Jaw Mechanisms: Rhipidistians, Amphibians, Reptiles

mechanisms considered in this study. Other functions also have a direct relationship to jaw morphology?for example, the actions of respiration in fish. These will be intro? duced only as they pertain directly to one or another specific point in the discussions. In performance of their primary functions, the jaws necessarily have effects upon other systems; and the actions of adjacent systems modify the movement of the jaws. It is not possible to isolate these various actions or to limit or precisely define limits of morpho? logical or functional systems. Jaw mecha? nisms must be considered from this point of view. We will look at them as a more or less coherent unit with specific functions, but keep in mind that actions and changes external to this system may have strong bearing upon the course of its evolution. This study is concerned with the rhipidistian ancestors of the tetrapods, the shifts in jaw mechanisms that took place in the de? velopment of primitive tetrapods, and the initiation of evolutionary patterns among amphibians and reptiles. It will be empha? sized that the principal changes in jaw mechanisms took place after the origin of tetrapods, changes that became possible only after other modifications had occurred. The actual rhipidistian ancestors of the tetrapods are not known. Jarvik (1942) suggested that there were two sources and that these can be recognized, in general, among the osteolepiform and prorolepiform rhipidistians. Cross (1941) held the opinion that the stegocephalians came from a subgroup of rhipidistians rather different from the ones we know and that the well known rhipidistians are rather distant in structure from the actual ancestors of tetra? pods. Nilsson (1944) agreed with this posi? tion, emphasizing dental differences be? tween stegocephalians and rhipidistians.