Relations between conodont provincialism and the changing palaeogeography during the Early Palaeozoic

Abstract

AbstractWorld-wide distribution patterns of conodont species during the Cambrian, Ordovician, and Silurian are assessed quantitatively using a Coefficient of Similarity (CS) formula. The validity of the idea is examined that there is a correlation between CS values and the geographic distance between the sample points, and that changes in CS values may, at least in part, reflect plate motions. To minimize effects of local palaeoecological control and the varying degree of exploration of the faunas, well known faunas are used from comparable depositional environments on the several continental plates.Conodont provincialism can be traced back to Upper Cambrian time, when there was an initial differentiation into low-latitude, warm-water (Midcontinent Faunal Region) and high-latitude, colder-water (Atlantic Faunal Region) faunas. The Lower Ordovician is characterized by great taxonomic diversification as well as striking provincial differentiation. Based on regional distribution patterns, it is possible to recognize a North American Interior Province, a Mediterranean Province, a North Chinese Province, a Siberian Province, a Baltic Province, and an Australian Province during Arenig time. A similar lateral differentiation prevailed during Llanvirn to lower Caradoc time. It is difficult to recognize a close correlation between CS values and inferred distances of plate separations in the Cambrian and Lower and Middle Ordovician.Unlike some megafossil groups that markedly decreased in provincialism during the Upper Ordovician, the conodont faunas continued to exhibit pronounced lateral differentiation despite the fact that the Baltic, Siberian, and North American plates were then relatively closely together in the equatorial zone. Near the Ordovician-Silurian boundary (probably in the uppermost Ordovician) there was a global, quite conspicuous turn-over in the conodont faunas. During that time, the taxa characteristics of the Atlantic Faunal Region disappeared and apparently, the ancestors of virtually all Silurian taxa are to be found in the Midcontinent Faunal Region.The Silurian conodont faunas have a cosmopolitan character although there are minor local differences that may be attributed to local environmental control. During Silurian time, the plates that have produced the best conodont faunas (North America, Baltica, Siberia) were all in the equatorial zone, and a general uniformity in environmental conditions may be one of several reasons behind the cosmopolitan character of the Silurian conodont faunas.Many Lower Palaeozoic conodont species were capable of crossing water bodies of oceanic dimensions, and the distribution of many taxa seems to have been more closely controlled by water temperature and other ecological parameters than by potential migration barriers related to size of water bodies and emerged continental blocks. Nevertheless, because of their pronounced latitudinal differentiation during the Ordovician, the conodont faunas may be used as tools to decipher some of the positions of continental plates.