguñà |first1=Jaume |last2=Martinez |first2=Pere |last3=Paps |first3=Jordi |last4=Riutort |first4=Marta |date=April 2008 |title=Back in time: a new systematic proposal for the Bilateria |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=363 |issue=1496 |pages=1481–1491 |doi=10.1098/rstb.2007.2238 |pmc=2615819 |pmid=18192186}} However, Lewis I. Held presents evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).{{cite book |last=Held |first=Lewis I. |author-link=Lewis I. Held |title=How the Snake Lost its Legs. Curious Tales from the Frontier of Evo-Devo |title-link=How the Snake Lost its Legs |date=2014 |publisher=Cambridge University Press |isbn=978-1-107-62139-8 |page=11 }}

Bilaterians, presumably including the urbilaterian, share many more Hox genes controlling the development of their more complex bodies, including of their heads, than do the Cnidaria and the Acoelomorpha.{{cite journal |last1=Hombría |first1=James C.-G. |last2=García-Ferrés |first2=Mar |last3=Sánchez-Higueras |first3=Carlos |title=Anterior Hox Genes and the Process of Cephalization |journal=Frontiers in Cell and Developmental Biology |volume=9 |date=5 August 2021 |pmid=34422836 |pmc=8374599 |doi=10.3389/fcell.2021.718175 |doi-access=free}}

File:Ikaria wariootia.jpg, living 571–539 million years ago, is one of the oldest bilaterians identified.{{cite journal |last1=Evans |first1=Scott D. |last2=Hughes |first2=Ian V. |last3=Gehling |first3=James G. |last4=Droser |first4=Mary L. |title=Discovery of the oldest bilaterian from the Ediacaran of South Australia |journal=Proceedings of the National Academy of Sciences |date=7 April 2020 |volume=117 |issue=14 |pages=7845–7850 |doi=10.1073/pnas.2001045117 |pmid=32205432 |pmc=7149385 |bibcode=2020PNAS..117.7845E |doi-access=free }}]]

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 {{Ma|555}}.{{Cite journal |last1=Fedonkin |first1=M. A. |last2=Waggoner |first2=B. M. |date=November 1997 |title=The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism |journal=Nature |volume=388 |issue=6645 |pages=868–871 |doi=10.1038/42242 |bibcode=1997Natur.388..868F|s2cid=4395089 |doi-access=free }} Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.{{cite journal |last1=Bengtson |first1=S. |last2=Budd |first2=G. |date=19 November 2004 |title=Comment on 'small bilaterian fossils from 40 to 55 million years before the Cambrian'. |journal=Science |volume=306 |pages=1291a |doi=10.1126/science.1101338 |pmid=15550644 |issue=5700 |doi-access=free }}{{cite journal |last1=Bengtson |first1=S. |last2=Donoghue |first2=P. C. J. |last3=Cunningham |first3=J. A. |author4=Yin, C. |year=2012 |title=A merciful death for the 'earliest bilaterian,' Vernanimalcula |journal=Evolution & Development |volume=14 |issue=5 |pages=421–427 |doi=10.1111/j.1525-142X.2012.00562.x |pmid=22947315 |s2cid=205675058 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-597 |url-access=subscription }} Fossil embryos are known from around the time of Vernanimalcula ({{Ma|580}}), but none of these have bilaterian affinities.{{Cite journal |last1=Hagadorn |first1=J. W. |last2=Xiao |first2=S. |last3=Donoghue |first3=P. C. J. |last4=Bengtson |first4=S. |last5=Gostling |first5=N. J. |last6=Pawlowska |first6=M. |last7=Raff |first7=E. C. |last8=Raff |first8=R. A. |last9=Turner |first9=F. R. |last10=Chongyu |doi=10.1126/science.1133129 |first10=Y. |last11=Zhou |first11=C. |last12=Yuan |first12=X. |last13=McFeely |first13=M. B. |last14=Stampanoni |first14=M. |last15=Nealson |first15=K. H. |s2cid=25112751 |title=Cellular and Subcellular Structure of Neoproterozoic Animal Embryos |journal=Science |volume=314 |issue=5797 |pages=291–294 |date=13 October 2006 |pmid= 17038620|bibcode=2006Sci...314..291H }} Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old.{{cite journal |title=Bilaterian burrows and grazing behavior at >585 million years ago |last1=Pecoits |first1=E. |last2=Konhauser |first2=K. O. |last3=Aubet |first3=N. R. |last4=Heaman |first4=L. M. |last5=Veroslavsky |first5=G. |last6=Stern |first6=R. A. |last7=Gingras |first7=M. K. |s2cid=27970523 |journal=Science |volume=336 |issue=6089 |pages=1693–1696 |doi=10.1126/science.1216295 |pmid=22745427 |date=June 29, 2012 |bibcode=2012Sci...336.1693P}} However, more recent evidence shows these fossils are actually late Paleozoic, not Ediacaran.{{cite journal |last1=Verde |first1=Mariano |title=Revisiting the supposed oldest bilaterian trace fossils from Uruguay: Late Paleozoic, not Ediacaran |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=15 September 2022 |volume=602 |doi=10.1016/j.palaeo.2022.111158 |bibcode=2022PPP...60211158V |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018222003285|url-access=subscription }}

