Higher-level metazoan relationships: recent progress and remaining questions

By G. Edgecombe, Gonzalo Giribet, Casey W. Dunn, A. Hejnol, R. Kristensen, R. Neves, G. Rouse, K. Worsaae, M. Sørensen. Organisms Diversity & Evolution, 2011

271 citations recorded by the source corpus.

Metazoa comprises 35–40 phyla that include some 1.3 million described species. Phylogenetic analyses of metazoan interrelationships have progressed in the past two decades from those based on morphology and/or targeted-gene approaches using single and then multiple loci to the more recent phylogenomic approaches that use hundreds or thousands of genes from genome and transcriptome sequencing projects. A stable core of the tree for bilaterian animals is now at hand, and instability and conflict are becoming restricted to a key set of important but contentious relationships. Acoelomorph flatworms (Acoela + Nemertodermatida) and Xenoturbella are sister groups. The position of this clade remains controversial, with different analyses supporting either a sister-group relation to other bilaterians (=Nephrozoa, composed of Protostomia and Deuterostomia) or membership in Deuterostomia. The main clades of deuterostomes (Ambulacraria and Chordata) and protostomes (Ecdysozoa and Spiralia) are recovered in numerous analyses based on varied molecular samples, and also receive anatomical and developmental support. Outstanding issues in protostome phylogenetics are the position of Chaetognatha within the protostome clade, and the monophyly of a group of spiralians collectively named Platyzoa. In contrast to the broad consensus over key questions in bilaterian phylogeny, the relationships of the five main metazoan lineages—Porifera, Ctenophora, Placozoa, Cnidaria and Bilateria—remain subject to conflicting topologies according to different taxonomic samples and analytical approaches. Whether deep bilaterian divergences such as the split between protostome and deuterostome clades date to the Cryogenian or Ediacaran (and, thus, the extent to which the pre-Cambrian fossil record is incomplete) is sensitive to dating methodology.

Subfields: Global and Planetary Change, Molecular Biology, Paleontology

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Citations in the corpus

Animal biodiversity: An introduction to higher-level classification and taxonomic richness (2011, background): “This evo-Linnaean classification of animal phyla general follows consensus phylogenetic relationships summarised in Edgecombe et al. (2011).”
Animal biodiversity: An update of classification and diversity in 2013. In : Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013) (2013, methodology): “The classification used here is the evo-Linnaean classification of animal phyla in Zhang (2011a), which reflects the consensus phylogenetic relationships in Edgecombe et al. (2011).”
Animal Phylogeny and Its Evolutionary Implications (2014, background): “This challenge comes down to testing the relationships among five clades: Porifera, Ctenophora, Cnidaria, Placozoa, and Bilateria (Edgecombe et al. 2011).”
The origin of annelids (2014, background): “Such analyses recover crown group ages ranging from mid-Cambrian (Edgecombe et al. 2011) to Early Ordovician (Erwin et al.”
Constraints on the timescale of animal evolutionary history (2015, background): “Traditionally, hemichordates were paired with chordates to the exclusion of echinoderms, but the consensus of morphological and molecular studies (e.g., Eernisse and Peterson, 2004; Delsuc et al., 2006; Swalla and Smith, 2008; Cannon et al., 2009; Edgecombe et al., 2011; Röttinger and Lowe, 2012) no”
Birth and early evolution of metazoans (2014, background): “Acoelomorpha is favorably placed at the basal position of the Bilateria (Edgecombe et al., 2011; Nielsen, 2012), which contains three small phyla, i.e. Acoela, Nemertodermatida, and Xenoturbellida, all escaping the fossil record.”