Editing Placozoa (section)

===Epitheliozoa hypothesis===
A concept based on purely morphological characteristics pictures the Placozoa as the nearest relative of the animals with true tissues ([[Eumetazoa]]). The taxon they share, called the [[Epitheliozoa]], is itself construed to be a sister group to the sponges ([[Porifera]]):

{{clade
   |label1= [[Metazoa]] 
   |1={{clade
      |1= [[Porifera]]
      |label2= [[Epitheliozoa]] 
      |2={{clade
         |label1=  
         |1= [[Placozoa]]
         |2= [[Eumetazoa]]
      }}
   }}
}}

The above view could be correct, although there is some evidence that the [[ctenophore]]s, traditionally seen as [[Eumetazoa]], may be the sister to all other animals.<ref>
{{cite journal
 |last1=Whelan |first1=Nathan V.     |last2=Kocot   |first2=Kevin M.
 |last3=Moroz  |first3=Tatiana P.    |last4=Mukherjee |first4=Krishanu 
 |last5=Williams |first5=Peter       |last6=Paulay  |first6=Gustav
 |last7=Moroz  |first7=Leonid L.     |last8=Halanych |first8=Kenneth M.
 |display-authors=6
 |date=2017-10-09
 |title=Ctenophore relationships and their placement as the sister group to all other animals
 |journal=Nature Ecology & Evolution
 |volume=1  |issue=11  |pages=1737–1746
 |doi=10.1038/s41559-017-0331-3 |pmid=28993654
 |pmc=5664179 |bibcode=2017NatEE...1.1737W |issn=2397-334X  |lang=en
}}
</ref>
This is now a disputed classification.<ref>
{{cite press release
 |title=Sponges and comb jellies
 |date=November 2015
 |department=News and features 
 |publisher=University of Bristol
 |website=www.bristol.ac.uk
 |url=https://www.bristol.ac.uk/news/2015/november/sponges-comb-jellies.html#:~:text=Recent%20genomic%20studies%20have%20suggested,are%20the%20oldest%20animal%20phylum.
 |access-date=2023-03-11
 |lang=en-GB
}}
</ref> Placozoans are estimated to have emerged 750–800 million years ago, and the first modern neuron to have originated in the common ancestor of cnidarians and bilaterians about 650&nbsp;million years ago (many of the genes expressed in modern neurons are absent in ctenophores, although some of these missing genes are present in placozoans).<ref>[https://www.eurekalert.org/news-releases/1001544 Tiny sea creatures reveal the ancient origins of neurons]</ref><ref>{{cite journal | url=https://doi.org/10.1016/j.cell.2023.08.027 | doi=10.1016/j.cell.2023.08.027 | title=Stepwise emergence of the neuronal gene expression program in early animal evolution | date=2023 | last1=Najle | first1=Sebastián R. | last2=Grau-Bové | first2=Xavier | last3=Elek | first3=Anamaria | last4=Navarrete | first4=Cristina | last5=Cianferoni | first5=Damiano | last6=Chiva | first6=Cristina | last7=Cañas-Armenteros | first7=Didac | last8=Mallabiabarrena | first8=Arrate | last9=Kamm | first9=Kai | last10=Sabidó | first10=Eduard | last11=Gruber-Vodicka | first11=Harald | last12=Schierwater | first12=Bernd | last13=Serrano | first13=Luis | last14=Sebé-Pedrós | first14=Arnau | journal=Cell | volume=186 | issue=21 | pages=4676–4693.e29 | pmid=37729907 | pmc=10580291 | hdl=10230/58738 | hdl-access=free }}</ref>

The principal support for such a relationship comes from special cell to cell junctions – belt [[desmosomes]] – that occur not just in the Placozoa but in all animals ''except'' the sponges: They enable the cells to join together in an unbroken layer like the epitheloid of the Placozoa. ''[[Trichoplax adhaerens]]'' also shares the ventral gland cells with most eumetazoans. Both characteristics can be considered evolutionarily derived features ([[apomorphies]]), and thus form the basis of a common taxon for all animals that possess them.{{cn|date=January 2022}} 

One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid in ''[[Trichoplax adhaerens]]'' evolved by reduction of the collars in the collar cells ([[Choanocyte|choanocytes]]) of sponges as the hypothesized ancestors of the Placozoa abandoned a filtering mode of life. The epitheloid would then have served as the precursor to the true epithelial tissue of the eumetazoans.{{cn|date=January 2022}}

In contrast to the model based on functional morphology described earlier, in the Epitheliozoa hypothesis, the ventral and dorsal cell layers of the Placozoa are homologs of endoderm and ectoderm — the two basic embryonic cell layers of the eumetazoans. The digestive ''[[gastrodermis]]'' in the Cnidaria or the gut epithelium in the bilaterally symmetrical animals ([[Bilateria]]) may have developed from endoderm, whereas ectoderm is the precursor to the external skin layer ([[epidermis]]), among other things. The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is unclear whether the calcium ions stored in the syncytium would be related to the lime skeletons of many cnidarians.{{cn|date=January 2022}}

As noted above, this hypothesis was supported in a statistical analysis of the ''Trichoplax adhaerens'' whole genome sequence, as compared to the whole-genome sequences of six other animals and two related non-animal species.<ref name=genome/>