120 D. E. SAVAGE 



the first group, and it is pointless to guess as to an origin area, even 

 an area of continental size. 



The remaining orders, not presently recognized in the Nearctic 

 record, can be discussed briefly. 



1. Monotremata, as Patterson (1956, p. 100) has suggested, may 

 be related to forms that are being uncovered in the late Triassic of 

 England. This Mesozoic group (Docodonta Kretzoi, fide Patterson) 

 has representatives in the late Jurassic of North America. 



2. Pholidota, Embrithopoda, Hyracoidea, and Tubulidentata 

 have no presently recognizable North American affinities. The rela- 

 tionships between hyracoids and the Holarctic, early Cenozoic 

 meniscothere condylarths are yet to be studied thoroughly. The 

 affinity of Tubulodon Jepsen from the early Eocene of North America 

 with the Tubulidentata is disputed (Jepsen, 1932; Colbert, 1941). 

 More fossil material referable to Tubidodon is badly needed. 



In summary, the evidence appears strongest for Nearctic origin of 

 the marsupials, edentates, tillodonts, taeniodonts, perissodactyls, 

 dermopterans, primates, and rodents. For none of these groups, 

 however, is the evidence compulsory. Figure 7 also demonstrates 

 that late Paleocene through Eocene was the time when most of the 

 modern orders of mammals arose. 



HISTORY AND AFFINITIES OF THE MINOR 

 TAXONOMIC GROUPS 



Simpson (1947) so meticulously covered the evidence for origin 

 and dispersal direction of families and certain lesser taxa of North 

 American mammals that it would be superfluous to do more than 

 summarize his conclusions. Figure 8, showing possible origins, is 

 presented with the belief that most of the participants in this sym- 

 posium will be interested primarily in the families that are now 

 living. It is to be remembered that an individual subfamily, genus, 

 or species does not necessarily correspond with the origin and dis- 

 persion of its family ; for example : 



1. Castor (beaver; Castoridae) evidently dispersed from Pale- 

 arctica to Nearctica; Miocene and middle Pliocene members of the 

 family dispersed in the opposite direction. 



2. Didelphis (opossum; Didelphidae) and Tayassu (peccary; 

 Tayassuidae) appear to be Neotropical autochthons and Nearctica 

 is recently marginal to their expanded or expanding range. 



