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PALEONTOLOGICAL GEOLOGY 61 
PALEONTOLOGICAL GEOLOGY 
Derivation of South American Faunas. The prime controling 
influence on the development of the mammals is food supply. 
Although mammals originate as early as Early Cretacic time, their 
developement in variety and numbers begins in Early Tertic time, 
and parallels the development of hardwood trees, and insect 
pollinated herbaceous plants. Previous to the Eocene deposition. 
South America and North America were connected. About the 
time that the Wasatch beds were laid down, South America was 
isolated and so long remained. On this island continent the 
mammals, at least, developed independently and adapted themselves 
to all the varied environments there, uninfluenced by the progress 
being made on the northern complex of continents. North America, 
Asia and Europe. Many times these adaptations are parallel, 
but in all such cases careful study emphasizes the independence of 
the southern forms. 
So much has been written postulating land-bridges across present 
ocean basins, and migrations especially from Africa and Austra¬ 
lia, that it is fundamental to look closely at the fauna to see if it 
can be explained without recourse to these tremendous revolutions 
of the land. 
The first South American fauna is that of the Notostylops beds. 
It includes marsupials, edentates, litopternas, typotheres, tox- 
odonts, etc. Marsupials in the south include true didelphids, 
sparassodonts, caenolestids, and diprotodonts. The didelphids 
run back to the Notostylops beds but are known equally early in 
North America and Europe. Without much doubt they found 
their way south before the isolation of South America. The 
sparassodonts include large carnivorous forms similar to the 
Tasmanian wolf, but their history extends back in South America 
to the Eocene and they have not been introduced since that time. 
The caenolestids are tiny forms, often called pseudo-diprotodonts. 
