Biogeography of Ordovician and Silurian faunas

Abstract

The new reconstructions generated for this volume fit well with the majority of Ordovician and Silurian faunal data and are a great improvement on previous attempts. The distribution of selected trilobites, brachiopods and graptolites are plotted on the new maps and confirm the importance of palaeolatitude in controlling the faunal distributions, particularly of the old cratons, which are shown for the Early Ordovician and Late Silurian. Two contrasting patterns of cratonic faunas are (a) disjunct or (b) gradational across a large palaeocontinent, e.g. Gondwana. Marginal and deeper-water biofacies show different patterns, which help to define the edges of palaeocontinents, but which are not so constrained in their palaeolatitudinal distributions. In contrast pelagic trilobites do not help to define palaeocontinents, but were sensitive to palaeo temperature and palaeolatitude. Specific case histories are considered, in particular the closing of Iapetus and the contemporary widening of the Rheic Ocean and the positioning of Avalonia, and the ancestry of the various associated trilobite and brachiopod genera during the later Ordovician. The Ashgill deeper-water Foliomena fauna is also plotted.