Under the proposed model, the vertebrate skull evolves from the segmented anterior radials of the axial skeleton shown in Fig.2. This explains why in embryonic growth the skull is formed through the fusion of many separate parts.
It is often assumed that fishes with jaws descended from fish without jaws, and that the agnathan ancestors of gnathostomes were similar to some of the known agnathans, living or extinct. This elaborative development of functional jaws is problematic for gradual Darwinian evolution. Gould, for one, has suggested that jaws evolved through the migration of gill arches (Gould1980b). This fits with the observed general pattern of reduction and distortion of skeletal segments, but, looking at the known gnathostomes, it is hard to imagine that natural selection would favor the intermediate stages, in which gill arches come loose and migrate into the mouth area.
With the proposed model, agnathans and gnathostomes each evolve separately from the vertebrate progenitor; there is no need for a gradual or sudden transformation from one to the other. It might, however, be feasible for an agnathan to evolve from a gnathostome, as this could occur through gradual atrophy of the jaw.
In the primitive form depicted in fig.2, the anterior radials of the axial skeleton function like the tentacles of a squid. Over a geologically brief time, these radials fuse to form a solid skull. In gnathostomes, the upper and lower sets of radials fuse to form the upper and lower jaw. A few segments are distorted to form auditory ossicles. And some species retain secondary jaw articulations and cranial articulations.
In some vertebrates, such as gars and crocodiles, the mandible and maxilla are very symmetrical, supporting the idea of their primitive homology. Conventional Darwinians might argue that selection pressure just happened to foster such shapes, after jaws evolved from gill arches. But this does not explain why, in these vertebrates, the early embryonic pattern of fusion of many segments is similar within both maxilla and mandible, even though the supposed evolutionary history of these parts is different.
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