Editing Ctenophora (section)

===Internal phylogeny ===

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{{clade
|1={{clade
 |1=[[Mertensiidae]] ([[Cydippida|cydippid]]s)
 |2={{clade
 |1=[[Platyctenida]]
 |2={{clade
 |1=[[Pleurobrachiidae]] (cydippids)
 |2={{clade
 |1=[[Lobata]]
 |2=[[Thalassocalycida]]
 |3=[[Cestida]]
 }}
 |3={{clade
 |1=[[Haeckeliidae]] (cydippids)
 |2=[[Beroida]]
 }}
 }}
 }}
 }}
}}
</div>Relationships within Ctenophora (2001).<ref name="PodarHaddockEtAl2001MolecularPhylogeneticFrameworkForCtenophora" /></div>

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{{clade
|1={{clade
 |1=''[[Euplokamis]]''
 |2={{clade
 |1=[[Platyctenida]]
 |2={{clade
 |1=[[Pleurobrachiidae]]
 |2={{clade
 |1=[[Mertensiidae]]
 |2={{clade
 |1={{clade
 |1=[[Dryodoridae]]
 |2=[[Beroida]]
 }}
 |2=[[Lobata]] <small>incl. [[Cestida]]</small>
 }}
 }}
 }}
 }}
 }}
}}
</div>Relationships within Ctenophora (2017).<ref name=Whelan2017/></div>

Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor that was itself a cydippid. Instead, he found that various cydippid [[family (biology)|families]] were more similar to members of other ctenophore [[order (biology)|orders]] than to other cydippids. He suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like.<ref>{{cite book|last=Harbison|first=G.R.|title=The Origins and Relationships of Lower Invertebrates|url=https://archive.org/details/originsrelations00morr|url-access=limited |editor=Conway Morris, S. |editor-link=Simon Conway Morris |editor2=George, J.D. |editor3=Gibson, R. |editor4=Platt, H.M.|publisher=Clarendon Press|year=1985 |pages=[https://archive.org/details/originsrelations00morr/page/n47 78]–100|chapter=On the classification and evolution of the Ctenophora|isbn=978-0-19-857181-0}}</ref> 

A molecular phylogeny analysis in 2001, using 26 species, including four recently discovered ones, confirmed that the cydippids are not monophyletic and concluded that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps survived the [[Cretaceous–Paleogene extinction event]] {{ma|65.5}} while other lineages perished. When the analysis was broadened to include representatives of other phyla, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain.<ref name=PodarHaddockEtAl2001MolecularPhylogeneticFrameworkForCtenophora>{{cite journal |last1=Podar |first1=Mircea |last2=Haddock |first2=Steven H.D. |last3=Sogin |first3=Mitchell L. |last4=Harbison |first4=G. Richard |date=November 2001 |title=A molecular phylogenetic framework for the phylum Ctenophora using 18S rRNA genes |journal=[[Molecular Phylogenetics and Evolution]] |volume=21 |issue=2 |pages=218–230 |doi=10.1006/mpev.2001.1036 |pmid=11697917 |bibcode=2001MolPE..21..218P |citeseerx=10.1.1.384.6705 }}</ref> A  2017 study corroborates the paraphyly of [[Cydippida]] but finds that [[Lobata]] is paraphyletic with respect to [[Cestida]].<ref name=Whelan2017/>