Acoelomorph flatworm monophyly is a long-branch attraction artefact obscuring a clade of Acoela and Xenoturbellida.

Acoelomorpha is a broadly accepted clade of bilaterian animals made up of the fast-evolving, morphologically simple, mainly marine flatworm lineages Acoela and Nemertodermatida. Phylogenomic studies support Acoelomorpha's close relationship with the slowly evolving and similarly simplistic , together forming the phylum Xenacoelomorpha. The phylogenetic placement of Xenacoelomorpha amongst bilaterians is controversial, with some studies supporting Xenacoelomorpha as the sister group to all other bilaterians, implying that their simplicity may be representative of early bilaterians. Others propose that this placement is an error resulting from the fast-evolving Acoelomorpha, and instead suggest that they are the degenerate sister group to Ambulacraria. Perhaps as a result of this debate, internal xenacoelomorph relationships have been somewhat overlooked at a phylogenomic scale. Here, I employ a highly targeted approach to detect and overcome possible phylogenomic error in the relationship between and the fast-evolving acoelomorph flatworms. The results indicate that the subphylum Acoelomorpha is a long-branch attraction artefact obscuring a previously undiscovered clade comprising and Acoela, which I name Xenacoela. The findings also suggest that Xenacoelomorpha is not the sister group to all other bilaterians. This study provides a template for future efforts aimed at discovering and correcting unrecognized long-branch attraction artefacts throughout the tree of life.