Editing Placozoa (section)

===Eumetazoa hypothesis===
A third hypothesis, based primarily on molecular genetics, views the Placozoa as highly simplified [[eumetazoans]]. According to this, ''Trichoplax adhaerens'' is descended from considerably more complex animals that already had muscles and nerve tissues. Both tissue types, as well as the basal lamina of the [[epithelium]], were accordingly lost more recently by radical secondary simplification.<ref>{{cite book |last=Pechenik |first=Jan |year=2015 |title=Biology of the Invertebrates |chapter=The Poriferans and Placozoans |edition=7 |page=90 |publisher=McGraw-Hill Education |isbn=978-0073524184 }}</ref>

Various studies in this regard so far yield differing results for identifying the exact sister group: In one case, the Placozoa would qualify as the nearest relatives of the [[Cnidaria]], while in another they would be a sister group to the [[Ctenophora]], and occasionally they are placed directly next to the [[Bilateria]]. Currently, they are typically placed according to the cladogram below:<ref>{{cite book |last=Layden |first=Michael J. |year=2018 |publication-date=14 February 2018 |chapter=Cnidarian Zic genes |department=Zic family in animal evolution and development |editor-first=Jun |editor-last=Aruga |title=Zic Family: Evolution, development, and disease |edition=1st |series=Advances in Experimental Medicine and Biology |volume=1046 |pages=27–39 |publisher=Springer Nature Singapore |place=Singapore |lang=en |doi=10.1007/978-981-10-7311-3_2 |pmid=29442315 |issn=0065-2598 |isbn=978-981-10-7310-6 }}</ref>

{{clade
  |label1= [[Metazoa]] 
  |sublabel1= 
  |1={{clade
     |1=[[Porifera]]
     |label2= [[Eumetazoa]] 
     |sublabel2= [[Diploblast]]s / <br/> [[Epitheliozoa]] 
     |2={{clade
        |1=[[Ctenophora]]
        |label2= [[ParaHoxozoa]] 
        |2={{clade
           |1=[[Placozoa]]
           |label2= [[Planulozoa]] 
           |2={{clade
              |1=[[Cnidaria]]
              |2=[[Bilateria]] / [[Bilateria|Triploblasts]]
           }}
        }}
     }}
  }}
}}

In this cladogram the [[Epitheliozoa]] and Eumetazoa are synonyms to each other and to the [[Diploblast]]s, and the [[Ctenophora]] are basal to them.

An argument raised against the proposed scenario is that it leaves morphological features of the animals completely out of consideration. The extreme degree of simplification that would have to be postulated for the Placozoa in this model, moreover, is only known for parasitic organisms, but would be difficult to explain functionally in a free-living species like ''Trichoplax adhaerens''.{{cn|date=January 2022}}

This version is supported by statistical analysis of the ''Trichoplax adhaerens'' whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species. However, Ctenophora was not included in the analyses, placing the placozoans outside of the sampled Eumetazoans.<ref name=genome/><ref>{{cite journal |last1=Wallberg |first1=Andreas |last2=Thollesson |first2=Mikael |last3=Farris |first3=James S. |last4=Jondelius |first4=Ulf |date=2004-12-01 |title=The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling |journal=Cladistics |lang=en |volume=20 |issue=6 |pages=558–578 |doi=10.1111/j.1096-0031.2004.00041.x |doi-access=free |pmid=34892961 |s2cid=86185156 |issn=1096-0031 }}</ref>