{{further|Protostome|Deuterostome}}

{{See also|List of bilateral animal orders}}

The Bilateria are now by far the most successful animal lineage, with over 98% of known animal species.{{cite web |title=Animalia |url=https://www.catalogueoflife.org/ |website=The Catalogue of Life |access-date=18 February 2025}} The group has traditionally been divided into two main lineages or superphyla.{{cite journal |last=Nielsen |first=Claus |title=Six major steps in animal evolution: are we derived sponge larvae? |journal=Evolution and Development |year=2008 |volume=10 |issue=2 |pages=241–257 |doi=10.1111/j.1525-142X.2008.00231.x |pmid=18315817 |s2cid=8531859 }} The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, molluscs, and flatworms. There are several 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 recognise at least two more superphyla among the protostomes, Ecdysozoa{{cite journal |last1=Halanych |first1=K. |author2=Bacheller, J. |author3=Aguinaldo, A. |author4=Liva, S. |author5=Hillis, D. |author6=Lake, J. |date=17 March 1995 |title=Evidence from 18S ribosomal DNA that the lophophorates are protostome animals |journal=Science |volume=267 |issue=5204 |pages=1641–1643 |doi=10.1126/science.7886451 |pmid=7886451 |bibcode=1995Sci...267.1641H |s2cid=12196991 }} and Spiralia.{{cite journal |last1=Paps |first1=J. |author2=Baguna, J. |author3=Riutort, M. |title=Bilaterian phylogeny: a broad sampling of 13 nuclear genes provides a new Lophotrochozoa phylogeny and supports a paraphyletic basal Acoelomorpha |journal=Molecular Biology and Evolution |date=14 July 2009 |volume=26 |issue=10 |pages=2397–2406 |doi=10.1093/molbev/msp150 |doi-access=free |pmid=19602542}}{{cite journal |last=Telford |first=Maximilian J. |date=15 April 2008 |title=Resolving animal phylogeny: A sledgehammer for a tough nut? |journal=Developmental Cell |volume=14 |issue=4 |pages=457–459 |doi=10.1016/j.devcel.2008.03.016 |pmid=18410719 |doi-access=free }} The arrow worms (Chaetognatha) have proven difficult to classify; recent studies place them in the Gnathifera.{{cite journal |last1=Helfenbein |first1=Kevin G. |last2=Fourcade |first2=H. Matthew |last3=Vanjani |first3=Rohit G. |last4=Boore |first4=Jeffrey L. |date=20 July 2004 |title=The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=101 |issue=29 |pages=10639–10643 |doi=10.1073/pnas.0400941101|pmid=15249679 |pmc=489987 |bibcode=2004PNAS..10110639H |doi-access=free }}{{cite journal |last1=Papillon |first1=Daniel |last2=Perez |first2=Yvan |last3=Caubit |first3=Xavier |last4=Yannick Le |first4=Parco |date=November 2004 |title=Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome |journal=Molecular Biology and Evolution |volume=21 |issue=11 |pages=2122–2129 |doi=10.1093/molbev/msh229|pmid=15306659 |doi-access=free }}{{Cite journal |last1=Fröbius |first1=Andreas C. |last2=Funch |first2=Peter |date=2017-04-04 |title=Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans |journal=Nature Communications |volume=8 |issue=1 |pages=9 |doi=10.1038/s41467-017-00020-w |pmid=28377584 |pmc=5431905 |bibcode=2017NatCo...8....9F }}

