THE VERTEBRATE BEFORE THE SILURIAN

Our knowledge of the structure and diversity of the pre-Silurian verte brates has been considerably increased during the past thirty years, t hrough the discovery of almost complete specimens from the Middle Ordo vician of North America, Australia and Bolivia, but also of numerous i solated remains of Arenig to Ashgill age. Porophoraspis appears now as the earliest known undisputable vertebrate. It is now established tha t jawed vertebrates (gnathostomes) occur in the Ashgill, and possibly as early as the Caradoc. In addition, Ordovician and Cambrian fragment s have been referred to the vertebrates on the ground of some histolog ical resemblance with the structure of the vertebrate dermal skeleton, but they remain controversial and the due can only come from the disc overy of articulated specimens. The overall morphology of euconodonts makes them likely to be vertebrates, yet the homology between the hist ological structure of their denticles and that of the vertebrate hard tissues remains vividly debated. Finally, some non-mineralized, Cambri an fossils may be reinterpreted as basal craniates or vertebrates, mor phologically comparable to hagfishes and lampreys.