Many textbooks list five classes of vertebrates: Pisces(28,000 species), Amphibia (4300), Reptilia (6000), Aves(9000), and Mammalia (4800). But as Nelson (1969)clearly demonstrated, this five-class system is anthropomorphic, with bird and mammal groups overemphasized by the mammal doing the classification – that is, us. The morphological and evolutionary gap between the Agnatha, the jawless vertebrates (lampreys and hagfishes), and other groups of fishes is much greater than between the classes of jawed fishes on the one hand and the tetrapodson the other hand. Thus fishes (or Pisces) is not a monophyletic group but a grade used for convenience for the Agnatha, Chondrichthyes, bony fishes, the fossil Acanthodii and several primitive, extinct jawless superclasses.
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