FOSSIL LAND MAMMALS AND WESTERN NEARCTIC FAUNA 123 



Hiljbard; but it is beyond the intent of this paper to consider the 

 taxonomic problems of Pleistocene mammals or the effects of "split- 

 ting" or of "lumping" on the species Hsts of fossils. Hibbard's list 

 indicates the general stocks that were available for evolutionary 

 modification and zoogeographic change through the last few thou- 

 sand years. Many of the late Pleistocene animals are very similar in 

 hard part structures to the living species and have been so identified. 

 Very few of the middle or early Pleistocene forms have been referred 

 to living species. 



Intercontinental faunal comparisons show probable trans-Hol- 

 arctic mammalian dispersal waves during most of the subepochs 

 from late Jurassic through late Pleistocene. Strongest evidence for 

 such dispersal is found in the late Jurassic, late Paleocene, early 

 Eocene, early Oligocene, middle to late Miocene, and middle Plio- 

 cene through late Pleistocene. Sharp peaks in the intercontinental 

 faunal resemblance curve indicate, as shown by Simpson (1947), 

 that these dispersals were discontinuous pulsations. 



SUMMARY 



1. A given area may be considered the most probable center of 

 origin of a group of animals if: (a) it contains the earliest record of 

 the group; (b) it contains the record of suitable progenitors; (c) it 

 contains greatest taxonomic differentiation within the group; (d) it 

 contains a large and continuous geographic range of the group. 



2. The extant climatic belts and districts are poor geographic 

 indices for late Mesozoic-early Cenozoic arrangements. Therefore, 

 to propose the Holarctic or Paleotropic or other regions as centers of 

 origin and dispersal for a given group of animals we must first 

 evaluate the climate and ecology of these regions at the time of 

 origin. 



3. The fossil sample may now give a good picture of the taxonomic 

 diversity of once living mammals for some districts, but the study 

 of paleobiocenoses, based upon significant numbers of individuals, 

 is in its infancy. 



4. The biostratigraphic record is not yet adequate to reveal pre- 

 cisely the districts of origin and directions of dispersal for many 

 groups of land mammals. 



5. Useful census and ecological interpretations may be derived 

 from a meticulously collected fossil sample made up of all identi- 



