DISTRIBUTIONAL PATTERNS OF VERTEBRATES 463 



The urodeles, by contrast with anurans, are typically a cool- or 

 cold-adapted group. The center of distribution of the large family 

 Plethodontidae, which includes a majority of the urodeles of the 

 eastern United States, is in the Appalachian highlands in remnants of 

 the Arcto-Tertiary forest, which the evidence shows to have shifted 

 southward in the Glacial stages of the Pleistocene. As might be 

 expected, there is for this group no strong pattern of east- west dis- 

 junction comparable to that of the warmth-adapted anurans. Of 

 some 56 species and species groups of urodeles in the eastern United 

 States, only 23 occur on the coastal plain, and only 11 are restricted 

 there. Only two of the coastal plain group, Diemictylus and Amphi- 

 uma, show evidence of east-west disjunction, as earlier mentioned. 

 The most obvious disjunctions in the eastern urodeles are ones that 

 are attributable to the southward and westward spread of the Arcto- 

 Tertiary forest and its urodele fauna under Glacial-stage climates 

 and to the isolation of relictual populations under locally tolerable 

 conditions, as the environment became warmer and dryer. This 

 would explain the relict populations of Plethodon, Hemidactylium, 

 and Eurycea in the Ozarks, of Plethodon and Eurycea on the Edwards 

 Plateau, and of Aneides and Plethodon in the mountains of New 

 Mexico. The absence of plethodontid relicts from the cloud forests 

 of Mexico, discussed by Martin and Harrell (1957), would be ex- 

 pected if these animals stayed with the Arcto-Tertiary forest in the 

 southern United States. The difference in the pattern of distribution 

 of anurans and urodeles results, then, from the fact that the urodeles 

 would have moved with the invading environment that fragmented 

 anuran ranges. Amelioration of the environment that permitted 

 reoccupation of the coastal plain by warmth-adapted anurans would 

 have led to the observed disjunctions in urodele ranges. Unlike the 

 situation in the anurans, there are numerous instances of north- 

 south or unoriented speciation in the urodeles of the eastern United 

 States. 



SUMMARY 



A large body of paleontological and zoogeographical data support 

 the thesis that present distributional patterns of the warmth- 

 adapted vertebrates of the Gulf of Mexico and southern Atlantic 

 coastal plains reflect Pleistocene splitting of this faunal group into 

 eastern and western populations. The agency of this splitting, as 



