Editing Bilateria (section)

=== Phylogeny ===

{{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.<ref name="Cat of Life">{{cite web |title=Animalia |url=https://www.catalogueoflife.org/ |website=The Catalogue of Life |access-date=18 February 2025}}</ref> The group has traditionally been divided into two main lineages or [[superphyla]].<ref name=Nielsen-2008>{{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 }}</ref> The deuterostomes traditionally include the [[echinoderm]]s, [[hemichordate]]s, [[chordate]]s, and the extinct [[Vetulicolia]]. The protostomes include most of the rest, such as [[arthropod]]s, [[annelid]]s, [[mollusc]]s, 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 [[Taxonomy (biology)|taxonomists]] now recognise at least two more superphyla among the protostomes, [[Ecdysozoa]]<ref name="Halanych 1641–1643">{{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 }}</ref> and [[Spiralia]].<ref name="Halanych 1641–1643"/><ref>{{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}}</ref><ref>{{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 }}</ref> The arrow worms ([[Chaetognatha]]) have proven difficult to classify; recent studies place them in the [[gnathifera (clade)|Gnathifera]].<ref>{{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 }}</ref><ref>{{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 }}</ref><ref name="Fröbius">{{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 }}</ref>

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.<ref name=Nielsen-2008/><ref name="Hejnol 2009">{{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}}</ref><ref>{{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 }}</ref> The latter clade was called Nephrozoa by Jondelius et al. (2002) and [[Eubilateria]] by Baguña and Riutort (2004).<ref name=Nielsen-2008/> 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.<ref name=Nielsen-2008/><ref name=Cannon-2016>{{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 }}</ref> Subsequently, the acoelomorphs were placed in phylum Xenacoelomorpha, together with the [[xenoturbellid]]s, and the sister relationship between Xenacoelomorpha and Nephrozoa supported in phylogenomic analyses.<ref name=Cannon-2016/>

A [[cladogram]] for Bilateria under the Nephrozoa hypothesis from a 2014 review by Casey Dunn and colleagues, is shown below.<ref name="Dunn Giribet Edgecombe Hejnol 2014">{{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}}</ref> The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).<ref>{{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 B|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}}</ref> 

{{clade |style=font-size:85%;line-height:85%
|label1='''Bilateria''' 
|1={{clade
   |label1=[[Xenacoelomorpha]] 
   |1={{clade
      |1=[[Xenoturbellida]] [[File:Xenoturbella japonica.jpg|70 px]]
      |2=[[Acoelomorpha]] [[File:Proporus sp.png|60 px]]
      }}
   |label2= [[Nephrozoa]] 
   |sublabel2=650 Mya
   |2={{clade
      |label1=[[Deuterostomia]] 
      |sublabel1=''anus before mouth''
      |1={{clade
         |1=[[Ambulacraria]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
         |2=[[Chordata]] [[File:Blueback herring fish (white background).jpg|60 px]]
         }}
      |label2= [[Protostomia]] 
      |sublabel2=610 mya
      |2={{clade
         |1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|60 px]]
         |2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|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).<ref>{{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 }}</ref> 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".<ref name="PhilippePoustka2019">{{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 }}</ref> As of 2024, the issue of which hypothesis is correct remains unresolved.<ref>{{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}}</ref><ref>{{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}}</ref>

Cladogram showing Xenambulacraria hypothesis with a [[Paraphyly|paraphyletic]] Deuterostomia:<ref name="Mulhair McCarthy 2022">{{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 }}</ref>

{{clade
|style=font-size:85%;line-height:85%
|label1= '''Bilateria''' 
|1={{clade
   |label1=[[Xenambulacraria]] 
   |sublabel1=
   |1={{clade
      |1=[[Xenacoelomorpha]] [[File:Xenoturbella japonica.jpg|70 px]]
      |2=[[Ambulacraria]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
      }}
   |label2=[[Centroneuralia]] 
   |sublabel2=
   |2={{clade
      |1=[[Chordata]] [[File:Blueback herring fish (white background).jpg|60 px]]
      |label2=[[Protostomia]] 
      |2={{clade
         |1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|60 px]]
         |2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|60 px]]
         }}
      }}
   }}
}}

Cladogram showing hypothesis of Xenambulacraria within a monophyletic Deuterostomia:<ref name="Mulhair McCarthy 2022"/>

{{clade
|style=font-size:85%;line-height:85%
|label1='''Bilateria'''
|1={{clade
   |label1=[[Deuterostomia]]
   |1={{clade
      |label1=[[Xenambulacraria]]
      |1={{clade
         |1=[[Xenacoelomorpha]] [[File:Xenoturbella japonica.jpg|70 px]]
         |2=[[Ambulacraria]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
         }}
      |2=[[Chordata]] [[File:Blueback herring fish (white background).jpg|60 px]]
      }}
   |label2=[[Protostomia]] 
   |2={{clade
      |1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|60 px]]
      |2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|60 px]]
      }}
   }}
}}