A small humerus from Eocene levels of Seymour Island, Antarctica is assigned here to <i>Aprosdokitos mikrotero</i> sp. and gen. nov. (Aves, Sphenisciformes), based predominantly on its small size. An ontogenetic series based on <i>Pygoscelis antartica</i> was established for comparative purposes, and evaluation of pathological conditions was also carried out in order to rule out other possible sources of size variation.Facultad de Ciencias Naturales y Museo