DISTRIBUTIONAL PATTERNS OF VERTEBRATES 441 



The latter are: the hoary marmot {Marmota flaviventris) , a microtine 

 {Microtiis longicaudus), the hoary packrat {Neotoma cinerea), the 

 white-tailed jackrabbit {Lepus townseudi), and the red fox (Vidpes 

 fulva). These species are associated today with yellow pine and 

 spruce-fir forests and with the exception of the microtine reach 

 their southern limits about 250 miles to the north at high elevations 

 in the mountains. An extinct caribou-like species, Rangifer fricki, 

 and an extinct bovid, Eiiceratheriimi collinum, of debated ecological 

 significance, are also listed. 



As listed by Sherman (1952), the Pleistocene mammalian fauna of 

 Florida, which is commonly regarded as a refuge for warmth-re- 

 quiring species during the glacial stages, contains virtually no mam- 

 mals of boreal affinity. Possible exceptions are an elk (Cerviis sp.) 

 and a bog lemming {Synaptomys australis). The latter, however, is 

 associated with a presumably Sangamon (Third Interglacial) fauna 

 in Kansas (Hibbard, 1955b). 



The evidence from plant fossils on the coastal plain, from pollen 

 profiles that show spruce, fir, and other northern species of plants as 

 far south as southeastern Texas and northern Florida, and from the 

 numerous Pleistocene occurrences of vertebrates and vertebrate 

 faunas far south of their present distributions, leads to the conclu- 

 sion that there were great ecological changes in the southern United 

 States in the Pleistocene. It is the thesis of the present discussion 

 that these changes, particularly those accompanying the glacial 

 stages, were sufficiently drastic to fragment the ranges of warmth- 

 adapted coastal plain species and to force their southeastward and 

 south westward withdrawal, respectively into Florida and Mexico. 



EFFECTIVENESS OF THE GRASSLANDS BARRIER 



It has been argued that the southern grasslands have existed as a 

 barrier to the interchange of forest biota between the eastern United 

 States and Mexico since the development of the grasslands under in- 

 creasing aridity in the Pliocene. Separation since pre- Pliocene times 

 and slow rates of evolution are explanations given for the strong 

 floral resemblance between the humid forests along the escarpment 

 of the Mexican Plateau and the eastern forests (Braun, 1955). 

 Martin and Harrell (1957) discussed this theory in the light of some 

 vertebrate distributions and suggested that "few temperate-forest 

 animals were able to cross the arid Texas barrier in the Pleistocene. 



