HERPETOLOGY OF ZUNI MOUNTAINS — GEHLBACH 317 



basin. Crotalus atrox and possibly Masticophis flagellum also reach 

 northern limits in the Zuni region. They are the only two species 

 from the transcontinental and western North America groups that 

 inhabit semiarid lowlands south of and, in the case of M. flagellum, 

 east and west of the Colorado Plateau. 



No species of reptile or amphibian present in the Zunis is restricted 

 to the Colorado Plateau, and none are definitely limited to it at their 

 southern edge of range. Cnemidophorus velox is distributed largely 

 within Plateau boundaries but also ranges into the Basin and Range 

 province. The main distribution of Sceloporus graciosus apparently 

 coincides with the southern border of the Plateau in New Mexico 

 although relict colonies exist much farther south. Scaphiopus inter- 

 montanus may reach its southern limits in the Zuni region, but is 

 known too poorly in New Mexico to be of zoogeographic value. 

 It appears, therefore, that Colorado Plateau environments have been 

 more effective in limiting northward dispersal than in preventing 

 movement southward. 



Physiographic continuity of the southern Plateau (Datil and 

 Navajo Sections) across the Zuni region is in sharp contrast to the 

 north-south break between the Plateau and Basin and Range south 

 and, to some extent, southeast of this area. This situation does not 

 favor east-west geographic differentiation but allows gene flow 

 between subspecies that previously might have been better isolated. 

 The intergradation patterns, thus, of Crotaphytus collaris and Crotalus 

 viridis might be expected to differ in degree from those of the afore- 

 mentioned species. There is reason to suspect that relatively smooth 

 character gradients are present in the east-west pattern and that 

 sharper breaks are involved in the more common north-south pattern. 



The remaining eurytopic species, Ambystoma tigrinum, Rana 

 pipiens, Phrynosoma douglassi, Thamnophis elegans, Diadophis 

 pundatus, and Masticophis taeniatus, are distributed throughout the 

 Zunis, the Colorado Plateau, and bordering Basin and Range province 

 with little apparent regional differentiation. While geographic 

 variation occurs elsewhere, the demonstrated local barriers to dispersal 

 are relatively imimportant in each of these species. 



Paleoecology and Dispersal 



As indicated by species adaptation and rather close restriction to 

 modern environments, development of the Zuni herpetofauna — indeed, 

 that of the entire Southwest — followed closelj^ upon the formation of 

 recent topography and semiarid vegetation in the late Cenozoic. 

 Differentiation of the Colorado Plateau from a relatively flat plain 

 began in the Miocene, and epeirogenic uplift has continued into the 

 Pliocene and Pleistocene, raising the Plateau above basins to the 



