Xenacoelomorpha

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Prior to molecular studies, xenacoelomorphs were considered to be flatworms based on their superficial similarities. Like flatworms, they do not have a coelom and are dorsoventrally flattened.^[6] With the advent of phylogenetics, Xenoturbella and Acoelomorpha were found to be sister groups and only distantly related to flatworms.^[5] Initially, this phylum was considered to be a member of the deuterostomes,^[4] but later transcriptome analyses have suggested that the phylum Xenacoelomorpha is a sister group to all the other bilateria. In this model, Xenacoelomorpha are neither protostomes nor deuterostomes, and the protostomes and deuterostomes together make up a new clade that has been named Nephrozoa. This makes Xenacoelomorpha the basalmost bilaterian clade.^[7][8]

Their larvae show similarities with cnidarian planula larvae and poriferan parenchyma larvae, but it is not clear if the similarities are ancestral or derived.^[9]

However, some studies point out that their basal placement may be caused by high mutation rates leading to long branch attraction (LBA). These analyses suggest that the xenacoelomorphs are instead the sister group of Ambulacraria, forming the clade Xenambulacraria, and that despite their simple body plans, they actually derive from a more complex ancestor.^[10][11] Having a larger number of species within this group would allow for better conclusions and analysis to be made within the phylum and in groups closely related to the phylum.

For multiple decades, the genus Xenoturbella contained only one species, X. bocki. In 2016, however, a team reported the discovery of four new species from the Gulf of California and sequenced each new species' mitogenome and, upon analysis, found that the two species that lived in shallow water (X. bocki and X. hollandorum) formed a "shallow" clade and that three deep water species formed a "deep" clade.^[12] The following year, another team discovered a sixth species, X. japonica, found off the coast of Japan. Their phylogenetic analysis confirmed the first team's hypothesis and placed X. japonica within the shallow clade.^[13]

The other two groups, Nemertodermatida and Acoela, have less clear relationships as species-level phylogenies have not been conducted. Nemertodermatida only has two families and six total genera. Ascopariidae contains two of these genera, while Nemertodermatidae has the other four. A 2016 study analyzed three of the four Nemertodermatid genera and found that Sterreria and Meara are closer to each other than to Nemertoderma, while Nemertinoides was left unplaced.^[7] Acoela phylogeny is even less certain, as it is by far the most diverse part of the phylum and is very understudied. A 2011 study attempted to solve this problem and recovered numerous traditional families as polyphyletic. They also recovered a tentative clade of various species from Actinoposthiidae and Isodiametridae, which is not shown in the cladogram below.^[14] Several small basal families were not included in their study, and their position is still uncertain.