CHAPTER III. 



WESTERN AMERICAN CENOZOIC HORIZONS. 



EOCENE. 



I. FIRST FAUNAL PHASE. 



Archaic Mesozoic mammals with partly South American, partly European 



affinities. 



POST-CRETACEOUS OR BASAL EOCENE (EUROPE, ETAGE THANETIEN).« 

 1. PUERCO FORMATION; POLYMASTODON ZONE. 

 (Fig. 1; PL I.) 

 HOMOTAXIS. 



North America. — Puerco formation (500 feet), San Juan basin, 

 northwestern New Mexico. 



South America. — A contemporary or previous (i. e., Cretaceous) 

 land connection with South America is indicated by the occurrence of 

 similar mammals in the Notostylops zone, Upper Cretaceous or basal 

 Eocene of Patagonia. Additional evidence of South American con- 

 nection is afforded by the subsequent occurrence of animals related to 

 the Edentata-Dasypoda in the American middle Eocene. 



Europe. — No European mammal fauna of this earliest stage is 

 known, therefore no conclusions as to homotaxis can be drawn. It 

 corresponds broadly to the Thanetien. 



FAUNA. ^ 



In New Mexico and Montana are found small archaic mammals'' 

 evolving from Cretaceous, Jurassic, and Triassic ancestors. Multi- 

 tuberculata, which originated in the Triassic, 3 families. Two orders 

 of archaic ungulates — (1) Amblypoda-Periptychidse, (2) Condylar- 

 thra-Phenacodontidse. Archaic Carnivora-Creodonta, 3 families: 

 (1) Oxyclsenidse, (2) Mesonychidse-Triisodontinse, (3) Arctocyonidae 



a Throughout this paper the French stages Thanetien, Spamacien, etc., are inserted merely to indi- 

 cate approximate homotaxis with Europe. 



b Matthew, W. D., A revision of the Puerco fauna: Bull. Am. Mus. Nat. Hist., vol. 9, 1897, pp. 259-323. 

 (See Appendix to this volume, p. 91.) 



c In the following pages "Archaic mammals " include members of the orders Multituberculata, Mar- 

 supialia, Insectivora, Taeniodonta-Edentata, Amblypoda, and Condylarthra, of Mesozoic origin (hence 

 Meseutheria Osbom) and typically of Mesozoic and early Eocene radiation. "Modernized mammals " 

 include members of the orders Primates, Camivora, Fissipedia, Rodentia, Hyracoidea, Proboscidea, 

 Perissodactyla, and Artiodactyla, which are in general of higher type and of Cenozoic origin and radia- 

 tion (hence Ceneutheria O^bom). 



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