Haeckel (1895 p. 631-632) introduced the name Protanthropus atavus as a stage in human evolution following Pithecanthropus erectus, representing "primitive speaking man" of the Diluvial period, i.e. Pleistocene. Haeckel does not provide a morphological desciription. The name is thus nomen nudum.
Campbell (1965) listed Protanthropus atavus as an objective junior synonym of Homo neanderthalensis and indicates the name is nomen nudum.
Groves (1989) listed Protanthropus atavus Haeckel, 1895 as an available junior synonym of Homo sapiens neanderthalensis.