CENOZOIC ERA: AGE OF MAMMALS 591 



areas of the earth's surface. It is to be noted, too, that the very 



earliest Tertiary upland deposits contain a rich mammalian fauna. 



It does not seem improbable, therefore, that on the higher land of 



Asia and North America mammals of considerable variety were in 



existence during the later days of the Mesozoic ; all proof of which 



has either been lost by the wiping out of the upland deposits by 



erosion, or else has not yet been discovered. 



Archaic Mammals of Ancient Ancestry. — In the earliest known 



Eocene beds (Puerco) the remains of small, archaic mammals 



(marsupials) associated with true mammals (Eutheria) occur, which 



clearly belong to animals whose ancestors lived in the Mesozoic, 



some of which (Plagiaulacidae) date back even to the Upper Trias- 



sic. These animals are characterized by large grinding teeth (Fig. 



534) with many elevations (multi- 



tuberculate), and elongated front 



teeth (incisors) ; the latter being, 



in some cases, chisel-shaped, as 



in the rabbit, and in others 



pointed. The " back teeth " of 



the lower jaw are, moreover, 



usually very different from those 



of the upper jaw. These animals Fig. 534. — An archaic mammal, Ptilo- 



were all small or of moderate ^/ < U PP er Cretaceous). (After Gidley.) 

 . . . . Ihe many-ridged teeth are especially to 



size, the largest known having be not i ce d. 



about the bulk of a beaver. 



Judging from the teeth, it seems probable that some were gnawing 



animals like rabbits or mice (but not true rodents), and that 



others were either fruit eaters (frugivores) or even insect eaters 



(insectivores). 



This entire class became extinct before the close of the Eocene 

 and should be considered as survivors from the Mesozoic, which 

 lingered for a time in the Tertiary. The question naturally arises 

 as to the cause of their extinction, since they were able to survive 

 the many changes, not only of the Mesozoic, but of those at the close 

 of the era as well. If the variety of their fossils in the Mesozoic 

 formations indicates the relative abundance of these archaic mam- 

 mals as compared with other forms during the era, conditions of 

 climate or of food, or competition with the dinosaurs must have pre- 

 vented their increase. If competition with the dinosaurs prevented 

 their increase in the Mesozoic, it would have been surprising had 



CLELAND GEOL. — 38 



