Anseriformes is an order of birds that comprise about 180 living species in three families: Anhimidae (the screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which includes over 170 species of waterfowl, among them the ducks, geese, and swans.
In fact, these living species are all included in the Anatidae except for the three screamers and the magpie goose.
All species in the order are highly adapted for an aquatic existence at the water surface. The males, except for the screamers, also have a penis, a trait that has been lost in the Neoaves. All are web-footed for efficient swimming (although some have subsequently become mainly terrestrial).
The Anseriformes and the Galliformes (pheasants, etc.) are the most primitive neognathous birds, and should follow ratites and tinamous in bird classification systems. Together they belong to the Galloanserae. Several unusual extinct families of birds like the albatross-like the pseudotooth birds and the giant flightless diatrymas and mihirungs have been found to be stem-anseriforms based on common features found in the skull region, beak physiology and pelvic region. The genus Vegavis for a while was found to be the earliest member of the anseriform crowned group but a recent 2017 paper has found it to be just outside the crowned-group in the family Vegaviidae.
Below is the general consensus of the phylogeny of anseriforms and their stem relatives.
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. Anatidae is traditionally divided into subfamilies Anatinae and Anserinae. The Anatinae consists of tribes Anatini, Aythyini, Mergini and Tadornini. The higher-order classification below follows a phylogenetic analysis performed by Mikko's Phylogeny Archive and John Boyd's website.
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)
^ abAndors, A. (1992). "Reappraisal of the Eocene groundbird Diatryma (Aves: Anserimorphae)". Science Series Natural History Museum of Los Angeles County. 36: 109-125.
^ abMurrary, P.F; Vickers-Rich, P. (2004). Magnificent Mihirungs: The Colossal Flightless Birds of the Australian Dreamtime. Indiana University Press.
^ abBourdon, E. (2005). "Osteological evidence for sister group relationship between pseudo-toothed birds (Aves: Odontopterygiformes) and waterfowls (Anseriformes)". Naturwissenschaften. 92 (12): 586-91. doi:10.1007/s00114-005-0047-0. PMID16240103.
^ abAgnolín, F. (2007). "Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina". Revista del Museo Argentino de Ciencias Naturales. 9: 15-25. doi:10.22179/revmacn.9.361.
^Gonzalez, J.; Düttmann, H.; Wink, M. (2009). "Phylogenetic relationships based on two mitochondrial genes and hybridization patterns in Anatidae". Journal of Zoology. 279: 310-318. doi:10.1111/j.1469-7998.2009.00622.x.
^Liu, G; Zhou, L; Zhang, L; Luo, Z; Xu, W (2013). "The complete mitochondrial genome of bean goose (Anser fabalis) and implications for anseriformes taxonomy". PLoS ONE. 8 (5): e63334. doi:10.1371/journal.pone.0063334.
Agnolin, F (2007). "Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina". Revista del Museo Argentino de Ciencias Naturales. 9: 15-25. doi:10.22179/revmacn.9.361.
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.