Cayaoa bruneti, coming from the early Miocene of Gaiman Formation (Chubut, Argentina) is interpreted as one of the earliest examples of diving behaviour and the earliest example of flight loss within Anseriformes, being also the only known diving duck from South America. Herein is presented the first phylogeny of Anatidae with Cayaoa bruneti. It was made by the methodology of parsimony and by using characters previously defined by other authors, redefined in the present work as well as new characters. The phylogenetic analysis using all the characters and without considering the information contributed by analyses using molecular characters puts Cayaoa bruneti as part of a single radiation of diving ducks within Anatinae, as sister group to the Erismaturinae. Analyzing the different bones as different partitions show that the humerus and femur have the most effect, while the skull characters result in groupings closer to those seen in analyses with molecular characters. When making the analysis with a molecular backbone for the topology, Cayaoa bruneti arises as a basal branch within the Erismaturinae. These results make Cayaoa bruneti an independent and early example of the recurring evolution of flightlessness in large marine Anatidae.