Hypotheses that provide explanations of major transitions in early vertebrate evolution can be tested by analysis of fossils. In this chapter I consider two widely held views of early vertebrate evolution: firstly, the hypothesis that jawless vertebrates were driven to almost complete extinction by competition with gnathostomes during the late Palaeozoic; and secondly, scenarios that link the origin of vertebrates and the origin of gnathostomes to a long term ecological trend towards increasingly active and predatory lifestyles.

Analysis of familial diversity suggests that there is no simple relationship between the decline of agnathans and the rise of gnathostomes. Rates of family extinctions in jawless vertebrates were highest in the Early Devonian, but gnathostome diversity peaked in the Late Devonian. Also, the ecology of many early vertebrates is poorly constrained. The hypothesis that the pattern of early vertebrate diversity reflects competition between agnathans and gnathostomes or between specific clades of jawless and jawed fish must be regarded as untested, and at present untestable, speculation.

Evidence from conodonts supports the hypothesis that a shift to predation occurred at the origin of vertebrates, but data concerning feeding in other groups of fossil agnathans, exemplified here by heterostracans, are currently inconclusive. If rigorous analysis demonstrates that any of the major clades of fossil agnathans were non-predatory, hypotheses that early vertebrate evolution was driven by a long-term trend towards increasing levels of activity and predacity may be overturned.

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