174 POPULATIONS OF THE SEA 



Identification of Equatorial and Polar Positions 



Given a particular distribution at the present surface of the 

 earth of reliably determined fossil organisms, communities, and 

 biotas whose ecologic significance is believed understood, the 

 special job of the paleobiogeographer is to relate these variables 

 consistently. Factors complicating this task include the parallelism 

 between the latitudinal and the vertical distribution of life, 

 heat transport by water currents, parallel origins of similar 

 associated phenomena, and the poleward migration and blending 

 of paleobiogeographic temperature indicators at times of less- 

 marked latitudinal temperature zonation. 



Brooks (1949, pp. 50-53, 189, 192-197) illustrated the depend- 

 ence of the present wind scheme on the pressure balance between 

 poleward temperature decrease in the lower 8 km of the atmosphere 

 (troposphere) and the poleward increase of temperature in the 

 stratosphere. He discounts the significance of the glacial anti- 

 cyclone per se. Lessening of the tropospheric temperature gradient 

 poleward would reduce polar air pressures and result in repression 

 of the polar easterlies and expansion poleward of the westerly wind 

 systems and the low-latitude trade winds. The global system of 

 ocean currents would necessarily undergo parallel changes, with 

 the result that all their boundaries would be moved poleward 

 (Figs. 5-7) and eventually eliminate the easterly polar currents. 

 This would accelerate the warming effect at the poles and further 

 reduce the actual temperature gradient between poles and equator, 

 as (Dubois, 1895, pp. 72-99) would also be the case in the event 

 of greater general input of solar heat. 



The accepted technique for the paleobiogeographical location of 

 equator and poles for a given time interval is to draw a line 

 through the middle of an areal distribution pattern of biological 

 indicators of warm water, and then to find the locus of points that 

 would have been about 90 degrees from such a line by moving 

 poles or continents until the best fit is found. If the paleobio- 

 geographer is to contribute critically to hypotheses of polar 

 migration and continental drift, he should also be constantly on 

 the lookout for the most reasonable interpretations of his evidence 



