ParaHoxozoa (or Parahoxozoa) is a clade of animals that consists of Bilateria, Placozoa, and Cnidaria.[1] The relationship of this clade relative to the two other animal lineages Ctenophora and Porifera is debated. Some phylogenomic studies have presented evidence supporting Ctenophora as the sister to Parahoxozoa and Porifera as the sister group to the rest of animals (e.g. [2][3][4][5][6][7]).[excessive citations] Other studies have presented evidence supporting Porifera as the sister to Parahoxozoa and Ctenophora as the sister group to the rest of animals (e.g. [8][9][10]), finding that nervous systems either evolved independently in ctenophores and parahoxozoans,[11] or were secondarily lost in poriferans.[12] If ctenophores are taken to have diverged first, Eumetazoa is sometimes used as a synonym for ParaHoxozoa.[13]

ParaHoxozoa
Temporal range: 605.2 –0 Ma Ediacaran-Present
Diversity of parahoxozoans
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Ryan et al., 2010
Taxa

Phylogeny edit

The tree below, which is congruent with the vast majority of these phylogenomic studies, conveys this uncertainty with a polytomy.

  Choanozoa  

 Choanoflagellata  

  Animalia  

 Ctenophora  

 Porifera  

  Parahoxozoa  

 Placozoa  

  Planulozoa  
     

 Cnidaria  

 Bilateria  

  
  
  
  

ParaHoxozoa or Parahoxozoa edit

Though "ParaHox" genes are usually referred to in CamelCase and the original paper that named the clade used "ParaHoxozoa", the single initial capital format "Parahoxozoa" has become more common in the literature as CamelCase is not standard in zoological nomenclature.[citation needed]

Characteristics edit

Parahoxozoa was defined by the presence of several gene (sub)classes (HNF, CUT, PROS, ZF, CERS, K50, S50-PRD), as well as Hox/ParaHox-ANTP from which the name of this clade originated. It was later proposed[14][15] and contested[16] that a gene of the same class (ANTP) as the Hox/ParaHox, the NK gene and the Cdx Parahox gene, is also present in Porifera, the sponges. Regardless of whether a ParaHox gene is ever definitively identified, Parahoxozoa, as originally defined, is monophyletic and therefore continues to be used as such.[17]

Planula-acoel, triploblasty, and bilaterian similarities edit

The original Bilateria are hypothesized to be a bottom dwelling worm with a single body opening.[18] A through-gut may already have developed with the Ctenophora however.[19] The through-gut may have developed from the corners of a single opening with lips fusing. E.g. Acoela resemble the planula larvae of some Cnidaria, which exhibit some bilaterian symmetry. They are vermiform, just as the cnidarian Buddenbrockia is.[20][21][22] Placozoa has been noted to resemble planula.[23] Usually, "Planulozoa" is to the exclusion of Placozoa, but not necessarily. In this case it appears synonymous with Parahoxozoa.[24] Triploblasty developed before the Cnidaria-Bilateria radiation as well.[25]

References edit

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