Jawed vertebrates gnathostomes

Apart from sharks, where the upper and lower teeth are relatively similar in shape and number, the upper and lower jaws of modern jawed vertebrates (gnathostomes) generally differ in the shape and number of their teeth or tooth-bearing dermal bones. In bony fishes (osteichthyans), and land vertebrates (tetrapods), this difference disappears as one considers early, Paleozoic groups, whose upper jaw bones are almost a mirror image of those of the lower jaw. This curious symmetry has been pointed out long ago by the American paleontologist A.S. Romer in early... 

Tetrapoda In some classifications, a superclass of jawed chordates ( Gnathostom-ata) comprising all vertebrate animals with four limbs, l.e. the amphibians, reptiles, birds, and mammals. The skeleton of the limbs of all tetrapods is based on the same five-digit pattern (see pentadactyl umb). 

In the absence of articulated specimens, it is problematic to prove the presence of jawed vertebrates in the Ordovician based upon scale morphology and histology alone. However, a number of scales recovered from the Harding Sandstone bear a remarkable resemblance to those from accepted stem and crown gnathostomes with jaws. They can be divided into two types Skiichthys halsteadi with scales bearing a... 

Concerning broader hypotheses of early vertebrate diversity, the view that diversity patterns and the extinction of most clades of jawless fish reflect competition between agnathans and gnathostomes or between specific clades of jawless and jawed fish, although widely held, must be regarded as untested, and at present untestable, speculation. The available data for feeding habits, palaeogeographic distribution, and habitats are inadequate for this purpose, and the possibility that members of two contemporary clades were potential competitors can be neither confirmed nor refuted. Competitive interactions are just one of a number of possible explanations of the pattern of early vertebrate diversity. 

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