Migration, origination and extinction of Southern Hemisphere brachiopods during the middle Carboniferous
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The migration patterns and latitudinal diversity gradients of early and middle Carboniferous articulate brachiopod genera from the paleoequatorial zone and Southern Hemisphere are consistent with the paleoclimatic history of these intervals interpreted from the study of paleoequatorial and Northern Hemisphere forms. Both poles apparently became warmer between the middle and late Visean. South Polar cooling associated with glaciation probably began in the late Visean. At this time, 44 of 92 Southern Hemisphere nonequatorial genera migrated toward the equator. The North Pole remained warm into the Namurian A and cooled during the Namurian A to B. Both Northern and Southern Hemisphere non-equatorial genera reacted to high-latitude warming with significant increases in percent origination; genera from all latitudinal zones responded to high-latitude cooling with significant increases in percent extinction. During the Namurian A to B, when both poles were cool, a small but statistically significant number of equatorial genera (8 out of 70) migrated out of the equatorial zone into the non-equatorial Northern Hemisphere. No similar migration into the Southern Hemisphere occurred at this time, perhaps due to the steep latitudinal temperature gradient. Several lines of evidence suggest that the latitudinal temperature gradient was steeper in the Southern Hemisphere than in the Northern Hemisphere: whereas percent origination in northern non-equatorial latitudes rose significantly following North Polar cooling, origination rates in southern non-equatorial latitudes did not rise significantly following South Polar cooling; Southern Hemisphere non-equatorial brachiopods suffered a greater loss of diversity during the interval of polar cooling than either equatorial or Northern Hemisphere non-equatorial brachiopods; and there may be Namurian tillites within 20S of the paleoequator (Veevers and Powell, 1987). 1991.