The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence supported a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha), and the remaining bilaterians.{{cite book |chapter-url=https://www.researchgate.net/publication/230766195 |chapter=The mouth, the anus, and the blastopore - open questions about questionable openings |title=Animal Evolution — Genomes, Fossils, and Trees |year=2009 |editor=M. J. Telford |editor2=D. T. J. Littlewood |last1=Hejnol |first1= A. |last2=Martindale |first2=M. Q. |pages=33–40}}{{cite journal |doi=10.1038/s41559-016-0005 |title=The developmental basis for the recurrent evolution of deuterostomy and protostomy |journal=Nature Ecology & Evolution| volume=1 |page=0005 |year=2016 |last1=Martín-Durán |first1=José M. |last2=Passamaneck |first2=Yale J. |last3=Martindale |first3=Mark Q. |last4=Hejnol |first4=Andreas |issue=1 |url=https://qmro.qmul.ac.uk/xmlui/handle/123456789/54816 |pmid=28812551 |s2cid=90795 }} The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004). The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in the Nephrozoa.{{Cite journal |last1=Cannon |first1=Johanna Taylor |last2=Vellutini |first2=Bruno Cossermelli |last3=Smith |first3=Julian |last4=Ronquist |first4=Fredrik |last5=Jondelius |first5=Ulf |last6=Hejnol |first6=Andreas |title=Xenacoelomorpha is the sister group to Nephrozoa |year=2016 |journal=Nature |volume=530 |issue=7588 |pages=89–93 |doi=10.1038/nature16520|pmid=26842059 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 |bibcode=2016Natur.530...89C |s2cid=205247296 }} Subsequently, the acoelomorphs were placed in phylum Xenacoelomorpha, together with the xenoturbellids, and the sister relationship between Xenacoelomorpha and Nephrozoa supported in phylogenomic analyses.

A cladogram for Bilateria under the Nephrozoa hypothesis from a 2014 review by Casey Dunn and colleagues, is shown below.{{cite journal |last1=Dunn |first1=Casey W. |last2=Giribet |first2=Gonzalo |last3=Edgecombe |first3=Gregory D. |last4=Hejnol |first4=Andreas |title=Animal Phylogeny and Its Evolutionary Implications |journal=Annual Review of Ecology, Evolution, and Systematics |volume=45 |issue=1 |date=23 November 2014 |doi=10.1146/annurev-ecolsys-120213-091627 |pages=371–395}} The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).{{Cite journal |last1=Peterson |first1=Kevin J. |last2=Cotton |first2=James A.|last3=Gehling |first3=James G. |last4=Pisani |first4=Davide |date=2008-04-27 |title=The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records |journal=Philosophical Transactions of the Royal Society of London B: Biological Sciences |volume=363 |issue=1496 |pages=1435–1443 |doi=10.1098/rstb.2007.2233 |pmid=18192191 |pmc=2614224 |df=dmy-all}}

