Early fossil reptiles retained the arrangement seen in more primitive vertebrates, but in mammals, the lower surface of the palatine became folded over during evolution, forming the horizontal plate, and meeting in the midline of the mouth. This forms the rear of the hard palate, separating the oral and nasal cavities, and making it easier to breathe while eating. A parallel development has occurred to varying degrees in many living reptiles, reaching its greatest extent in crocodilians. In birds, the palatine bones remain separate, long the sides of the rear part of the upper jaw, and typically have a mobile articulation with the cranium.