Xenacoelomorpha, a Key Group to Understand Bilaterian Evolution: Morphological and Molecular Perspectives

The Xenacoelomorpha is a clade of mostly marine animals placed as the sister group of the remaining Bilateria (Nephrozoa) in most phylogenomic and morphological analyses, although alternative hypotheses placing them within deuterostomes have been proposed. This key phylogenetic position has raised recently a great interest in the study of their constitutive clades, since they can provide us with character states that illuminate different aspects of the origin of bilateral animals. Moreover, the recent availability of genomic and transcriptomic data from different species has been used in inferring the internal relationships among xenacoelomorph clades and the deciphering of molecular mechanisms that contribute to the evolution of metazoan genomes. Having access to molecular data paves the way to the systematic analysis of the genetic control of xenacoelomorph development and, additionally, to a better-informed study of bilaterian innovations. Here we revisit what has been learned over the last decades on the morphology, genomics and phylogenetic relationships of the Xenacoelomorpha.