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).
The earliest known Anseriform is the recently discovered Vegavis
, which lived during the Cretaceous
period. It is thought that the Anseriformes originated when the original Galloanserae
(the group to which Anseriformes and Galliformes
belong) split into the two main lineages. The extinct dromornithids
represent early offshoots of the anseriform line, possibly derived from screamer
-like ancestors, and so maybe Gastornis
(if it is an Anseriform). 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.
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.
The Anseriformes and the Galliformes
(pheasants etc) are the most primitive neognathous birds, and should follow ratites
in bird classification systems.
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:
- Anatalavis (Late Cretaceous/Early Paleocene - Early Eocene) - Anseranatidae or basal. Includes "Telmatornis" rex.
- Proherodius (London Clay Early Eocene of London, England) - Presbyornithidae?
- Romainvillia (Late Eocene/Early Oligocene) - Anseranatidae or Anatidae
- Paranyroca (Rosebud Early Miocene of Bennett County, USA) - Anatidae or own family?
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.
- Apatornis (Smoky Hill Chalk Late Cretaceous of Twin Butte Creek, USA)
- Vegavis (Late Cretaceous) - closer to Presbyornithidae and Anatidae than to Anseranatidae
- "Presbyornithidae" gen. et sp. indet. (Barun Goyot Late Cretaceous of Udan Sayr, Mongolia) - Presbyornithidae?
- UCMP 117599 (Hell Creek Late Cretaceous of Bug Creek West, USA)
- Petropluvialis (Late Eocene of England) - may be same as Palaeopapia
- Agnopterus (Late Eocene - Late Oligocene of Europe) - includes Cygnopterus lambrechti
- "Headonornis hantoniensis" BMNH PAL 4989 (Hampstead Early Oligocene of Isle of Wight, England) - formerly "Ptenornis"
- Palaeopapia (Hampstead Early Oligocene of Isle of Wight, England)
- "Anas" creccoides (Early/Middle Oligocene of Belgium)
- "Anas" skalicensis (Early Miocene of "Skalitz", Czechia)
- "Anas" risgoviensis (Late Miocene of Bavaria, Germany)