Calanoid copepods are extremely successful inhabitants of marine, brackish and freshwater plankton. Morphological data are used to arrive at a first, parsimony-based phylogeny for the calanoid copepods. Each family is represented by at least one exemplar, typically a recognised plesiomorphic species. The Epacteriscidae are sister to all other calanoid copepods – an observation that brings into question the concept of an Epacteriscoidea that includes the Ridgewayiidae. The monophyly of the Augaptiloidea, Centropagoidea, Clausocalanoidea and Pseudocyclopoidea is corroborated. The current analysis suggests there may be two major clades, one containing the Augaptiloidea, Centropagoidea and possibly the Pseudocyclopoidea and Ridgewayiidae and the other including the Megacalanidae, Calanidae/Paracalanidae, Bathypontioidea, Eucalanoidea, Ryocalanoidea, Spinocalanoidea and Clausocalanoidea. The relationships of the Pseudocyclopidae, Boholinidae, and Ridgewayiidae to the Centropagoidea/Augaptiloidea clade received low nodal support. Monophyly of an enlarged Bathypontioidea (including the Fosshageniidae) is proposed. A monophyletic Megacalanoidea is not retrieved. The nature of the inferred ancestral Calanoida is discussed. New evolutionary series are proposed for the female genitalia (including several losses and regaining of seminal receptacles, and independent losses of the genital operculum – once lost it is never regained) and leg 1 endopod. Paedomorphosis appears to be a dominant process in the evolution of the Calanoida.

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