Abstract

After a long mid-Ordovician to late Silurian existence in shallow marine waters the agnatha were equatorially widespread. They entered brackish and freshwater environments in late Silurian and Devonian times and very many new taxa appeared. Biogeographical ‘provinces’ (Laurussia, Angara, South China, East Gondwana) may be distinguished, each a response to local conditions and ecological opportunities resulting from local terrestrial and biological events. The influence of continental plant life must have been rapid and beneficial to the vertebrates. Previously continuous coastal marine habitats were now largely replaced by a zone of transitional environments wherein vegetational and tidal factors were important. Thus for a while access to inland waters was favored rather than longshore migration. Eventually these restrictions faded and the ‘endemic’ populations gave way to the cosmopolitan. Migration by marine vertebrates and by those now strongly preferring brackish and freshwater habitats was fully restored by the end of Early Devonian time. Continental distributions in recent global reconstructions are consistent with this.