Birds (Avialae, meaning "bird wings") are the only dinosaurs to have survived the K-Pg extinction event, and are characterized by their flying abbilities, absent in other dinosaurs. The definition of avialans is highly regarded as being all dinosaurs more closely related to modern birds than to dromaeosaurids or troodontids. The earliest avialans arrived in the Middle-Late Jurassic, already capable of some form of powered flight or at least gliding.

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