The order Anseriformes contains about 150 species of birds in three families...

The order Anseriformes contains about 150 species of birds in three families: the Anhimidae (the screamers), Anseranatidae (the Magpie-goose), and the Anatidae, which includes over 140 species of waterfowl, among them the ducks, geese, and swans.
All species in the order are highly adapted for an aquatic existence at the water surface. All are web-footed for efficient swimming (although some have subsequently become mainly terrestrial).

The earliest known Anseriform is the recently discovered Vegavis, which lived during the Cretaceous period...

The earliest known Anseriform is the recently discovered Vegavis, which lived during the Cretaceous period. It is thought that the Anseriformes originated when two groups of specialised filter feeders diverged from the Charadriiformes (plovers, gulls and allies) by evolving different mechanisims to filter small food particles from water. The flamingos developed a filter structure in the lower part of the bill; the ancestors of the Anseriformes developed the characteristic bill structure that they still share. The combination of the internal shape of the bill and a modified tongue acts as a suction pump to draw water in at the tip of the bill and expel it from the sides and rear; an array of fine filter plates called lamellae traps small particles, which are then licked off and swallowed.
Magpie Goose Anseranas semipalmata
All Anseriformes have this basic structure, but many have subsequently adopted alternative feeding strategies: geese graze on plants, the saw-billed ducks catch fish; even the screamers, which have bills that seem on first sight more like those of the game birds, still have vestigal lamellae.