|Whooper Swan (Cygnus cygnus), a member of the Anatidae|
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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.
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. Some unusual fossil groups, such as the Gastornithidae and Dromornithidae, are often found to be at the base of the Anseriformes family tree, or at least their closest relatives. The higher-order classification below follows a phylogenetic analysis performed by Angolin, 2007.
- Order Anseriformes
- †Family Dromornithidae?: mihirungs
- †Family Gastornithidae?: "diatrymas"
- Anseres (true anseriformes)
- Family Anhimidae: screamers
- Family Anseranatidae: the Magpie Goose
- Superfamily Anatoidea
- Family Anatidae
- Subfamily Dendrocygninae: Whistling ducks (sometimes given full family status as the Dendrocygnidae).
- Subfamily Thalassorninae: the White-backed Duck.
- Subfamily Anserinae: Swans and geese.
- Subfamily Stictonettinae: the Freckled Duck.
- Subfamily Plectropterinae: the Spur-winged Goose.
- Subfamily Tadorninae: Shelducks and sheldgeese - probably paraphyletic
- Subfamily Anatinae: Dabbling ducks and moa-nalos
- Subfamily Aythyinae: Diving ducks (sometimes included in Anatinae)
- Subfamily Merginae: eiders, scoters, mergansers and other sea-ducks.
- Subfamily Oxyurinae: Stiff-tailed ducks and allies.
- †Family Presbyornithidae: several genera of wading-"geese"
Some fossil anseriform taxa not assignable with certainty to a family are:
- Anatalavis (Late Cretaceous/Early Paleocene - Early Eocene) - Anseranatidae or basal.
- 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.
- "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)
- ^ Clarke et al. (2005)
- ^ a b Agnolin, F. (2007). "Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina." Revista del Museo Argentino de Ciencias Naturales, n.s. 9, 15-25.
- ^ Livezey, B.C. and Zusi, R.L. (2007). "Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion." Zoological Journal of the Linnen Society, 149: 1-95.
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- Clarke, J.A.; Tambussi, C.P.; Noriega, J.I.; Erickson, G.M. & Ketcham, R.A. (2005): Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433: 305-308. doi:10.1038/nature03150 PDF fulltext Supporting information