These two groups only occupied two ecological niches during the Mesozoic, living in water and on the ground, while the toothed Enantiornithes were the dominant birds that ruled the trees and air.

The asteroid that ended the Mesozoic destroyed all trees as well as animals in the open, a condition that took centuries[citation needed] to recover from.

[5][6][7][8][9][11] †Pelagornithidae (pseudo-tooth birds) †Gastornithidae †Dromornithidae (mihirungs) †Vegaviidae Anseriformes (screamers and waterfowl) Anatidae systematics, especially regarding placement of some "odd" genera in the dabbling ducks or shelducks, is not fully resolved.

The higher-order classification below follows a phylogenetic analysis performed by Mikko's Phylogeny Archive[13][14] and John Boyd's website.

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.

[15] Anhima Chauna Anseranas Dendrocygna Thalassornis Plectropterus Stictonetta Nettapus Biziura Heteronetta Nomonyx Oxyura Malacorhynchus Coscoroba Cereopsis Sthenelides Cygnus Branta Anser Merganetta Chloephaga Oressochen Neochen Radjah Tadorna Alopochen Histrionicus †Camptorhynchus Clangula Polysticta Somateria Melanitta Bucephala Mergellus Lophodytes Mergus Cairina Aix Callonetta ?Salvadorina Chenonetta Hymenolaimus Sarkidiornis Pteronetta Cyanochen Marmaronetta Asarcornis Netta Metopiana Aythya Anas Sibirionetta Spatula Tachyeres Lophonetta Amazonetta Speculanas Studies of the mitochondrial DNA suggest the existence of four branches – Anseranatidae, Dendrocygninae, Anserinae and Anatinae – with Dendrocygninae being a subfamily within the family Anatidae and Anseranatidae representing an independent family.