OUR ANCIENT RELATIVES 



The known mammalian remains from these 

 great formations consist mostly of very fragmen- 

 tary jaws, with a few teeth in them, of tiny mam- 

 mals. Most of these mammals were no bigger 

 than mice, but in the closing stages of the Age 

 of Reptiles a few of them became as large as 

 beavers. Some of the mammals of the Age of 

 Reptiles in Europe and North America are believed 

 by certain authorities to be related to that most 

 archaic of mammals, the egg-laying Platypus of 

 Australia. Others seem to have been remotely 

 related to the existing marsupials or pouched 

 mammals, which today live chiefly in Australia. 



The most primitive marsupial of today, how- 

 ever, is the common opossum of North America, 

 which is one of our oldest "living fossils." It is, 

 in fact, the little-changed descendant of a group of 

 mammals that lived in the latter part of the Age 

 of Reptiles. One of these ancestral opossums, 

 represented by a fossil jaw and parts of the skull 

 (Fig. 27), was found by Barnum Brown embedded 

 beneath a large dinosaur skull in Upper Cretaceous 

 rocks of Montana. This form, named Eodelphis 

 (dawn-opossum) by Dr. W. D. Matthew, has the 



known jaw and skull parts so nearly like those of 



47 



