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A REVIEW OF THE UPPER EOCENE 



ARTIODACTYLA OF NORTH 



AMERICA 



By C. lewis GAZIN 



Curator, Division of Vertebrate Paleontology 



United States National Museum 



Smithsonian Institution 



(With i8 Plates) 



INTRODUCTION 



Perhaps the most significant feature of life during upper Eocene 

 time in North America is the striking diversity and relative abundance 

 of the Artiodactyla among the mammalian groups. At this time the 

 even-toed ungulates made their first bid for a dominant role in the 

 Tertiary sequence. Their new prominence is in marked contrast to 

 the insignificant position occupied in the preceding middle Eocene 

 Bridgerian interval, during which the Perissodactyla appear to have 

 been unchallenged as the predominating ungulates. The reason for 

 this new deployment is not immediately evident, but it may be de- 

 scribed as a noticeable and perhaps rapid diversification of the buno- 

 dont types in situ together with the sudden appearance, possibly 

 through migration, of a host of selenodont types. Remains of the 

 latter exhibit a basic resemblance throughout but are already clearly 

 divisible into several of the major groups, such as the hypertragulids, 

 agriochoerids, camelids, and leptomerycids. Explanation for such a 

 marked shift in the proportions of the fauna may lie in environmental 

 changes that were taking place, as indicated in part by the dwindling 

 and disappearance of the extensive lake system which provided the 

 Green River formation. Perhaps it was a setting of the stage causing 

 or permitting a faunal readjustment, encouraging the introduction of 

 new types through migration. 



In somewhat greater detail the changes that were accomplished 

 following Bridgerian time include the development and expansion of 

 the homacodont dichobunids into the genera Bunomeryx, Hylomeryx, 

 Mesomeryx, Pentacemylus, and Mytonomeryx, The helohyids exhibit 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 128, NO. 8 



