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

Abstract

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 Acoelomorphas close relationship with the slowly evolving and similarly simplistic Xenoturbella, 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 a long branch attraction artefact resulting from the fast-evolving Acoelomorpha, and instead suggest that they are the secondarily simplified sister group of the deuterostome clade Ambulacraria. Perhaps as a result of this debate, internal xenacoelomorph relationships have been somewhat overlooked at a phylogenomic scale. Here, I employ both empirical and simulation approaches to detect and overcome phylogenomic errors to reassess the relationship between Xenoturbella and the fast evolving acoelomorph flatworms. I conclude that subphylum Acoelomorpha is a long-branch attraction artefact obscuring a previously undiscovered clade comprising Xenoturbella and Acoela, for which I propose the name Xenacoela. These analyses are also consistent with the Nephrozoa hypothesis deriving from systematic error, and instead generally favour a close, but unclear, relationship of Xenacoelomorpha with deuterostomes. This study provides a template for future efforts aimed at discovering and correcting unrecognised long-branch attraction artefacts throughout the tree of life.