iS6 AMERICAN MESOZOIC MAMMALIA 



centers. But these complications do not vitiate the general idea of primary mammalian 

 radiations which we owe to Osborn and which is such a helpful concept in considering 

 the course of mammalian history as a whole. 



Because of these intricate lesser factors and the general inadequacy of the present 

 data, the problem of the relationship between Cretaceous and Paleocene mammals is 

 the most difficult of all from a faunal point of view. Marsh's belief in a great time gap 

 between the Lance and the Puerco must be abandoned. Increasing stratigraphic knowl- 

 edge makes it more and more apparent that there is, geologically speaking, no con- 

 siderable time interval in the Late Cretaceous and Paleocene unrepresented by sedi- 

 ments in the North American West. It is also clear, on stratigraphic grounds alone, 

 that the Lance and Puerco were very close to one another in time. This is strongly sup- 

 ported by the mammalian evidence as now interpreted. The representatives in the 

 Paleocene of the Multituberculata, Marsupialia, and Insectivora are hardly appre- 

 ciably advanced over their Lance forerunners. A new and rather distinctive group of 

 multituberculates, the Taeniolabididae, appears, and the marsupials become much less 

 numerous and less varied. The former fact is due to immigration or change of facies or 

 both, the latter to extinction or change of facies or, most likely, both. 



As is now universally recognized, the essential difference between the Lance and 

 Paleocene faunas lies in the presence in the latter of very numerous new groups. This 

 difference is very marked, the new groups comprising almost the entire later fauna. 

 There are three possible explanations : ( i ) that the apparently new mammals are the 

 result of evolution from the known Lance forms or their close allies, (2) that their 

 ancestors were actually present here in Lance time but are unknown because they were 

 of a very different facies from that of any late Cretaceous deposit, or (3) that the 

 majority of the Paleocene mammals originated elsewhere than in the known fossilif- 

 erous areas and appeared suddenly by migration from some unknown center conse- 

 quent with some geographic change permitting their spread. The first explanation 

 must be abandoned in view of the now reasonably certain knowledge of the true affini- 

 ties of the Lance mammals and of the extreme shortness of the time which could be 

 allotted for their further evolution before the Paleocene. 



The second view, that the Lance and Puerco differ sharply in facies but little if 

 any in age and that the Paleocene mammals were already present in the known Paleo- 

 cene areas in the Cretaceous, seems to be inevitably involved in Osborn's view of a 

 mesoplacentalian, Cretaceous-Lower Tertiary radiation. It is also the most essential 

 feature in Matthew's explanations of this break in faunas. This authority states (1928, 

 p. 953) that "the Paleocene mammals belong to Cretaceous orders. It is essentially the 

 culmination of a Cretaceous mammal fauna, although living in the beginning of the 

 Tertiary period as the lines are usually drawn." Such an expression might mean only 

 that the Paleocene orders had Cretaceous ancestors and were already differentiated in 

 the Cretaceous; but all mammals naturally had Cretaceous ancestors, and it is highly 

 probable that the Eocene orders also were already differentiated in the latest Cre- 

 taceous. Every mammal fauna is the continuation, if not necessarily the culmination, 

 of some fauna of the preceding period or epoch. What must be, and from Dr. Mat- 



