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Qilinyu had three bones, the dentary, maxilla and premaxilla, that characterize the modern vertebrate jaw seen in bony fish, amphibians, reptiles, birds and mammals, though they are absent in the cartilaginous sharks and rays.

The maxilla and premaxilla are bones of the upper jaw.

"In us, the lower jaw is made entirely from the dentary. Most of the upper jaw is composed from the maxilla, but the bit that carries the incisor teeth is the premaxilla," Ahlberg said.

They reasoned that reptile and dinosaur snouts develop from premaxilla in a similar way, and that the developmental pathways that form the snout were altered in the course of bird evolution.

But in the early stages of bird evolution, the twin bones that formed the snout in dinosaurs and reptiles — called the premaxilla — grew longer and joined together to produce what is now the beak.