Abstract
Appearing for the first time in Upper Cambrian deposits (Repetski, 1978), the fossil agnathans underwent extensive evolutionary radiation, flourishing in the Silurian then decreasing in numbers, finally to disappear from the fossil record at the close of the Devonian. Since the first jawed vertebrates, the acanthodians were not to be found earlier than the upper Silurian, it is hardly surprising that the agnathans and gnathostomes should have been regarded as a phyletic sequence, representing the evolution of hinged jaws from the more primitive jawless condition of some agnathan vertebrate ancestor. Many now reject this point of view (not least because of the difficulties of deriving the visceral arches of the gnathostomes from the fundamentally different structures of the agnathans) preferring instead to regard the agnathans and gnathostomes as sister groups of equal antiquity, but sharing a hypothetical and more remote common ancestor.
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