24 



TITANOTHEEES OF ANCIENT WTfOMING, DAKOTA, AND NEBRASKA 



Phyla of the odd-toed ungulates 



In North America the horses (Eohippus) were the 

 first perissodactyls to arrive. They were followed by 

 the tapirs (Systemodon) , which in turn were succeeded 

 by the lophiodonts {Heptodon). It is possible that 

 ancestral titanotheres were living iu northern parts 

 of the American continent, but apparently thej^ did 

 not reach the region near the fortieth parallel until 

 it had become well populated with horses, tapirs, and 

 lophiodonts. By middle Eocene time three more 

 families had appeared — the paleotheres, in Europe 

 only; the rhinoceros-like amynodonts (semiaquatic 

 forms), which first appear in North America and 

 subsequently in Europe; and the cursorial rhinocer- 

 oses known as hyracodonts (Hyrachyus), which appear 

 in North America only and preceded the amynodonts. 

 Toward the beginning of upper Eocene time there 

 first appear in North America, as well as in Europe, 

 ancestors (Eomoropus) of the chalicotheres, animals 

 closely related in tooth structure to the titanotheres, 

 which were separated into a distinct order (Ancylo- 

 poda) by Cope and are here regarded as forms some- 

 what parallel to the Titanotheroidea. 



WIDE GEOGRAPHIC DISTRIBUTION OF THE 

 PERISSODACTYLA 



We are first struck with the remarkably wide 

 holarctic distribution of the perissodactyls in Eocene 

 and lower Oligocene time, a fact which points to 

 facihty of migration over the whole Northern Hemis- 

 phere. Only one family, the paleotheres, is exclu- 

 sively European, and one other, the hyracodonts, is, 

 so far as known, exclusively North American. The 



titanotheres were formerly beheved to be exclusively 

 North American, but two forms have been found in 

 eastern Europe, which correspond very closely with 

 the titanotheres of upper Eocene age from the Uinta 

 Basin in northern Utah. 



Members of all the other perissodactyl families — 

 the chalicotheres, tapirs, lophiodonts, amynodonts, 

 and rhinoceroses — probably ranged freely to and fro 

 over the great northern continent of Em-asia and 

 North America combined, the geographic region 

 known as Holarctica. 



The second important fact regarding the Peris- 

 sodactyla is that, although the environment dining 

 middle and upper Eocene time, after the extinction 

 of the archaic imgulates — the Condylarthra and 

 Amblypoda — was especially favorable to the existence 

 of the Perissodactyla, this order reached its maxi- 

 mum expansion in the lower Ohgocene epoch, when 

 all the nine families were existing and apparently 

 flourishing at the same time. It would appear that 

 in upper Eocene and lower Oligocene time Holarctica 

 was dominated by perissodactyls. This period was 

 immediately followed by a period when either the 

 environment was adverse to the existence of the peris- 

 sodactyls or competition with other types of imgu- 

 lates was disastrous to them, because at or before the 

 end of the lower Oligocene epoch five perissodactyl 

 families suddenly disappeared — the titanotheres, paleo- 

 theres, lophiodonts, amynodonts, and hyracodonts. 

 The aberrant chalicotheres, apparently through retreat 

 to forested regions, survived in Europe and probably 

 also in North America until the Pliocene epoch. 



