ABSTRACT
The last 20 years have seen a growth of evidence that vertebrates originated during the Cambrian followed by an evolutionary radiation through the Ordovician, during which many of the progenitors of major groups appeared. These diversification events are some 50 million years earlier than previously considered. This shift has resulted largely from the consideration of microremains of vertebrates, their teeth and scales, and has greatly improved the fossil record of early vertebrates. Recently described articulated specimens have also played a fundamental role in our understanding of some primitive lineages. The identification of major vertebrate radiations in the Late Cambrian and in the Middle Ordovician is considered to be a consequence of extrinsic environmental factors (sea level change) allowing the exploita-tion of earlier, intrinsic, Hox cluster duplication events. Although conodonts are key components of the Cambro-Ordovician radiation of vertebrates (cf. Donoghue and Aldridge, this volume), we here confine our review specifically to recent advances in the understanding of non-conodont representatives.
