Evolution and taxonomy

Main article: Evolution of birds
Slab of stone with fossil bones and feather impressions
Archaeopteryx, the earliest known bird
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[2] Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[3] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.[4] Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[5]
Archaeopteryx, from the Tithonian stage of the Late Jurassic (some 150–145 million years ago), is the earliest known bird under this definition. Others, including Jacques Gauthier and adherents of the Phylocodecrown group. This has been done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae[6] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs. system, have defined Aves to include only the modern bird groups, the
All modern birds lie within the subclass Neornithes, which has two subdivisions: the Palaeognathae, containing birds that are flightless (like ostriches) or weak fliers, and the wildly diverse Neognathae, containing all other birds.[4] These two subdivisions are often given the rank of superorder,[7] although Livezey and Zusi assigned them "cohort" rank.[4] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[8] to 10,050.[9]