WHAT IF "POLYCHAETA" AND "ANNELIDA" ARE NOT MONOPHYLETIC? A NEW PROPOSAL FOR A PHYLOGENY OF THE METAMERIA
Christoffersen, M. L.1, Amorim, D. S.2, Almeida, W. O.1, Silva, G. S.2 & Garraffoni, A. R. S.2
1Departamento de Sistemática e Ecologia, Universidade Federal da Paraíba, 58059-900, João Pessoa, Paraíba, Brazil 2Departamento de Biologia, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040-901 Ribeirão Preto, São Paulo, Brazil.
Polychaetes are metameric animals traditionally recognised for having parapodia, setae, and nuchal organs. However, in some Ecdysozoa (moulting animals including onychophorans, arthropods, tardigrades, pentastomids, and aschelminths) there are parapodia modified into lobopodia or into fully articulated legs; Echiura and some Ecdysozoa also present supposedly homologous setae. We interpret nuchal organs, parapodia and setae to be groundplan features of the Metameria. Recent authors have extended the "Annelida" to include Pogonophora and Echiura. This solution still does not generate a monophyletic group, since it excludes Ecdysozoa and Radialia (lophophorate and deuterostome phyla). Under this traditional oversight, the presence of metamery and a coelom have been interpreted as homoplasies. A phylogenetic analysis, with the polychaete families, Clitellata, Pogonophora, Echiura, Ecdysozoa and Radialia as terminal taxa and the remaining non-metameric coelomate phyla (Sipuncula, Mollusca and Nemertea) as outgroups, was performed to test the monophyly of the "Polychaeta", and "Annelida". Empirical data was obtained mainly from the primary literature. The data matrix was run on the Hennig86 program. The most relevant conclusions are: (1) protostomates are clearly non-monophyletic; (2) Coelomata and Metameria are valid, nested clades; (3) "Polychaeta", even including Pogonophora, is not monophyletic; (4) Metameria includes all taxa traditionally placed in the "Annelida", as well as Pogonophora, Radialia, Echiura, and Ecdysozoa; (5) oweniids, pogonophorans, and Radialia compose a monophyletic group related to tubicolous sedentary polychaetes; (6) Clitellata is related to deposit feeding sedentary polychaetes, of which Questidae is sister group. Cladistics is not considered a straightforward and simple procedure, as sometimes implied, since the construction of a list of characters is a complex process directly affecting the conclusions. Hence, most effort must be concentrated on discussions of the characters. Hypotheses of some clades may still change, but there is a high rate of congruence between characters at different levels. The resolution of several problems of homology among different metazoan phyla indicates that our proposed scenario composes an integrated set of explanations and provides important novel solutions for the evolution of the higher Metazoa.