NO. 8 UPPER EOCENE ARTIODACTYLA — GAZIN 9 



The overall picture would appear to be one of increasing aridity and 

 one in which there was surely a floristic change effecting the propor- 

 tions of the various elements that go to make up the food supply 

 of herbivorous groups. Whether this change was a causative factor 

 in a transition in place to a more-selenodont type of dentition in several 

 of the artiodactyl groups represented, or whether the environmental 

 change permitted a faunal readjustment through migration, has not 

 been determined. Nevertheless, there is a correlation between these 

 facts which must be regarded as more than casual. 



While there is similar evidence of increasing aridity in a change 

 from lacustrine to fluviatile deposition in the Washakie Basin during 

 Eocene time, this change seems to have occurred somewhat earlier, 

 as the Green River lake there evidently disappeared during the middle 

 Eocene. In the Wind River Basin, on the other hand, although the 

 middle and upper Eocene sequence has rather limited surface distri- 

 bution, there would appear, nevertheless, to be no evidence of a lake 

 accumulation comparable to that of the Green River formation. Pre- 

 sumably, however, the climatic change indicated for the Uinta Basin 

 was of more than local importance. 



Faunally, not only was there a shift to more-selenodont types 

 among the Artiodactyla but, as has already been noted, the Artio- 

 dactyla, extremely rare in the middle Eocene, have nearly or quite 

 supplanted the Perissodactyla as the more-populous ungulates in the 

 fauna. Associated with the artiodactyls were a rather marked di- 

 versity of titanotheres, various rhinos, tapiroids, and horses of the 

 genus Epihippus among the perissodactyls ; waning groups of creo- 

 donts, together with miacid forerunners of modern carnivores; a 

 rather notable assemblage of sciuromorph rodents ; and, interestingly 

 enough, the first North American lagomorphs. 



RELATIONSHIPS 



Undoubtedly one of the more interesting aspects of this study has 

 been the attempt to determine the relationships between the various 

 kinds of artiodactyls that lived during Eocene time; to try to 

 visualize something of the phylogenetic arrangement, and relate, where 

 possible, their phyletic groupings to the better-known families of the 

 Oligocene. Heretofore, almost no attempt has been made to demon- 

 strate these relationships on any tangible basis and show pictorially the 

 conclusions obtained. Wortman called attention to the camel-like 

 features of Protylopiis, and, noting the striking resemblances between 

 the various Eocene forms, regarded most of them as camelid. Scott 

 made an outstanding contribution in his report on the selenodont 



