Aequornithes

Aequornithes (, from Latinaequor, expanse of water + Greekornithes, birds), or core water birds, are defined in the PhyloCode as "the least inclusive crown clade containing Pelecanus onocrotalus and Gavia immer", that is, the last common ancestor of the great white pelican and the common loon, as well as its descendants, extinct and extant.

Aequornithes, as currently understood, includes the cladesGaviiformes (loons), Sphenisciformes (penguins), Procellariiformes (petrels and albatrosses), Ciconiiformes (storks), Suliformes (gannets and cormorants) and Pelecaniformes (pelicans and herons). The monophyly of this grouping is supported by several molecular phylogenetic studies. It does not include several unrelated groups of aquatic birds such as flamingos and grebes (Mirandornithes), shorebirds, gulls, and auks (Charadriiformes), or the Anseriformes (waterfowl). The group first appeared during the Maastrichtian stage of the Late Cretaceous, with the earliest known member being Pujatopouli from the López de Bertodano Formation of Seymour Island, Antarctica.

Based on a whole-genome analysis of the bird orders, the kagu and sunbittern (Eurypygiformes) and the three species of tropicbirds (Phaethontiformes) together styled as the Eurypygimorphae are the closest sister group of the Aequornithes, in a clade later named Phaethoquornithes.