RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 147 



we have on this group indicates a late Tertiary crossing from 

 North America to Asia. A rather heavy vegetative cover is required. 



Jumping mice (Family Zapodidae). Conditions suitable for 

 the Microtinae would be suitable for this group. Late crossings 

 are indicated. 



Pikas (Family Ochotonidae). The fossil record would indicate a 

 middle Tertiary crossing for this group, and present relationships 

 indicate late crossings also. The pika now lives in talus slopes near 

 timberline. For the first ancestral crossings, when climates were 

 more moderate, a change in the habits of these little lagomorphs is 

 called for. In the Pleistocene, environmental conditions suitable 

 to present day pikas would have been more likely. 



Rabbits and hares (Family Leporidae). Either forest or savanna 

 would have been suitable for the crossing of these mammals. Climate 

 would not, in itself, have been a factor. Fossil evidence and present 

 relationships indicate early and late crossings. 



Peccary (Family Tayassuidae). Indications are of an early 

 Tertiary crossing. The peccary was then restricted to the North 

 American (and later South American) continent from about middle 

 Tertiary on. Open or wooded areas would have been suitable for 

 the crossing. 



Deer (Family Cervidae). From about middle Tertiary on, 

 members of this family probably crossed over the land bridge 

 several times. The elk, moose, and caribou now have close relatives 

 on the two land masses, an indication of late crossing. Most any 

 vegetative cover would have been suitable for the crossing over. 



Bovines (Family Bovidae). The bison and big horn sheep both 

 have close relatives on the two continents, an indication of late 

 interchange. There is less certainty about the mountain goat 

 (Oreamnos). From present relationships with Old World antelopes, 

 the indication is that no late passages occurred. 



It will be noted that the groups in this section required only 

 soil (moles) and a vegetative cover of savanna and forest (others) 

 for a suitable corridor. Temperatures were at no time (except pos- 

 sibly during the glacial stages in the Pleistocene) prohibitively low. 



LAND MAMMALS FOR WHICH LAND BRIDGE APPARENTLY 

 DID NOT SERVE AS A CORRIDOR 



Raccoons (Family Procyonidae). There is no indication of 

 intercontinental exchange since the evolutionary phase in which 



