The continents and terranes of the greater Iapetus Ocean have provided critical biotic and palaeomagnetic data for the reconstruction of Early Palaeozoic crustal configurations. During the last 40 years, simple orthogonal models for the opening and closing of the Iapetus Ocean have been replaced by more complex, multiplate models involving rotations and rapid translations. Moreover the ocean was probably filled by islands associated with microcontinents and volcanic arcs, particularly during the Ordovician. Although faunal provinces are well established and testable for the continental platforms of Baltica, Gondwana, Laurentia and Siberia, less certain are the positions of the many small island terranes associated with the ocean and its margins. Here, data derived from commonly poorly-preserved and often deformed material from the Caledonian mountain belts provide important evidence for the evolution of the ocean and its margins. These faunas, assembled in, for example, the Celtic and Toquima-Table Head provinces, are usually characterized by a high proportion of endemic taxa, some cosmopolitan elements together with a mixture of inhabitants from the platform provinces. A range of multivariate distributional datasets is replacing the use of key taxa in defining and positioning the smaller terranes. Interrogations of these large databases using Principal Component and Cluster analyses together with cladistic and seriation methods are providing a more accurate picture of the evolving ocean, its terranes and biotas. Moreover the roles of these terranes in creating and conserving biodiversity together with biotic staging posts in cross-oceanic migrations are becoming clearer as more data are available from both the Caledonian mountain belts and the adjacent platforms.
This complex biogeographic pattern was, however, further disrupted by climatic and environmental changes, for example, high sea levels associated with greenhouse conditions during the Sandbian generated more cosmopolitan taxa, whereas icehouse conditions during the Hirnantian encouraged a more-simple, better-defined biogeographic configuration against a closing Iapetus Ocean system. Biodiversity fluctuated markedly during the Ordovician Period suggesting that evolution of the Iapetus Ocean and its terranes provided an important geographic dimension to both the Great Ordovician Biodiversification and the End-Ordovician extinction events.