© The Norwegian Academy of Science and Letters • Zoologica Scripta, 31, 2, April 2002, pp201–215 201 Jondelius, U., Ruiz-Trillo, I., Baguñà, J. & Riutort, M. (2002). The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes. Zoologica Scripta, 31, 201– 215. Recent hypotheses on metazoan phylogeny have recognized three main clades of bilaterian animals: Deuterostomia, Ecdysozoa and Lophotrochozoa. The acoelomate and ‘pseudocoe- lomate’ 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 bila- terian 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 bila- terian 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 Nemer- todermatida 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 with- out 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. Ulf Jondelius, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE-752 36 Upp- sala, Sweden. E-mail: ulf.jondelius@ebc.uu.se Iñaki Ruiz-Trillo, Jaume Baguñà & Marta Riutort, Departament de Genetica, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain Blackwell Science Ltd The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes ULF JONDELIUS, IÑAKI RUIZ-TRILLO, JAUME BAGUÑÀ & MARTA RIUTORT Accepted: 19 September 2001 Introduction Platyhelminth flatworms have often played a crucial role in discussions of metazoan phylogeny as the sister group or even ‘ancestors’ of other Bilateria (e.g. Willmer 1990). Although monophyly of the Platyhelminthes has long been assumed, the relationships between the three major platyhelminth clades, Acoelomorpha ( Nemertodermatida + Acoela), Catenulida and Rhabditophora are controversial. Difficulties identifying robust morphological synapomorphies uniting Rhabditophora with Catenulida and with the Acoelomorpha led Smith et al. (1986) to question the monophyly of the Platyhelminthes, although they did not provide an alternative hypothesis for the sister groups of the three clades. A critical reassessment of morpho- logical evidence within the context of metazoan evolution led Haszprunar (1996) to consider Platyhelminthes a paraphyletic group, with Acoelomorpha being the earliest bilaterian off- shoot followed by the Rhabditophora, the Catenulida and the rest of the Bilateria. Another analysis of metazoan phylogeny using a large set of morphological characters in combination with 18S ribosomal DNA (rDNA), also indicated the Platy- helminthes to be a paraphyletic assemblage (Zrzavy et al. 1998). However, in most zoology textbooks, flatworms are still fea- tured as a monophyletic group. Recent analyses of molecular data have challenged the long- held position of Platyhelminthes as ancestral bilaterians as well as flatworm monophyly. 18S rDNA sequence data and Hox gene molecular signatures form the basis for the recognition of three main clades of bilaterian animals: Deuterostomia, Ecdysozoa and Lophotrochozoa (e.g. Aguinaldo et al. 1997; Adoutte et al. 1999). The acoelomate and ‘pseudocoelomate’ metazoans, including the bulk of the Platyhelminthes (the Rhabditophora, or probably Rhabditophora + Catenulida), are now referred to positions within these clades by many authors (Balavoine 1997, 1998; Carranza et al. 1997; Bayascas et al. 1998; Littlewood et al. 1999; De Rosa et al. 1999; Adoutte et al. 2000; Saló et al. 2001); but see Giribet et al. (2000) for a different view. Thus, Platyhelminthes are now generally con- sidered lophotrochozoans. However, a recent study based on