SUCCESSION OF FAUNAS 157 



thew's other writings apparently is, meant is that the spread of the Paleocene orders 

 took place in the Cretaceous, that the Paleocene distribution is the result of a Cre- 

 taceous radiation. As Matthew has explicitly recognized, this necessarily involves a 

 belief that the Lance and Paleocene faunas differ chiefly in facies. 



The evidence is indirect or negative, and consequently hard to disprove, but in 

 the present writer's opinion this view does not square well with the known facts. 

 Some difference in facies there may be, but the more remarkable faunal differences 

 probably cannot be explained on this basis. The uppermost Cretaceous and Paleo- 

 cene continental sediments suggest no definite difference in facies. The floras are 

 almost identical. Animals of such limited facies as the champsosaurs occur with both 

 t)pes of mammalian faunas, and the Cretaceous mammals themselves all have close 

 ecological analogues in the Paleocene faunas. Yet on the one hand are three mam- 

 malian families associated with abundant and varied dinosaurs, on the other no trace 

 of dinosaurs and the same three mammalian families along with some ten other 

 families unknown in any previous fauna. Such differences are quite unknown among 

 different facies of land faunas in the same region at any other time in the history of 

 vertebrates. I gnorance of the Lance fauna cannot be urged as a complete explanation. 

 The remains are so fragmentary as to be dubious in many respects, but they clearly are 

 not ancestral to the great majority of Paleocene mammals, many individual specimens 

 are known — several hundreds — and the fauna is known to have been essentially the 

 same in Lance time over a large area, for specimens from Montana, South Dakota, and 

 Wyoming are known. Furthermore the earlier Belly River has yielded a few remains 

 which indicate that its mammalian fauna was of this same type. 



Without ignoring the alternative hypothesis, it seems more reasonable ( i ) that 

 there was a Cretaceous type of mammalian fauna in the American West just as there 

 was a Paleocene type and an Eocene type regardless of strictly evolutionary changes, 

 (2) that the Paleocene fauna was not the culmination of any previous fauna in the 

 same region, but (3) that the beginning of the Paleocene was here marked by migra- 

 tory movements of the first magnitude involving the introduction of new groups which 

 had been evolving in some unknown center while the multituberculate-marsupial- 

 insectivore fauna, resultant of an earlier radiation, was dominant. 



RESUME OF FAUNAL SUCCESSION 



So far as can now be judged from the evidence of the known early mammals, the 

 following may have been the major events in mammalian history: 



1. The origin of true mammals from the cynodont reptiles or closely related 

 forms, probably in the Triassic. This may have been followed by a sort of proto-mam- 

 malian Rhaetic radiation of the tritylodontids, microcleptids, and possibly some other 



groups. 



2. A Jurassic radiation, world-wide in scope, involving the Multituberculata, 



Triconodonta, Symmetrodonta, and Pantotheria. 



