[Jaws of birds]

Birds (Class Aves) have been hypothesized to have evolved from theropod dinosaurs, which belong to the group known as diapsid reptiles, during the Mesozoic era. They subsequently acquired derived characters relating to flying ability in all parts of their body, including the jaws, which were toothed in the ancestral forms but beaked in later members. When the fore limbs became modified as wings specialized for flight, they became incapable of grasping food, which function was then assumed by the avian bill. Avian bills consequently evolved great morphological diversity reflecting very divergent modes of feeding. To illustrate the progression of "the evolution of vertebrate jaws," the comparative osteology of avian cranial bones is exemplified by reference to Mesozoic birds and the morphological and functional variations observed among the bills of Neornithes.