x 



182 THE AGE OF MAMMALS 



grand evolutionary changes in mammalian life; in other words, it is chiefly 

 the vast evolutionary changes in the American mammals (horses, titano- 

 theres, and rhinoceroses) which enable us to realize the enormous duration 

 and grandeur of Oligocene times in America. 



As the Oligocene atlvances, the countries again diverge and become dis- 

 similar in their faunal aspect. The correlation of the great tiine divisions 

 and depositions in which these changes occur are as follows: 



Life Zones European Stages American Formations 



Diceratherium Zone Aquitanian Harrison 



John Day Formations 

 White River (Upper) 



Metamynodon ( = Cadurcotherium) Stampian White River (Middle) 



Zone 



Ancodus (Titanotherium) Zone Sannoisian White River (Base) 



Palceogeography 



Continental connections. — With the Oligocene began an emergence of 

 the continents from their prolonged Eocene submergence. The land masses 

 of Europe, Asia, and North America became connected.' This is the theoret- 

 ical explanation of the intermigrations which followed and of the invasion 

 of a new fauna into North America and Europe, coming presumably from 

 the circumpolar region. Whether the connection between the Old and 

 New Worlds was by way of Alaska or across the whole breadth of the great 

 polar continent is uncertain. In the accompanying map by Matthew the 

 connection is indicated by way of Alaska and eastern Asia. In general 

 the southern continental masses (South America and AustraUa) appear to 

 have been disconnected. Of the great Lower Oligocene fauna now known 

 in northern Africa, the larger part is exclusively African in type, but a smaller 

 part includes a few mammals, such as the hysenodonts, anthracotheres, 

 certain suillines, and smaller rodents, which are also kno\\Ti in the Upper 

 Eocene and Lower Oligocene of France. 



The fluviatile and estuarine sea-cows, or sirenians, were probably common 

 to all the Mediterranean borders, African, Asiatic, European, and even the 

 western Atlantic in Oligocene times. The Eocene Egyptian types {Eo- 

 therium, Eosiren, Protosiren) are more primitive in the possession of hind limbs. 

 The earliest of the European forms is HaUtherium (H. veronense) from Middle 

 Eocene limestones of northern Italy (Monte Zuello). The most primitive 

 form in skull and tooth structure is Prorastomus from the (?) Eocene of the 

 island of Jamaica, West Indies. The Oligocene stage is HaUtherium scliinzi 

 from marine sands near Basel, Paris, Bordeaux, and Belgium.^ The fact 



' De Lapparent, A., Traite de Geologie,. 1906, p. 1547. 



^ Abel, O., Die Sirenen der mediterranen Tertiilrljildungen Osterreiehs. Ahh. K. K. Geol. 

 Reichsanst., Vol. XIX, no. 2, Vienna, 1904. See especially pp. 214-223. 



