An order of birds comprising two families, the screamers (Anhimidae) of South America and the worldwide waterfowl (Anatidae). They are closely related to the Galliformes (fowls), with the screamers being a rather intermediate group. The waterfowl and the fowls are closely related within the Aves and are often placed within a distinct superorder, the Galloanserae. The large, flightless diatrymids (giant ground birds) of the early Tertiary appear to be specialized offshoots of the Anseriformes.