244 LAND MAMMALS IN THE WESTERN HEMISPHERE 



Cruz fauna was, it did not contain everything that we should 

 expect to find in it; several recent families of undoubtedly- 

 indigenous South American origin have left no ancestors in 

 the early Miocene formations. For this, there are several 

 obvious reasons. In part, these gaps in the history are merely 

 due to the accidents of collecting and some of them will almost 

 certainly be filled by future exploration. Other absentees will 

 probably never be found, because the Santa Cruz beds are 

 known only in the very far south, and the Miocene climate of 

 the region, though much milder and more genjal than the 

 present one, must have been unsuitable for many tropicail 

 animals. Again, the Patagonia of that time appears to have 

 been a country of open plains, with few trees, and hence ar- 

 boreal forms were rare. 



While great numbers of large, flightless birds, some of them 

 of enormous size, were entombed in the volcanic ash and 

 dust which were spread over such wide areas and to such 

 great depths, the extreme scarcity of reptiles is surprising ; a 

 few remains of lizards have been found, but no snakes, croc- 

 odiles, or tortoises, and we have no information as to the 

 plant-life of the region at that time. The mammals were al- 

 most all of small or moderate size ; only one or two species were 

 really large. 



One very striking and characteristic feature of the Santa 

 Cruz fauna is the great abundance of marsupials which it 

 contained and which resembled more or less those of modern 

 Australia. There were no true Carnivora and their places 

 were taken by a variety of carnivorous marsupials, some of 

 which {e.g. ^Prothylacynus) were as large as wolves and were 

 closely similar to the so-called Tasmanian Wolf (Thylacynus). 

 Another genus {^Borhyoena) had a short, bullet head, not un- 

 like a small Puma in appearance and, besides, there were many 

 smaller beasts of prey, in size like badgers and minks. Opos- 

 sums were common and there were many very small herbivorous 

 marsupials, which resembled, though perhaps but superficially, 



