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

  • Ulf JondeliusEmail author
  • Olga I. RaikovaEmail author
  • Pedro MartinezEmail author


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.



Funding from The Swedish Research Council (project 2018-05191) is gratefully acknowledged by Ulf Jondelius. The work of Olga Raikova was supported by the Ministry of Education and Science of the Russian Federation (project no. AAAA-A19-119020690076-7) and the RFBR (project numbers 16-04-00593a and 20-04-01006a). We would also like to thank Dr. Pierre Pontarotti (Marseille) for organizing the yearly “Evolutionary Biology Meeting” in Marseille and for inviting us to submit this chapter.


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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Swedish Museum of Natural HistoryStockholmSweden
  2. 2.Department of ZoologyStockholm UniversityStockholmSweden
  3. 3.Zoological Institute RASSt. PetersburgRussia
  4. 4.Faculty of Biology, Chair of Invertebrate ZoologySt. Petersburg State UniversitySt. PetersburgRussia
  5. 5.Department de Genètica, Microbiologia i EstadísticaUniversitat de BarcelonaBarcelonaSpain
  6. 6.ICREA (Institut Català de Recerca i Estudis Avancats)BarcelonaSpain

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