The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes

Abstract

Recent hypotheses on metazoan phylogeny have recognized three main clades of bilaterian animals: Deuterostomia, Ecdysozoa and Lophotrochozoa. The acoelomate and ‘pseudocoelomate’ metazoans, including the Platyhelminthes, long considered basal bilaterians, have been referred to positions within these clades by many authors. However, a recent study based on ribosomal DNA placed the flatworm group Acoela as the sister group of all other extant bilaterian lineages. Unexpectedly, the nemertodermatid flatworms, usually considered the sister group of the Acoela together forming the Acoelomorpha, were grouped separately from the Acoela with the rest of the Platyhelminthes (the Rhabditophora) within the Lophotrochozoa. To re‐evaluate and clarify the phylogenetic position of the Nemertodermatida, new sequence data from 18S ribosomal DNA and mitochondrial genes of nemertodermatid and other bilaterian species were analysed with parsimony and maximum likelihood methods. The analyses strongly support a basal position within the Bilateria for the Nemertodermatida as a sister group to all other bilaterian taxa except the Acoela. Despite the basal position of both Nemertodermatida and Acoela, the clade Acoelomorpha was not retrieved. These results imply that the last common ancestor of bilaterian metazoans was a small, benthic, direct developer without segments, coelomic cavities, nephrida or a true brain. The name Nephrozoa is proposed for the ancestor of all bilaterians excluding the Nemertodermatida and the Acoela, and its descendants.

Back in time: a new systematic proposal for the Bilateria

“…Under this proposal, Cnidaria are considered true bilaterians and the sister group to a less-inclusive bilaterian clade, here named Triploblastica, which comprises present-day bilaterians with a true mesoderm. Within the Triploblastica, molecular evidence (figures 2 and 3) favours the early branching of a paraphyletic 'Acoelomorpha' (acoels first and nemertodermatids second) sister group to the traditional protostomeCdeuterostome clade, or Nephrozoa (sensu Jondelius et al 2002). Apomorphies of the new Bilateria would be the establishment and consolidation of a new D-V axis and the ensuing bilateral symmetry, and the appearance of a basic HOX cluster (2 HOX/2 ParaHOX genes) and a minimal set (two out of three genes) of miRNAs.…”

Acoel development supports a simple planula-like urbilaterian

“…The placement of the unsegmented Chaetognatha as the sister group to the Lophotrochozoa (Matus et al 2006a) has a huge impact on our understanding of the lophotrochozoan stem species, and recent work placing the enigmatic Xenoturbella inside the Deuterostomia as sister to the Ambulacraria (Bourlat et al 2003(Bourlat et al , 2006 indicates that both protostomes and deuterostomes have rather simple unsegmented worms with only one body opening and a non-centralized nervous system near their base. In particular, the placement of the Acoela and Nemertodermatida as the possible sister group of the remaining Bilateria (Ruiz-Trillo et al 1999, 2002Jondelius et al 2002;Telford et al 2003; allows a more critical evaluation of the statements made about the morphology of the urbilaterian. The first obvious result of this placement is that the stem species of the Bilateria does not correspond to the protostomedeuterostome ancestor, but to the last common ancestor of the Acoelomorpha and the remaining Bilateria (the Eubilateria or Nephrozoa; figure 1).…”

Assessing the root of bilaterian animals with scalable phylogenomic methods

“…This is therefore one of the most important remaining problems in animal phylogenetics. Acoela has been recovered as the sister group to all other bilaterian animals in direct sequencing analyses, though their placement with respect to Nemertodermatida has been inconsistent (Ruiz-Trillo et al 1999Jondelius et al 2002;Wallberg et al 2007;Paps et al 2009). The position of acoels has not been resolved satisfactorily in previous EST-based analyses (Philippe et al 2007;Dunn et al 2008;Egger et al 2009).…”