Paleolatitudes in the Devonian of Brazil and the Frasnian-Famennian mass extinction

Abstract

Differences in faunal composition of contemporary Devonian marine communities in Brazilian sedimentary basins suggest variation because of climatic gradients. Temperatures may have ranged from subarctic conditions in the Parana Basin, a type of Devonian Hudson's Bay, to temperate in the Amazon and adjoining basins, analogous to a modern “north Atlantic” climate. These gradients were paralleled in other parts of South America. The Malvinokaffric cold climate fauna was dominated by groups oforganisms that survived the worldwide late Devonian mass extinctions. Invertebrate groups absent in the Malvinokaffric regions of Brazil, Bolivia and Argentina (i.e. those restricted to Devonian equatorial belts of North America, Eurasia and Australia) were decimated in the late Devonian. These two parallel observations suggest that late Devonian extinction may have been the result of drastic cold spells that killed off reefal and peri-reefal life. By elimination of crucial benthic community components, the trophic structure of the shallow marine ecosystem of the time was upset. Subsequent repopulation of the Carboniferous seas was accomplished by hardy, eurythermal invertebrate taxa present in cold as well as tropical regions. Cold spells and mass mortality in very shallow waters may go some way to explain the production of the widespread black shale environments so typical of marine regressive phases in the late Devonian of the western hemisphere.