Broadly speaking, avian skulls consist of many small, non-overlapping bones. Pedomorphosis, maintenance of the ancestral state in adults, is thought to have facilitated the evolution of the avian skull. In essence, adult bird skulls will resemble the juvenile form of their theropod dinosaur ancestors. As the avian lineage has progressed and as pedomorphosis has occurred, they have lost the postorbital bone behind the eye, the ectopterygoid at the back of the palate, and teeth. The palate structures have also become greatly altered with changes, mostly reductions, seen in the ptyergoid, palatine, and jugal bones. A reduction in the adductor chambers has also occurred These are all conditions seen in the juvenile form of their ancestors. The premaxillary bone has also hypertrophied to form the beak while the maxilla has become diminished, as suggested by both developmental and paleontological studies. This expansion into the beak has occurred in tandem with the loss of a functional hand and the developmental of a point at the front of the beak that resembles a "finger". The premaxilla is also known to play a large role in feeding behaviours in fish.
