104 D. E. SAVAGE 



whether "tropical" means past or present, for the geographic con- 

 notation is much different. 



CHARACTER OF THE FOSSIL LAND MAMMAL RECORD 



Mayr, Linsley, and Usinger (1953, pp. 14-15) have reemphasized 

 that the study of many living groups of animals has hardly begun. 

 If the knowledge of living groups is in its infancy, the knowledge of 

 fossil groups is embryonic; for we yet lack complete study series 

 in the fossil sample. Darwin (1859, end of Chap. X), remarking on 

 the poorness of the paleontological record, said: "... I look at the 

 geological record as a history of the world imperfectly kept, and 

 written in a changing dialect; of this history we possess the last 

 volume alone, relating only to two or three countries. Of this vol- 

 ume, only here and there a short chapter has been preserved; and 

 of each page, only here and there a few lines. Each word of the 

 slowly-changing language, more or less different in the successive 

 chapters, may represent the forms of life, which are entombed in our 

 consecutive formations, and which falsely appear to have been 

 abruptly introduced." Matthew (1915, 1939 edition, pp. 13-14) 

 noted: "We know more about fossil mammals in proportion to 

 their modern numbers than about any other of the larger groups of 

 land animals, yet the number of species of which we have any ade- 

 quate knowledge is but a minute fraction of the number which must 

 have lived since the class first came into existence." Even in a 

 thoroughly studied district such as Crazy Mountain Field of Mon- 

 tana, Simpson (1937, p. 69) found that 25% of the species in the 

 sample were known from only one specimen each. And new animals 

 are yet being discovered in the frequently prospected Chadron 

 formation of Nebraska (Cook, 1954). At the present time, and un- 

 doubtedly for a long time to come, we can claim little more than 

 did Darwin or Matthew. The total taxonomic diversity may be 

 usefully represented for some stratigraphic intervals, but only a 

 few fossil assemblages adequately portray the populations that they 

 represent. 



Locations of the pre-Pleistocene land mammal localities of 

 Nearctica are shown on Fig. 2. The record of earliest mammals is 

 from two upper Jurassic localities, one in Wyoming and one in 

 Colorado. (Late Triassic representatives of the class are found in 

 Europe.) Fortunately the Wyoming site produced a splendid as- 



