The fossil record indicates that birds evolved from feathered ancestors within the theropod group, which are traditionally placed within the saurischian dinosaurs, though a 2017 paper has put them in a proposed clade Ornithoscelida, along with the Ornithischia. True birds first appeared during the Cretaceous period, around 120 million years ago. DNA-based evidence finds that birds diversified dramatically around the time of the CretaceousâPalaeogene extinction event 66 million years ago, which reduced the Pterosaurs, and killed off all the non-avian dinosaur lineages. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.
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