No abstract

Vertebrate remains from Ordovician formations indicate a conservative mode of life in marine environments but which seems to have become more diverse by the invasion of varied non-marine aquatic habitats in Siluro-Devonian time. The spread of Old Red Sandstone facies results from Caledonian tectonism and provides the earliest Phanerozoic example of an adaptive radiation of both agnathan and gnathostome vertebrates. Prior to the late stages in the closure of Iapetus the 'European' and 'American' vertebrate faunal assemblages were distinct. Closure brought an immediate influx of 'American' agnatha into the European area, and a rapid increase in numbers of species. Subsequent earth movements and changes in palaeogeography favoured the development of internal basins with accompanying differentiation of vertebrate stocks. Some primitive forms, perhaps relatively isolated geographically, persisted until the Late Devonian by which time nonmarine gnathostomes had spread to most of the continents.