The order Anseriformes contains about 150 living species of birds in three extant 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).
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. The prehistoric wading presbyornithids and the huge and possibly carnivorous dromornithids were even more bizarre.
Anatidae systematics, especially regarding placement of some "odd" genera in the dabbling ducks or shelducks, is not fully resolved. See the Anatidae article for more information, and for alternate taxonomic approaches.
The fossil Gastornithidae (diatrymas) are also occasionally included herein as a family.
Some fossil anseriform taxa not assignable with certainty to a family are:
In addition, a considerable number of mainly Late Cretaceous and Paleogene fossils have been described where it is uncertain whether or not they are anseriforms. This is because almost all orders of aquatic birds living today either originated or underwent a major radiation during that time, making it hard to decide whether some waterbird-like bone belongs into this family or is the product of parallel evolution in a different lineage due to adaptive pressures.
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