Systematics, evolution and phylogeny of Annelida – a morphological perspective

Purschke, G., Bleidorn, C. and Struck, T. 2014. Systematics, evolution and phylogeny of Annelida – a morphological perspective . Memoirs of Museum Victoria 71: 247–269. Annelida, traditionally divided into Polychaeta and Clitellata, is an evolutionary ancient and ecologically important group today usually considered to be monophyletic. However, there is a long debate regarding the in-group relationships as well as the direction of evolutionary changes within the group. This debate is correlated to the extraordinary evolutionary diversity of this group. Although annelids may generally be characterised as organisms with multiple repetitions of identically organised segments and usually bearing certain other characters such as a collagenous cuticle, chitinous chaetae or nuchal organs, none of these are present in every subgroup. This is even true for the annelid key character, segmentation. The first morphology-based cladistic analyses of polychaetes showed Polychaeta and Clitellata as sister groups. The former were divided into Scolecida and Palpata comprising Aciculata and Canalipalpata. This systematisation definitely replaced the old concept of dividing polychaetes into Errantia and Sedentaria, whereas the group Archiannelida had already been abandoned. The main critics came from a contradicting hypothesis relying on scenario based on plausibility considerations regarding Clitellata as highly derived annelids nesting within polychaetes and rendering the latter paraphyletic. In this hypothesis the absences of typical polychaete characters were regarded as losses rather than as primary absences. However, to date attempts to unambiguously identify the sister group of Clitellata on the basis of morphological characters have failed. Thus, two hypotheses on the last common annelid ancestor have been put forward either being an oligochaete-like burrowing animal or a parapodia-bearing epibenthic worm. These attempts to understand the major transitions in annelid evolution are reviewed and discussed in the light of new morphological evidence such as photoreceptor cell and eye evolution as well as the evolution of the nervous system and musculature. We also discuss the plausibility of these scenarios with regard to recent advances in molecular phylogenetic analyses.