FOSSIL LAND MAMMALS AND WESTERN NEARCTIC FAUNA 115 



3. Approximately 45 million years, a sufficient interval for great 

 evolution of morphologies and change of ecological-physiological 

 tolerances, elapsed between early Eocene and late Miocene. 



4. There are possible subtle differences between the similar litho- 

 facies of the two areas. Similar lithofacies were not necessarily 

 formed under the same environmental conditions. The color differ- 

 ences in the Caliente formation may mean little more than differing 

 sediment-source, and both the red and the gray facies may have 

 been deposited in the same physical environment. 



Detailed sedimentological studies have not yet been accomplished, 

 and much work lies ahead. Nevertheless, the example from Cuyama 

 suggests that conclusions as to relative abundance of taxonomic units 

 and adaptive types in certain lithologies are premature and may be 

 completely misleading, especially when based on samples collected 

 by techniques of disparate refinement. 



HISTORY AND AFFINITIES OF THE ORDERS OF NEARCTIC 



LAND MAMMALS 



Earliest records of the class Mammalia are from the late Triassic 

 of England, but generalized reptilian progenitor stocks are known 

 from South Africa as well as in various Holarctic districts. There- 

 fore, on the basis of the stratigraphic record, the possibility of either 

 Holarctic or Paleotropic origin for the class Mammalia must be 

 conceded. 



Pre-Paleocene land mammal faunas are so poorly known on most 

 continents that little can be determined as to intercontinental dis- 

 persal and affinities. From late Paleocene to the present, however, 

 the fauna of North America is clearly dominated by groups common 

 to many parts of Holarctica, especially such forms as shrews and 

 moles, rabbits and pikas, sciuromorph and myomorph rodents, 

 creodont and fissiped carnivores, condylarths, uintatheres pro- 

 boscideans, perissodactyls, and artiodactyls. All zoogeographers 

 know that the Nearctic Cenozolc mammalian fauna differs markedly 

 from the prototherian-metatherian fauna of Australasia and is only 

 slightly less distinct from the pre- Pleistocene metathere-edentate- 

 archaic ungulate assemblage of South America. 



Nearctica is dominant in the recorded range and dispersal of the 

 33 recognized orders of land mammals (Fig. 7), but only 9 orders 

 (with 27 families) are represented here by living forms: Marsupialia 



