SUCCESSION OF FAUNAS 15S 



even contemporaneous, and (3) that the imperfectly known Lance fauna may have 

 included the immediate ancestors of the Puerco fauna so that it may be that there is no 

 important distinction between them due either to facies or to immigration of new forms 

 and extinction of old. 



Some minor points have since been altered by Osborn, such as his more definite 

 recognition that many of the Puerco mammals have no apparent Cretaceous ancestry 

 and are of unknown origin, but his later generalizations have been chiefly based on 

 these views. In his most important study devoted specifically to the problems here 

 under consideration (1905) Osborn advanced the view that there had been four great 

 mammalian radiations: (i) a Jurassic radiation of doubtful relationships to the later 

 mammals, (2) a Cretaceous and Tertiary radiation of Marsupialia, chiefly in Aus- 

 tralia, Antarctica (hjpothetical), and South America, (3) a Cretaceous and lower 

 Tertiary radiation of archaic, chiefly non-ancestral placentals, and (4) a Tertiary 

 radiation of higher or modernized placentals. The members of the third radiation he 

 had previously called the Meseutheria or archaic placentals, those of the fourth radia- 

 tion the Ceneutheria or modernized placentals. 



The view, first advanced by Osborn, that there is a vast break between the Jurassic 

 faunas of Europe and America and the known upper Cretaceous faunas has not been 

 seriously questioned and is on an even stronger basis today than when first proposed. 

 The Jurassic triconodonts and symmetrodonts disappeared without issue in the course 

 of lower Cretaceous time or at the beginning of the Cretaceous. The multituberculates 

 continued, but with modifications rather more important than was realized twenty or 

 thirty years ago. These modifications may have been due entirely to evolution more or 

 less in place during the long gap in our knowledge, but it is somewhat more probable 

 that the origin of the Cretaceous and Paleocene multituberculates is to be sought in 

 some post-Jurassic center of radiation and that they are immigrants where found. 

 Marsupials and placentals apparently are not present as such in the Jurassic faunas 

 and here, too, it is probable that they originated from a Jurassic group (the Panto- 

 theria in a broad sense) in some definite center of evolution and hence spread through- 

 out the world in the Cretaceous. Recognizing the essentially tentative and inaccurate 

 nature inherent in the broadest generalizations, it is nevertheless useful and permis- 

 sible to consider, as did Osborn, that the Jurassic and Cretaceous mammalian faunas 

 as known represent two great separate mammalian radiations. 



The known members of the Cretaceous radiation belonged to only three orders : 

 Multituberculata, Marsupialia, and Insectivora. This involved the original radiation 

 of the Marsupialia and the present writer cannot follow Osborn's older view that the 

 marsupial radiation was separate from that of the placentals primarily. They spread 

 over the world in company with the other Cretaceous mammals. Their later radiations 

 in South America and in Australia are of secondary scope, not on the same plane as 

 the three or four great primary mammalian radiations here discussed. The whole prob- 

 lem is greatly complicated by the fact that secondary radiations of this sort, confined 

 to single land masses, and still more limited local radiations were of constant occur- 

 rence, along with intermittent migrations between the various continental and local 



