Early Paleozoic Conodont Biostratigraphy in the Atlantic Borderlands

Abstract

Abstract Cambrian conodonts, still relatively poorly known but apparently widespread in the Atlantic Borderlands, include some 15 multielement genera. Described faunas are mainly from Scandinavia, Poland, and Germany (erratics) whereas little information is available from, for instance, eastern North America and the British Isles. Although no succession of formally defined conodont zones has as yet been established throughout the system, Cambrian conodonts have considerable potential as guide fossils, particularly in the Upper Cambrian, where many forms show limited vertical ranges and very wide horizontal distributions. Ordovician conodonts, which include about 80 multielement genera are much better known than the Cambrian ones, and show striking provincial differentiation throughout the period. In the North American Midcontinent Province, some 17 biostratigraphic units of zonal type have been recognized and in the North Atlantic Province, some 15 zones, and more than 10 subzones, have been formally defined. Ordovician conodonts have proved very useful biostratigraphically, in many instances providing a stratigraphic resolution superior to that of any other fossil group. Silurian conodont faunas exhibit far less taxonomic diversity (about 15 multielement genera) and provincial differentiation than Ordovician ones. The Silurian conodont succession, best known in the Atlantic Borderlands from Great Britain, Scandinavia, Virginia, and the eastern part of the North American Midcontinent, form the basis for about 12 zones. Many of these zones have been widely recognized not only in Europe and North America but also in Australia, Asia, and northern Africa. Although published reports on Early Paleozoic conodonts of Africa and South America include only a few papers, the available data suggest that the conodonts faunas from those continents are, by and large, very similar to those known from the northern hemisphere. Co-occurrence of stratigraphically diagnostic conodonts and graptolites has made it possible to tie together conodont and graptolite zonal units in the Ordovician and Silurian at a relatively large number of stratigraphic levels. Geographic distance apparently was of relatively minor importance in comparison with ecologic factors for the establishment of patterns in the Early Paleozoic conodont biogeography. This conclusion, along with the fact that very similar conodont faunas were present on both sides of the Proto—Atlantic, make conodonts of little use for evaluations of the size and development of the Proto—Atlantic Ocean, at least at the present time.