{{clade |style=font-size:85%;line-height:85%

|label1=Bilateria

|1={{clade

|label1=Xenacoelomorpha

|1={{clade

|1=Xenoturbellida 70 px

|2=Acoelomorpha 60 px

}}

|label2= Nephrozoa

|sublabel2=650 Mya

|2={{clade

|label1=Deuterostomia

|sublabel1=anus before mouth

|1={{clade

|1=Ambulacraria 60 px

|2=Chordata 60 px

}}

|label2= Protostomia

|sublabel2=610 mya

|2={{clade

|1=Ecdysozoa 60 px

|2=Spiralia 60 px

}}

}}

}}

}}

A different hypothesis is that Ambulacraria is sister to Xenacoelomorpha, together forming Xenambulacraria. Xenambulacraria may be sister to Chordata or to Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or, as shown here, to Chordata + Protostomia).{{Cite journal |last1=Kapli |first1=Paschalia |last2=Natsidis |first2=Paschalis |last3=Leite |first3=Daniel J. |last4=Fursman |first4=Maximilian |last5=Jeffrie |first5=Nadia |last6=Rahman |first6=Imran A. |last7=Philippe |first7=Hervé |last8=Copley |first8=Richard R. |last9=Telford |first9=Maximilian J. |display-authors=5 |date=2021-03-19 |title=Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria |journal=Science Advances |volume=7 |issue=12 |pages=eabe2741 |doi=10.1126/sciadv.abe2741 |pmc=7978419 |pmid=33741592 |bibcode=2021SciA....7.2741K }} A 2019 study by Hervé Philippe and colleagues presents the tree, cautioning that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy".{{cite journal |last1=Philippe |first1=Hervé |last2=Poustka |first2=Albert J. |last3=Chiodin |first3=Marta |last4=Hoff |first4=Katharina J. |last5=Dessimoz |first5=Christophe |last6=Tomiczek |first6=Bartlomiej |last7=Schiffer |first7=Philipp H. |last8=Müller |first8=Steven |last9=Domman |first9=Daryl |last10=Horn |first10=Matthias |last11=Kuhl |first11=Heiner |last12=Timmermann |first12=Bernd |last13=Satoh |first13=Noriyuki |last14=Hikosaka-Katayama |first14=Tomoe |last15=Nakano |first15=Hiroaki |last16=Rowe |first16=Matthew L. |last17=Elphick |first17=Maurice R. |last18=Thomas-Chollier |first18=Morgane |last19=Hankeln |first19=Thomas |last20=Mertes |first20=Florian |last21=Wallberg |first21=Andreas |last22=Rast |first22=Jonathan P. |last23=Copley |first23=Richard R. |last24=Martinez |first24=Pedro |last25=Telford |first25=Maximilian J. |display-authors=5 |title=Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria |journal=Current Biology |volume=29 |issue=11 |date=2019 |doi=10.1016/j.cub.2019.04.009 |doi-access=free |pages=1818–1826.e6|pmid=31104936 |bibcode=2019CBio...29E1818P |hdl=21.11116/0000-0004-DC4B-1 |hdl-access=free }} As of 2024, the issue of which hypothesis is correct remains unresolved.{{Cite journal |last1=Abalde |first1=Samuel |last2=Jondelius |first2=Ulf |date=2025-02-10 |editor-last=Whelan |editor-first=Nathan |title=A Phylogenomic Backbone for Acoelomorpha Inferred From Transcriptomic Data |url=https://academic.oup.com/sysbio/article/74/1/70/7841810 |journal=Systematic Biology |volume=74 |issue=1 |pages=70–85 |doi=10.1093/sysbio/syae057 |issn=1063-5157 |pmc=11809588 |pmid=39451056}}{{Cite journal |last1=Robertson |first1=Helen E. |last2=Sebé-Pedrós |first2=Arnau |last3=Saudemont |first3=Baptiste |last4=Loe-Mie |first4=Yann |last5=Zakrzewski |first5=Anne-C. |last6=Grau-Bové |first6=Xavier |last7=Mailhe |first7=Marie-Pierre |last8=Schiffer |first8=Philipp |last9=Telford |first9=Maximilian J. |last10=Marlow |first10=Heather |date=2024-03-19 |title=Single cell atlas of Xenoturbella bocki highlights limited cell-type complexity |journal=Nature Communications |volume=15 |issue=1 |page=2469 |bibcode=2024NatCo..15.2469R |doi=10.1038/s41467-024-45956-y |issn=2041-1723 |pmc=10951248 |pmid=38503762}}

