Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives ( Paleognathae), ducks and relatives ( Anseriformes), ground-living fowl ( Galliformes), and 'modern birds' ( Neoaves). According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked ' reptile' clade, the Archosauria. Modern phylogenies place birds in the dinosaur clade Theropoda. For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird. Birds are categorized as a biological class, Aves. The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropod dinosaurs named Paraves.
