FOSSIL LAND MAMMALS AND WESTERN NEARCTIC FAUNA 99 



and dispersal of organisms and with tiie ideas tiiat pertain thereto. 

 Certain criteria for determination of origin, as Hsted below, apply 

 particularly to the expanded field of paleozoogeography — a field 

 extending through millions of years. 



Criteria for Determination of Origin 



1. The earliest record of the group in the proposed area of origin. In 

 the absence of strong contradictory data, the district in which there is the 

 oldest, that is, earliest, occurrence of a group may be taken as the area of 

 origin. This procedure is elementary but essential, just as Steno's ride of 

 superposition is an essential in the science of stratigraphy (Steno's rule 

 states that in an undisturbed succession of sedimentary rocks, the oldest 

 formation is at the bottom, the youngest on top.) For example, on this 

 criterion, we may claim that Africa, perhaps the restricted area in North 

 Africa that was the southern coastal plain of the old Tethyan Seaway, was 

 the area of origination of the Proboscidea and that the armadillos origi- 

 nated in South America. These statements are based primarily on the 

 earliest records. In the case of the proboscideans such claims should be 

 employed only as exploratory models of interpretation because of the 

 paucity of the fossil record of early forms. Proboscideans may have 

 originated in central Asia, as some workers suggest, rather than in North 

 Africa; but if the record of late Eocene warm-temperate, savanna, 

 gallery-forest habitats in Asia becomes better known, yet does not pro- 

 duce remains of proboscideans or pre-proboscideans, the probability 

 becomes greater that Africa was indeed the homeland. Already the rela- 

 tively complete Eocene records in Europe and in the United States make 

 it probable that these territories were not centers of proboscidean origin. 

 In the case of the armadillos, contemporary and comparable environ- 

 ments are relatively well represented in North America, and we believe 

 that armadillos would have been found If they had lived here as early as 

 in South America. 



2. An earlier record of progenitors in the proposed area of origination. 

 From the standpoint of paleontology, this, along with the first criterion, is 

 the ultimate basis for definition of center of origin and dispersal. These 

 criteria demonstrate that final statements as to the origin of any taxo- 

 nomlc unit of animals whose classification is supported by morphology of 

 geologically preservable parts must be based on the complete stratigraphic 

 record of the unit. 



3. A group probably originated in the area wherein It has greatest 

 taxonomic differentiation (see, also, Emerson's conclusion 13 (1952, p. 224). 

 Taxonomic differentiation is here taken as the best available Index of 

 general evolutionary differentiation. 



This criterion Is based on the reasoning that with given equality of 

 opportunity to diversify, the group has had more time to adapt to the 

 various niches, hence has greater antiquity in the area of greatest diversi- 