Cladogram showing Xenambulacraria hypothesis with a paraphyletic Deuterostomia:{{Cite journal |last1=Mulhair |first1=Peter O. |last2=McCarthy |first2=Charley G.P. |last3=Siu-Ting |first3=Karen |last4=Creevey |first4=Christopher J. |last5=O’Connell |first5=Mary J. |date=December 2022 |title=Filtering artifactual signal increases support for Xenacoelomorpha and Ambulacraria sister relationship in the animal tree of life |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982222016840 |journal=Current Biology |volume=32 |issue=23 |pages=5180–5188.e3 |doi=10.1016/j.cub.2022.10.036|pmid=36356574 |bibcode=2022CBio...32E5180M }}

{{clade

|style=font-size:85%;line-height:85%

|label1= Bilateria

|1={{clade

|label1=Xenambulacraria

|sublabel1=

|1={{clade

|1=Xenacoelomorpha 70 px

|2=Ambulacraria 60 px

}}

|label2=Centroneuralia

|sublabel2=

|2={{clade

|1=Chordata 60 px

|label2=Protostomia

|2={{clade

|1=Ecdysozoa 60 px

|2=Spiralia 60 px

}}

}}

}}

}}

Cladogram showing hypothesis of Xenambulacraria within a monophyletic Deuterostomia:

{{clade

|style=font-size:85%;line-height:85%

|label1=Bilateria

|1={{clade

|label1=Deuterostomia

|1={{clade

|label1=Xenambulacraria

|1={{clade

|1=Xenacoelomorpha 70 px

|2=Ambulacraria 60 px

}}

|2=Chordata 60 px

}}

|label2=Protostomia

|2={{clade

|1=Ecdysozoa 60 px

|2=Spiralia 60 px

}}

}}

}}

The Bilateria were named by the Austrian embryologist Berthold Hatschek in 1888. In his classification, the group included the Zygoneura, Ambulacraria, and Chordonii (the Chordata).{{cite journal |last=Nielsen |first=Claus |title=The 'new phylogeny'. What is new about it? |journal=Annual Review of Ecology, Evolution and Systematics |volume=35 |year=2009 |pages=229–256 |url=https://www.palaeodiversity.org/pdf/03Suppl/Supplement_Nielsen.pdf}}{{cite book |last=Hatschek |first=Berthold |author-link=Berthold Hatschek |year=1888 |title=Lehrbuch der Zoologie |edition=1st |language=de |pages=1–144 |location=Jena |publisher=Gustav Fischer}} In 1910, the Austrian zoologist Karl Grobben renamed the Zygoneura to Protostomia, and created the Deuterostomia to encompass the Ambulacraria and Chordonii.{{cite book |last1=Grobben |first1=Karl |author1-link=Karl Grobben |last2=Claus |first2=Carl Friedrich Wilhelm |author2-link=Carl Friedrich Wilhelm Claus |year=1910 |title=Lehrbuch der Zoologie |edition=2nd |location=Marburg |publisher=Elvert'sche Verlagsbuchhandlung}}

• Embryological origins of the mouth and anus

{{notelist}}

{{Reflist|30em}}

• [http://tolweb.org/tree?group=Bilateria&contgroup=Animals Tree of Life web project — Bilateria] {{Webarchive|url=https://web.archive.org/web/20201116111252/http://tolweb.org/tree?group=Bilateria&contgroup=Animals |date=2020-11-16 }}
• [http://www.ucmp.berkeley.edu/phyla/metazoasy.html University of California Museum of Paleontology — Systematics of the Metazoa]

{{Eukaryota classification}}

{{Animalia}}

{{Taxonbar|from=Q5173}}

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Bilaterians

Category:Subkingdoms

Category:Ediacaran first appearances

Category:Taxa named by Berthold Hatschek