==== Cnidaria-sister hypothesis ====
DNA comparisons suggest that placozoans are related to [[Cnidaria]], derived from [[planula]] larva (as seen in some Cnidaria).<ref name=Laumer2019>{{cite journal |last1=Laumer |first1=C.E. |last2=Fernández |first2=R. |last3=Lemer |first3=S. |last4=Combosch |first4=D. |last5=Kocot |first5=K.M. |last6=Riesgo |first6=A. |last7=Andrade |first7=S.C.S. |last8=Sterrer |first8=W. |last9=Sørensen |first9=M.V. |last10=Giribet |first10=G. |display-authors=6 |year=2019 |title=Revisiting metazoan phylogeny with genomic sampling of all phyla |journal=Proc. Biol. Sci. |volume=286 |issue=1906 |page=20190831 |doi=10.1098/rspb.2019.0831 |pmc=6650721 |pmid=31288696}}</ref> The [[Bilateria]] also are thought to be derived from planuloids.<ref name=Aleshin2002>{{cite journal |last1=Aleshin |first1=V.V. |last2=Petrov |first2=N.B. |year=2002 |title=Molecular evidence of regression in evolution of metazoa |journal=Zh. Obshch. Biol. |volume=63 |issue=3 |pages=195–208|pmid=12070939 }}</ref><ref>{{cite journal |last1=Laumer |first1=Christopher E. |last2=Gruber-Vodicka |first2=Harald |last3=Hadfield |first3=Michael G. |last4=Pearse |first4=Vicki B. |last5=Riesgo |first5=Ana |last6=Marioni |first6=John C. |last7=Giribet |first7=Gonzalo |date=2018-10-30 |df=dmy-all |title=Support for a clade of Placozoa and Cnidaria in genes with minimal compositional bias |journal=eLife |lang=en |volume=7 |doi=10.7554/elife.36278 |pmid=30373720 |pmc=6277202 |issn=2050-084X |doi-access=free }}</ref><ref>{{cite journal |last1=Syed |first1=Tareq |last2=Schierwater |first2=Bernd |date=June 2002 |title=The evolution of the placozoa: A new morphological model |journal=Senckenbergiana Lethaea |lang=en |volume=82 |issue=1 |pages=315–324 |doi=10.1007/bf03043791 |s2cid=16870420 |issn=0037-2110}}</ref><ref>{{cite journal |last1=Hejnol |first1=Andreas |last2=Martindale |first2=Mark Q. |date=2008-04-27 |df=dmy-all |title=Acoel development supports a simple planula-like urbilaterian |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |lang=en |volume=363 |issue=1496 |pages=1493–1501 |doi=10.1098/rstb.2007.2239 |issn=0962-8436 |pmc=2614228 |pmid=18192185}}</ref><ref>{{cite journal |last1=Alzugaray |first1=María Eugenia |last2=Bruno |first2=María Cecilia |last3=Villalobos Sambucaro |first3=María José |last4=Ronderos |first4=Jorge Rafael |year=2019 |title=The evolutionary history of the orexin / allatotropin GPCR family: From Placozoa and Cnidaria to Vertebrata |journal=Scientific Reports |volume=9 |issue=1 |page=10217 |biorxiv=10.1101/403709 |doi=10.1038/s41598-019-46712-9 |pmid=31308431 |pmc=6629687 |bibcode=2019NatSR...910217A |s2cid=256990037 }}</ref><ref>{{cite journal |last1=da&nbsp;Silva |first1=Fernanda Britto |last2=Muschner |first2=Valéria C. |last3=Bonatto |first3=Sandro L. |year=2007 |title=Phylogenetic position of Placozoa based on large subunit (LSU) and small subunit (SSU) rRNA genes |journal=Genetics and Molecular Biology |volume=30 |issue=1 |pages=127–132 |doi=10.1590/S1415-47572007000100022 |doi-access=free |issn=1415-4757 }}</ref><ref>{{cite journal |last1=Adl |first1=Sina M. |last2=Bass |first2=David |last3=Lane |first3=Christopher E. |last4=Lukeš |first4=Julius |last5=Schoch |first5=Conrad L. |last6=Smirnov |first6=Alexey |last7=Agatha |first7=Sabine |last8=Berney |first8=Cedric |last9=Brown |first9=Matthew W. |display-authors=6 |date=2018-09-26 |df=dmy-all |title=Revisions to the classification, nomenclature, and diversity of eukaryotes |journal=Journal of Eukaryotic Microbiology |volume=66 |issue=1 |pages=4–119 |lang=en |doi=10.1111/jeu.12691 |pmid=30257078 |pmc=6492006 |issn=1066-5234}}</ref><ref>{{cite book |last1=Giribet |first1=Gonzalo |last2=Edgecombe |first2=Gregory D. |date=2020-03-03 |df=dmy-all |title=The Invertebrate Tree of Life |publisher=Princeton University Press |isbn=978-0-691-19706-7 |lang=en |url=https://books.google.com/books?id=anetDwAAQBAJ&q=cnidaria+placozoa&pg=PP7 }}</ref> The Cnidaria and Placozoa body axis are overtly similar, and placozoan and cnidarian cells are responsive to the same [[neuropeptide]] [[Antibody|antibodies]] despite extant placozoans not developing any neurons.<ref>{{Cite journal |last1=duBuc |first1=Timothy Q. |last2=Ryan |first2=Joseph |last3=Martindale |first3=Mark Q. |date=2019-02-06 |df=dmy-all <!-- |editor-last=True |editor-first=John --> |title="Dorsal-ventral" genes are part of an ancient axial patterning system: Evidence from ''Trichoplax adhaerens'' (Placozoa) |journal=Molecular Biology and Evolution |volume=36 |issue=5 |pages=966–973 |lang=en |doi=10.1093/molbev/msz025 |pmid=30726986 |pmc=6501881 |issn=0737-4038}}</ref><ref>{{cite journal |last=Schuchert |first=Peter |date=1993-03-01 |df=dmy-all |title=''Trichoplax adhaerens'' (phylum Placozoa) has cells that react with antibodies against the neuropeptide RFamide |journal=Acta Zoologica |lang=en |volume=74 |issue=2 |pages=115–117 |doi=10.1111/j.1463-6395.1993.tb01227.x |issn=1463-6395 }}</ref>

{{clade
<!-- |style=font-size:80%; line-height:80% -->
 |label1= [[Choanozoa]] 
 |sublabel1=950 mya
 |1={{clade
    |1=[[Choanoflagellate|Choanoflagellata]] [[File:Desmarella moniliformis.jpg|60 px]]
    |label2= [[Animalia]] 
    |sublabel2=760 mya<br/> 
    |2={{clade
       |1=[[Porifera]] [[File:Reef3859 - Flickr - NOAA Photo Library.jpg|60 px]]
       |label2= [[Eumetazoa]] 
       |2={{clade
          |1=[[Ctenophora]] [[File:Comb jelly.jpg|60 px]]
          |label2= [[ParaHoxozoa]] 
          |sublabel2=680 mya
          |2={{clade
             |label1=  
             |1={{clade
                |label1=  
                |1=[[Placozoa]] [[File:Trichoplax adhaerens photograph.png|60 px]]
                |2=[[Cnidaria]] [[File:Cauliflour Jellyfish, Cephea cephea at Marsa Shouna, Red Sea, Egypt SCUBA.jpg|60 px]]
                }}
             |2=[[Bilateria]] / [[Triploblasts]] [[File:Sorocelis reticulosa.jpg|60 px]]
          }}
       }}
     }}
  }}
}}