FOSSIL LAND MAMMALS AND WESTERN NEARCTIC FAUNA 101 



in the Cretaceous and early Cenozoic in the northern hemisphere of the 

 pleurodire turtles, which now live in the southern hemisphere. Gavials 

 now live in Asia, but are known from the Oligocene of South America 

 according to Langston (1953). Cryptobranchid amphibians live in eastern 

 North America and in eastern Asia, but they are represented by the 

 renowned Andrias scheuchzeri ("Homo dilmni testis") in the Miocene of 

 Switzerland (Schmidt, 1946, p. 149; Romer, 1945, p. 592). These little- 

 changing types are in various degrees characterized by fragmented or by 

 recessed representation of a once greater geographic range that was 

 established during an expanding phase of their dispersal history. 



5. Vagility of the group must be considered. This factor adds im- 

 measurable complexity and uncertainty to the interpretation of the fossil 

 record. Migratory birds may, theoretically, become a part of the fossil 

 record at any resting point on their flyway. Rapidly dispersing land mam- 

 mals might be first trapped in the sediments at great distance from their 

 district of origin. These uncertainties tend to be overcome by an in- 

 creasing probability that correlates with an increasing paleontologic 

 sample. Several concordant examples lend an inference greater credence. 



Darlington (1948) and Stebbins (1950, Chap. XIV) have dis- 

 cussed other approaches to interpretation of the origin and dis- 

 persal of organisms: numbers of animals and of taxonomic units; 

 size and continuity of geographic range; distribution of related, com- 

 peting, or associated groups. Size and continuity of geographic 

 range appear to comprise an especially useful neontologic criterion 

 for interpreting origin of the smaller taxa. A large and continuous 

 range tokens the origin area. 



Thus to confirm an interpretation of origin for a group of animals, 

 we must first explore the diverse paleobiotopes for earliest oc- 

 currence. This documentation is fundamental to the historical 

 zoogeography of the phyla and is then enriched by the succeeding 

 criteria. If, then, an area contains the earliest record, shows a bio- 

 stratigraphic sequence from progenitors to the group concerned, 

 contains most differentiated subgroups, and exemplifies a large and 

 continuous group geographic range, it is probably the center of 

 origin and dispersal. 



Matthew (1915) claimed that the more advanced and progressive 

 members of a group should be nearer the center of origin because 

 evolution was more progressive at this point. He also concluded that 

 the less advanced members tend to disperse radially and will be 

 peripheral. Because of the known climatic changes through time in 

 Holarctica, and because of the concentration of primitive animal 



