To evaluate and assess the ontogenetic background for paedomorphosis in phocoenids, samples of 144 harbour porpoises, 81 white-beaked dolphins, and 130 Commerson’s dolphins were compared in terms of the development of epiphyseal fusion, cranial suture fusion, and ontogeny of cranial shape. Harbour porpoises and Commerson’s dolphins terminated growth and development of all investigated traits sooner than white-beaked dolphins, leading to lesser degrees of fusion of skeletal elements and less postnatal allometric development. The latter occurred even though shape in the two paedomorphic species developed at twice the rate relative to the size of white-beaked dolphins. These observations imply that progenetic evolution has occurred convergently in phocoenid and Cephalorhynchus ancestors. The truncated ontogenies allow sexual maturity to be attained earlier and provide a greater reproductive potential. Both species inhabit similar temperate productive habitats and, hence, ecological factors are proposed to have supplied the selection pressures leading to progenesis. Constant prey availability must be a prerequisite for the observed phenomena because frequent food-intake is necessitated by the limited capacity for energy storage and high heat-loss entailed by the resulting small body sizes. Progenesis has rarely been proposed in mammal species. This may reflect rarity or that mammalian expressions of progenesis are less obvious.