download link:The skull of the earliest tetrapods closely resembles that of their ancestors amongst the lobe-finned fishes. The skull roof is formed of a series of plate-like bones, including the maxilla, frontals, parietals, and lacrimals, among others. The various separate bones that compose the temporal bone of humans are also part of this series. A further plate composed of four pairs of bones forms the roof of the mouth; these include the vomer and palatine bones. The base of the cranium is formed from a ring of bones surrounding the foramen magnum and a median bone lying further forward; these are homologous with the occipital bone and parts of the sphenoid in mammals. Finally, the lower jaw is composed of multiple bones, only the most anterior of which (the dentary) is homologous with the mammalian mandible. In living tetrapods, a great many of the original bones have either disappeared, or fused into one another in various arrangements. In mammals and birds, in particular, there have been modifications of the skull to allow for the expansion of the brain. The fusion between the various bones is especially notable in birds, in which the individual structures may be difficult to identify . Living amphibians typically have greatly reduced skulls, with many of the bones either absent or wholly or partly replaced by cartilage.
