The early palaeozoic of the North Atlantic region as a test case for the use of fossils in continental reconstruction

Fossils have an important contribution to make in the reconstruction of early Palaeozoic palaeogeography. The North Atlantic regions include well-known early Palaeozoic fossil faunas, which should relate to the dispositions of continental plates during the period. However, different versions of Ordovician palaeogeography based on fossils are described in the literature, and in this paper we evaluate the influences which produce these discrepancies. The most reliable results are obtained by combining analysis of faunal similarity (especially shelf faunas) with mapping the distribution of those biofacies which relate to former deep-water sites. Planktic organisms may not be reliable palaeocontinent discriminants. Cladistic analyses of different groups of organisms living at the same (Early Ordovician) time point to the usefulness of trilobite data for palaeogeographic reconstruction. Such data indicate a three-fold division of the North Atlantic region into Baltica, Laurentia and Gondwana from the Cambrian to at least the mid-Ordovician. Paris and Robardet (1990) denied the existence of a Tornquist's Sea separating Baltica from Gondwana in the Early Ordovician, it is considered that their different reconstruction was because the planktic organisms on which they based their maps were not affected by this important oceanic separation to the same extent as shelly benthos. On the other hand, there is no evidence of deeper-water biofacies along the supposed line of Rheic Ocean separation of Avalonia from Gondwana until the Late Ordovician. However, there is some evidence of deeper biofacies along the Nantes-Sardic Line (new term) extending from southern Armorica to Sardinia.