AMERICAN MESOZOIC MAMMALIA 



I. INTRODUCTION 



The known specimens of Mesozoic mammals are among the most precious and impor- 

 tant remains of extinct life which have yet been discovered. They are the sole direct 

 evidence of the fundamental first two-thirds of the evolution of the Class Mammalia, 

 which is now dominant on the earth and to which we ourselves belong. This impor- 

 tance has long been rather vaguely recognized, but it can hardly be said to have been 

 properly evaluated. The Mesozoic forms are usually briefly dismissed as being rare, 

 fragmentary, and poorly understood — accusations which are true, but not in the ac- 

 cepted degree. The Mesozoic mammals are now represented by many hundreds of 

 specimens derived from all the continents save Australia and coming from various 

 horizons from the close of the Triassic to the first appearance of abundant mammals in 

 the lowest Paleocene. 



The larger part of this material is from Europe and North America. That from 

 North America is almost all in the Marsh Collection, now divided between the Pea- 

 body Museum of Natural History of Yale University and the United States National 

 Museum. Smaller, but still very important, collections are preserved in the American 

 Museum of Natural History and some other institutions have a few specimens. Espe- 

 cially as regards the earlier and more important Morrison fauna, this material is of 

 unexpected richness. Professor O. C. Marsh studied his great collection only in a very 

 preliminary and cursory way. He planned to return to it and to monograph it thor- 

 oughly, but that plan was destined never to be fulfilled. The study of this material has 

 thus remained an outstanding necessity. That a restudy and revision of Marsh's types, 

 in the light of the advances of so many years, were necessary has long been evident, 

 but it was not suspected that a large part of the collections had never been studied or 

 that they contained so much that is entirely new. 



It is now well over one hundred years since Buckland announced the discovery of 

 Mesozoic mammals (1824). The lively discussion that followed that announcement 

 and ended in establishing beyond question the presence of mammals in the Secondary 

 was based altogether on English material, chiefly from Stonesfield (middle Jurassic). 

 Further discoveries in the Stonesfield "slate," in the late Jurassic Purbeck beds, and in 

 the earlier Rhaetic beds followed, until by 1871, the date of Sir Richard Owen's 

 monumental memoir on the fossil mammals of the Mesozoic, almost as many European 

 specimens were known as at the present time. The discovery and description of the 

 equally important and somewhat more abundant American forms, on the contrary, has 

 been wholly subsequent to that date. The year 1871 thus forms a convenient and natu- 

 ral dividing line between two very distinct phases of the history of the subject.' 



^ The history of the discovery of the European specimens is more fully given in the writer's Cator 

 logue of Mesozoic Mammalia in the . . . British Museum (1928). 



