FOSSIL LAND MAMMALS AND WESTERN NEARCTIC FAUNA 119 



b. A horizontally classified group in which subdivisions are pre- 

 sumably derived from a ferungulate stock; some of the families 

 are a probable late Cretaceous differentiation 



c. Most of the families (Hyopsodontidae, Phenacodontidae, and 

 Periptychidae) may have originated in North America, although 

 some paleomammalogists would propose origin elsewhere because 

 of the "sudden" appearance of differentiated groups in the 

 lower Paleocene here. Didolodontidae, as presently recognized, 

 appear to have a South American history, but are only slightly 

 divergent from phenacodonts (McKenna, 1956). Meniscotheri- 

 idae might have originated in Palearctica 



12. Pantodonta (extinct) 



a. Earliest record in middle Paleocene 



b. An early unique differentiation within the Eutheria, possibly a 

 derivative of the ferungulate radicle but annectants with earlier 

 eutherians unknown 



c. Probably of Holarctic origin 



13. Dinocerata (uintatheres, extinct) 



a. Earliest record in upper Paleocene of North America and Asia 



b. Unique giants of uncertain phyletic origin within the Eutheria, 

 possibly derived from a ferungulate stock 



c. Probably of Holarctic origin 



14. Perissodactyla (horses, tapirs, rhinos) 



a. Earliest record in lower Eocene of North America and Europe 



b. Probably derived from advanced phenacodontid condylarths of 

 the type seen in the middle and late Paleocene of North America 



c. Possibly originated in North America but the pre-perissodactyl 

 record is poorly known in Eurasia 



15. Artiodactyla (pigs, deer, bovids) 



a. Earliest record in lower Eocene of North America and Europe. 



b. Probably derived from a generalized eutherian group of Paleocene 



c. Probably of Holarctic origin, possibly of Paleotropic origin 



Eight orders of mammals known in North America have earlier 

 records elsewhere: Triconodonta, Docodonta, Symmetrodonta, 

 Pantotheria, Insectivora, Lagomorpha, Proboscidea, and Notoungu- 

 lata. On the basis of prospecting and collecting data for North 

 America these orders may be divided into two groups: those that 

 may have a yet unknown earlier record in North America and those 

 that more probably do not have an earlier record in North America. 

 The lagomorph, proboscidean, and notoungulate beds of this con- 

 tinent are underlain by relatively well-explored mammal-bearing 

 strata; hence, these orders fall into the second group. The other 

 orders, all with earliest record in the Mesozoic, may be included in 



