EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



275 



SUMMARY 



A summary of the life conditions in the Wind River 

 and Big Horn Basins during lower Eocene time shows 

 that there is no evidence of climatic change throughout 

 lower Eocene time; that the conditions through the 

 Systemodon zone and the succeeding Heptodon zone, 

 into the overlying Lambdoiherium zone, remain sub- 

 stantially similar. This fact accords with the sub- 

 stantial similarity in the general character of the mam- 

 malian fauna throughout the lower Eocene. The 

 fauna evolves during this very long period; old forms 

 give way to new; but it does not change in its 

 general adaptation to conditions. 



Thus in the great mountain valleys sediments were 

 being continually derived by erosion from the older 

 rocks of the mountains and deposited in these great 

 basins. Throughout Wind River time fluviatile dep- 

 osition is indicated by numerous channels filled with 

 coarse sandstones which irregularly traverse the finer 

 clays or interstratify with them in the form of lenses. 

 In the clays are found fish, crocodiles, and turtles, and 

 occasionally beds of JJnio. Local swamps are indi- 

 cated by the presence of lignitic areas in the blue clays 

 and in the sandstones, but never among the red clays. 

 The feldspars that wash down from the surrounding 

 granitic mountains are fresh and angular, a fact that 

 suggests rapid transportation of the fragments for 

 short distances and burial beyond the reach of car- 

 bonated waters. These conditions do not favor the 

 idea of luxuriant Eocene tropical forests or of a warm, 

 humid climate with the formation of a deeply decay- 

 ing humus, but suggest rather a dry although not nec- 

 essarily arid climate, with rapid changes of tempera- 

 ture favorable to splintering the ledges of the granite 

 cliffs. There were no frosts, but the climate may have 

 been stimulating to a vigorous and actively competing 

 fauna. In the Big Horn Basin fluviatile deposition 

 is indicated throughout the entire Eocene epoch. 

 The lignitic shales that cap the Lambdoiherium zone, 

 as indicated by the fresh-water moUusks and plant 

 contents, are both fluviatile and palustrine. 



The above picture of the physiography and the cli- 

 mate of these Rocky Mountain basins of Wyoming in 

 early Eocene time accords thoroughly with the analysis 

 of the chief adaptive types of mammals whose re- 

 naains are found in the lower Eocene rocks. These 

 mammals are broadly divided into three types — flu- 

 viatile, or river-living; river border, or palustrine; 

 upland, or plains-living. 



The upland type sought and found hard ground, to 

 which their narrow feet and compressed hoofs were 

 adapted. Conspicuous among these dry-ground forms 

 is Lambdoiherium itself, a swift-moving, or cursorial 

 animal broadly analogous in structure to the horses 

 of that time (Eohippus) and lophiodonts (Heptodon), 

 as weU as to the archaic condylarths (Phenacodus) . 



The Wind River fauna of the Lambdotherium zone 

 represents the closing chapter of lower Eocene mam- 

 malian life. It is closely affiliated with the fauna of 

 the typical lower Eocene or Systemodon zone, because 

 the two contain 25 genera and 1 1 species of mammals 

 in common. It is doubtful whether a single family 

 of mammals of the Systemodon zone becomes extinct 

 in the Lambdotherium zone, yet some of the archaic 

 mammals begin to show a numerical reduction. 



On the other hand, the Wind River fauna is pro- 

 gressive; the first appearance of these two genera of 

 titanotheres, Lambdotherium and Eotitanops, and of 

 seven other new genera of mammals is prophetic of 

 the oncoming middle Eocene or Bridger life. 



During this transition of the Wind River mammals 

 from their Wasatch forerunners to their Bridger suc- 

 cessors the physiography and the climate apparently 

 remained the same as in earlier Wasatch time, a fact 

 attested not only by the geologic and physiographic 

 evidence just considered but by the similar relative 

 abundance of the adaptive types of mammals found in 

 these two formations. 



The extent of the collections in the American 



j Museum, the total number of specimens collected, and 



the field records of Granger show that fossil mammals 



are about four times as abundant in the Systemodon 



zone as in the Lambdotherium zone. 



The relative numbers of the mammals in the 

 Systemodon zone of the Big Horn Basin of Wyoming, 

 as indicated by the number of specimens collected, 

 are shown below: 



Mammals in the Systemodon zone 



Specimens 



Perissodactyla (horses, 1,202; tapirs, 370) 1, 572 



Insectivora (Hyopsodus, 254) 306 



Condylarthra (mostly Phenacodus) 264 



Amblypoda (corj'phodons only) 209 



Creodonta (various carnivores) 203 



Primates (Pel3'codus, Anaptomorphus, etc.) 151 



Artiodactyla (mostly Trigonolestes) 120 



TiUodontia (Esthonyx) 73 



Rodentia (Paramys) 16 



Taeniodonta (?edentates, Calamodon) 2 



Edentata (new type) 1 



2,917 



This table naturally is only approximately repre- 

 sentative. The rodents, for example, were probably 

 far more abundant numerically than the horses. 

 The great number of horses, tapirs, and hyopsodonts 

 in the assemblage listed above is due in part to the 

 abundance of these animals in the "red beds." The 

 table is valuable chiefly in expressing the relative 

 abundance of the adaptive types of ungulates. 



In the Lambdotherium zone the relative abundance 

 of the remains of ungulates undergoes a marked 

 change: both of the archaic types of mammals, the 

 condylarths and the amblypods, are relatively less 

 abundant than in the Systemodon zone. 



