276 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Relative frequency of mammals in the Lambdotherium zone of 

 Wyoming as indicated by the number of specimens collected 



Specimens 

 Perissodactyla (horses, 191; titanotheres, 124; heptodonts, 



56; hyrachyids, 2) 373 



Primates (pelycodonts, 42; anaptomorphs, 9; Microsyops, 



30) 81 



Insectivora (Hyopsodus, 71) 75 



Condylarthra (Phenacodus, 47; Meniscotherium, 1) 48 



Rodentia (Paramys, 39) 42 



Creodonta (various carnivores) 35 



Amblypoda (Coryphodon, 29; Bathyopsis, 2) 31 



Artiodactyla (Trigonolestes) 20 



Tillodontia (Esthonyx) 12 



Edentata (Taeniodonta) (Stylinodon, Calamodon) 5 



722 



It will be observed that the Condylarthra {Phena- 

 codus) here drop to the fourth place in relative fre- 

 quency, and the Amblypoda {Coryphodon) drop from 

 the fourth to the seventh place. This reduction is 

 partly in accord with the reduction of the archaic types 

 of mammals generally, as shown also in the following 

 faunistic comparison; in the Systemodon zone both the 

 genera and species of archaic mammals numerically 

 exceed those of modern type, whereas in the Lambdo- 

 therium zone the genera and species of archaic and of 

 modern types are evenly balanced. 



The relative frequency of the various adaptations 

 to cursorial, ambulatory, and arboreal life is still more 

 significant. The comparison of the adaptive types of 

 the Systemodon and Lambdotherium zones is as follows : 



System- Lambdo- 

 odon therimn 

 zone zone 



Ungulates; cursorial, small, light-limbed 1,692 373 



Ungulates; mediportal, medium in size 264 63 



Ungulates; graviportal, large, heavy-limbed 209 31 



Primates; arboreal, climbing -types 194 81 



Insectivores; rodents, etc., ambulatory, small 



terrestrial and fossorial mammals 395 139 



Carnivores; larger and smaller creodonts 203 35 



2,957 



722 



It will be observed that although the fossils collected 

 from the Lambdotherium zone are only one-third as 

 numerous as those of the Systemodon zone the relative 

 abundance of the adaptive types is approximately the 

 same, a fact that sustains the inferences as to geologic 

 and physiographic continuity or the absence of any 

 marked changes of environment during lower Eocene 

 time. Also, in the Lambdotherium zone, as in the 

 Systemodon zone, there is still a numerical predomi- 

 nance among the ungulates of cursorial types, the 

 horses {Eohippus), the lophiodonts {Eeptodon), the 

 titanotheres {Lambdotherium). Singularly, no primi- 

 tive tapirs {Systemodon) have been found. Among 

 the mediportal types may be noted the remains of 

 Phenacodus and the titanothere Eotitanops. The 

 graviportal coryphodonts, which may have been am- 

 phibious or partly aquatic in habit, are comparatively 



The first author to analyze the Wasatch fauna with 

 reference to adaptive types in their bearing on physi- 

 ography and climate was Loomis (1907.1), who divided 

 the Wasatch fauna into percentages, substantially as 

 follows : 



Terrestrial and arboreal types 75 



Aerial 3 



Amphibious 12 



Aquatic, including crocodiles, turtles, and fishes 10 



The present analysis of the adaptations of foot 

 structure in Wasatch and Wind River time combined 

 gives the following relative degrees of abundance 

 among the hoofed mammals: 



Ungulates; small, cursorial, light-Umbed types 2, 065 



Ungulates; medium, mediportal types, proportioned like 



the tapir 327 



Ungulates; graviportal, heavy-hmbed types {Cory- 

 phodon) , proportioned hke the hippopotamus 240 



The analysis both of the geologic and paleontologic 

 evidence appears to show that in Wind River time 

 there was a warm but relatively dry and invigorating 

 climate in the Rocky Mountain region; that there 

 were streams, swamps, and river borders for the coryph- 

 odonts, forests and meadow borders for the true 

 titanotheres {Eotitanops), and open spaces with harder 

 ground for the diminutive horses, lambdotheres, and 

 heptodonts. In the forests there were numerous lemu- 

 roid or monkey types, as well as arboreal rodents, and 

 on the borders of the savannas there were terrestrial 

 and partly fossorial edentate-like mammals. Periods 

 of aridity and areas of drier ground favored the de- 

 velopment of the light-limbed ungulates. 



CONTRASTS AND RESEMBLANCES BETWEEN LAMBDO- 

 THERIUM AND EOTITANOPS 



Geologic and biologic evidence of the existence of 

 areas of dry, hard ground in Wind River time is thus 

 adduced to explain the surprising fact that the feet 

 and limbs of the little Lambdotherium are more highly 

 specialized for cursorial locomotion than the feet of 

 any of the known middle Eocene titanotheres. An 

 alternative interpretation is that the ancestral peris- 

 sodactyls wei'e small cursorial forms with narrow feet 

 like Heptodon and Systemodon and that the widening 

 of the feet is a secondary adaptation to mediportal 

 habits. (See p. 586.) The skull of Lambdotherium 

 is elongate and relatively Eohippus-like. Lambdo- 

 therium was probably an early specialized cursorial 

 member of the great titanothere family, a member 

 that died out without leaving descendants. Under 

 the law of local adaptive radiation it may have lived 

 in the drier uplands; at all events its remains are 

 especially abundant in the "red beds," in which all 

 together no less than 111 specimens have been found 

 in comparison with 14 of the bulkier Eotitanops. 

 (See fig. 230.) 



Judging by the 14 specimens of Eotitanops that 

 have been found in the typical Wind River Lambdo~ 



