This is not surprising since different paralogs of effector genes are deployed at different times and places among different gnathostome models, likely conferring some species-specific properties.
The data suggested that several genes were expressed in patterns analogous to those in gnathostomes (Newth, 1950; Myojin et al, 2001; Neidert et al, 2001; Meulemans and Bronner-Fraser, 2002; Meulemans et al.
Support for the forebrain in gnathostomes is provided by the paired polar bodies and anterior trabeculae.
The two structures of primary importance in the transition from agnathan (jawless) fish to gnathostomes were the mandibular arch (gill arch 2) and the hyoid arch (gill arch 3).
Other paleontologists, however, argue that gnathostomes may not have evolved that early.
He asks why, if gnathostomes existed in the Ordovician, scientists haven't found complete fossils of these animals.
When the first gnathostomes
evolved during the Silurian, not only were teeth already formed, but the ability to form denticles like those found over the external surface of cartilaginous fishes had evolved and was found to extend throughout the entire oral mucosa (Kemp, 1999).
The myelinated nervous system arose in a common ancestor of all modern-day gnathostomes
Remarks about the cladistic analysis in connection with myopterygian hypothesis and the problem of the origin of gnathostomes