ENVIEONMENT OF THE TITANOTHERES 



97 



SUMMARY OF FAUNAS OF UINTA B AND C 



Though the whole later Eocene section of the 

 Uinta is 1,900 feet thick it includes 500 feet of un- 

 fossiliferous beds both at its base and at its summit, 

 so that the fossiliferous beds cover only about 900 feet. 

 The stages of evolution are best measured in the suc- 

 cessive species of DolichorJiinus, which are found both 

 at low and at high levels in the fossiliferous part of 

 the section. 



The archaic mammals that play so large a part 

 through lower and middle Eocene time diminish in 

 number and approach extinction at the end of Eocene 

 time. The numerical inferiority of the waning archaic 

 mammals and the rapid increase in the numbers of 

 modernized mammals are indicated in the following 

 table, prepared in 1910: 



Transition in mammalian life at end of Eocene time 



The Amblypoda culminate in the gigantic Eobasileus, 

 which disappears at the end of Uinta B, when the 

 gigantic creodont Mesonychidae and the catlike 

 Oxyaenidae appear for the last time. It is note- 

 worthy that these animals attain their largest size in 

 this, their waning period. The lemuroid primates 

 are found in greatly diminished numbers as compared 

 with those in the Bridger, possibly because the con- 

 ditions were unfavorable to the fossilization of re- 

 mains of arboreal animals; in fact, we know nothing 

 of the forest or the arboreal fauna during the entire 

 period of Washakie B and Uinta B because of pre- 

 vailing fluviatile conditions of deposition. 



ADAPXrVE RADIATION OF THE TITANOTHERES IN THE UINTA BASIN 

 GENERA AND SPECIES HEPRESENTED 



Through these 650 feet of fossiliferous sediments 

 the titanothere fauna of the Uinta Basin is revealed 

 as extraordinarily large and varied, no less than 11 

 genera and 22 species having been described. The 

 animals range in size from the small Metarhinus flu- 

 viatilis, some of which were not so large as a tapir, 

 to the huge ProtitanotJierium superbum. 



The titanothere Metarhinus is abundant and char- 

 acteristic in Uinta B 1, ranging from the base to the 

 summit but not extending into Uinta B 2 as here de- 

 fined. (In previous reports horizon B 2 has been 

 included in Uinta A.) According to Riggs (1912.1, p. 

 27) the genus includes two phyla — the first comprising 

 the small MetarJiinus fluviatilis Osborn and M. riparius 

 Riggs, with long, narrow skull; the second including 

 the broad-skulled forms M. earlei Osborn (which is 

 also found in Washakie B) and M. cristatus Riggs. 

 Metarhinus was a companion of its long-skulled rela- 

 tive Dolichorhinus in and near the rapidly flowing 

 streams, its remains being usually found in coarse 

 and semigravelly sandstones. (Riggs, op. cit., p. 24.) 

 In Uinta B 2 rapid streams, apparently the favorite 

 haunt of Metarhinus, were less abundant than in 

 Uinta B 1 (Riggs, op. cit., p. 25), which partly ac- 

 counts for the apparently sudden disappearance of 

 these animals from the sediments. 



Sphenocoelus uintensis, which is also probably from 

 the Metarhinus zone (Uinta B 1), is known only from 

 the hinder half of a skull. This strange animal is 

 clearly a member of the Metarhinus-Dolichorhinus 

 group and may be closely related to the long-skulled 

 Metarhinus riparius. The Metarhinus series as a 

 whole is clearly related to the older and more primitive 

 Mesatirhinus megarhinus of Washakie A and Bridger 

 C and D, which is also structurally ancestral to 

 Dolichorhinus . 



The name Heterotitanops parvus Peterson has been 

 applied to the skeleton of a very young animal from 

 Uinta B 1. It was found, articulated, in a hard sand- 

 stone concretion and lower down in Uinta B 1 than 

 any mammalian remains heretofore described from 

 that horizon. (Peterson, 1914.2.) In the opinion of 

 Gregory the characters of the deciduous dentition 

 and of the facial region of the skull of this animal 

 indicate that it probably represents the newly born 

 young of some of the Metarhinus-Rhadinorhinus group. 



Rhadinorhinus is distinguished from Metarhinus by 

 its tapering nasals and by the reduced infraorbital 

 process of the malar bones. One species, R. abhotti 

 Riggs, is found in Uinta B 1 , and another, R. diploconus 

 Osborn, is recorded from Uinta B 2. Riggs suggests 

 that Rhadinorhinus was an upland rather than semi- 

 aquatic form. Gregory noted in 1902 that it fore- 

 shadows the long-horned titanothere Megacerops 

 (Symiorodon) of the lower Oligocene in the abbrevia- 

 tion of the face and in the characters of the dentition. 



The long-skulled Dolichorhinus is represented by 

 two species in Uinta B 1 (one of which, D. longiceps 

 Douglass, extends into the base of Uinta B 2) and by 

 four species in Uinta B 2. The most primitive species, 

 D. superior, is in general intermediate in structure 

 between the ancestral Mesatirhinus and the later 

 species of Dolichorhinus. The most advanced species, 

 D.fluminalis Riggs, is from the upper levels of Uinta 



