PALEOBIOGEOGRAPHY OF THE MARINE REALM 159 



Temperature, which is effectively one expression of light, 

 provides us with the most concrete basis of biogeographic sub- 

 division and the most convenient circumglobal terms. Global 

 temperature and temperature zonation has, of course, varied in 

 the past, from causes both terrestrial (e.g., Plass, 1956) and 

 extraterrestrial (e.g., Brooks, 1951, pp. 1016-1017); and, for the 

 paleobiogeographer, it is essential not to confuse temperature 

 subdivisions with geodetic terms such as tropical, polar, or 

 equatorial. The sequence warm, temperate, and cold, with the 

 temperature ranges indicated (Fig. 3), describes the local condi- 

 tions and biotal affinities as closely as the paleobiogeographer can 

 usually recognize them, and without commitment as to orientation, 

 subsequent movement, and geodesy. In paleobiogeography also it 

 is rarely meaningful to attempt to approximate depth zones more 

 closely than shore, shallow, and deep, with deep as here used 

 including everything below the photic zone (or "wave base"). 

 Quantitative depth and temperature limits naturally should be 

 estimated wherever the evidence permits, but the usual kinds of 

 paleontological and sedimentological evidence are apt to limit 

 depth evaluation of ancient seas to more ambiguous terms such as 

 shelf, shallow shelf, deep shelf, shallow slope, deep slope, basin, or 

 shore zone. The limiting effects of temperature and other variables 

 may affect the organism differently at different stages of existence; 

 Hutchins (1947) showed how upper and lower seasonal tempera- 

 ture limits on installation and survival produce four basic types 

 of latitudinal zonation of interest to zoogeographers. 



Salinity affects the inhabitants of the hydrosphere through 

 osmosis and secondarily through density, light penetration, pH, 

 and the distribution of mineral nutrients. The effects of salinity 

 variations in the older Tertiary of the Fergana Gulf were described 

 at this Congress by Hecker, Ossipova, and Belskaya (1959). 



The distribution of life in the marine realm under conditions 

 existing at any given time is a reflection of the variables mentioned, 

 plus depth, pressure, gas content, and turbidity of the water and 

 substrate conditions. The paleobiogeographer has only the fossils 

 and the enclosing sediments, which may or may not have been 

 associated in life. He must deduce the rest from the biological 



