CLIMATE SINCE THE LATE CRETACEOUS 69 



pollen has not surely been found earlier than the Lower Miocene. 

 It is probable that the components of the existing prairies had their 

 beginnings about the Middle Miocene, although the prairies as 

 such do not seem to have developed until well into the Pliocene 

 (Elias, 1942; MacGinitie, 1953, p. 59). The climatic change (Chaney 

 and Elias, 1936, pp. 25-34) that stimulated the growth of herbaceous 

 vegetation had a profound, indirect effect on the evolution of the 

 mammals, since abundant new supplies of nutritious food became 

 available for herbivores. The fossil localities of the later Miocene 

 and Pliocene offer striking evidence of the astonishing abundance 

 of mammalian life on the Great Plains and westward. 



By making comparisons between the present deciduous forest 

 floras of eastern Asia and North America we can gain some idea of 

 the herbs that came southw^ard with the dispersal of the Arcto- 

 Tertiary flora. Here we find closely similar or identical species of 

 herbs on the two continents, and these are almost entirely broad- 

 leafed, perennial forms with heavy rootstocks. Hui-Lin Li (1952, 

 pp. 385-405) has presented this evidence in his excellent paper on 

 the related floras of Asia and America. 



The numerous Upper Miocene and Lower Pliocene floras of the 

 West have been studied by Axelrod (1956, 1957) and their climatic 

 significance has been well set forth. He has recognized about a half- 

 dozen climatic provinces — beginnings of the sharply demarked 

 climatic provinces now found in the area, but by no means as clearly 

 defined. He has also shown that the average temperatures were not 

 much higher than those of today, iDut that the summer maximum 

 temperatures were reduced and the winter minima considerably 

 raised; in other words, there was a much more equable climate. 

 The frost-free season in west-central Nevada was probably three or 

 four months longer than at present. Axelrod has also emphasized the 

 strong evidence for a shift in the pattern of seasonal distribution 

 and kind of precipitation, from summer showers and winter rain to 

 the present regime of dry summers and winter snows. 



The late Miocene and early Pliocene floras from Nebraska to 

 California still indicate comparatively mild conditions, with, on the 

 whole, moderate rainfall. The presence of such trees as Cedrela 

 (cedro) and Per sea (avocado) in the latest Miocene suggests the 

 absence of severe winters, but the majority of the species are 

 essentially modern in aspect and are not greatly displaced north of 



