736 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Measurements, in ■millimeters, and ratios of limbs of ungulates of primitive types 



Ambulatory : 



Pantolainbda bathmodon 



Meniscotherium terraerubrae — 



Subcursorial: Phenacodus primaevus 



Cursorial: 



Euprotogonia puercensis 



Eohippus sp 



149 

 100 

 234 



105 

 162 



114 



91 



198 



107 

 162 



Tibio- 

 femoral 

 ratio 



76 

 91 



84 



101 

 100 



Meta- 



tarso- 



femoral 



ratio 



124 



82 



167 



Radio- 

 humeral 

 ratio 



82 



58 



146 



110 



Meta- 

 carpo- 



35 

 37 

 13 



Phyletic progression (Eocene) to graviportal type. — 

 An extreme instance of progression into the heavy- 

 bodied, slow-moving (graviportal) type is that found 

 in the comparison of the three Amblypoda, Panto- 

 lainbda of the basal Eocene, CorypJiodon of the lower 

 Eocene, and Uintatherium of the upper Eocene. The 



relative abbreviation of the tibia, of the metatarsus, 

 and of the metacarpus is in contrast with the slight 

 relative elongation of the radius. This illustrates a 

 frequently repeated principle of graviportal adaptation, 

 namely, that the radius is not abbreviated step by step 

 with the tibia but may actually be elongated. 



Measurements, in millimeters, and ratios showing progression of Eocene ungulates from the ambulatory to the 



graviportal type 



Primitive cursorial condylarths and perissodactyls. — 

 Even in lower Eocene time the condylarths and 

 perissodactyls embrace many primitive swift and 

 light-limbed types. These cursorial types, which 

 include a lower Eocene titanothere, had what may be 

 called high-speed ratios for the tibia and radius, but 



low-speed ratios for the metatarsus and metacarpus. 

 The most primitive Eocene titanothere, Eotitanops, 

 was subcursorial, and Lambdotherium was cursorial in 

 limb proportions, but the material available for these 

 genera does not afford the required indices. 



Measurements, in millimeters, and ratios of limbs of primitive cursorial condylarths and perissodactyls 



Tibio- 

 femoral 

 ratio 



Meta- 

 tarso- 



femoral 

 ratio 



Radio- 

 humeral 

 ratio 



Meta- 



carpo- 



humeral 



ratio 



Phenacodontidae: Euprotogonia puercensis. 



Lophiodontidae : Heptodon calciculus 



H yracodontidae : Hyrachy us agrarius 



Equidae: Eohippus sp 



105 

 175 

 254 

 167 



107 

 175 

 243 

 162 



101 



100 



95 



100 



45 

 »75 

 110 

 '■82 



115 

 197 

 121 



114 

 197 

 110 



99 



100 



90 



5S 

 47 

 53 



Mediportal or intermediate group. — The mediportal 

 group includes the middle Eocene titanotheres as well 

 as the modern tapirs and some of the older types of 

 rhinoceroses, all animals of intermediate size with an 

 intermediate rate of speed. Here we observe a strik- 

 ing uniformity in the limb segment ratios. As com- 

 pared with the primitive ungulates shown in the first 

 table above, the mediportal titanotheres were all 

 apparently abbreviating the tibia in connection with 



their increasing weight. The metatarsals and meta- 

 carpals were also abbreviating more rapidly in adapta- 

 tion to weight. The radius, however, in relation to 

 the humerus, remains relatively long; in several other 

 phyla which are progressing toward the weight-bear- 

 ing type the radius yields a less constant weight index 

 than the tibia. As indications of speed the tibial and 

 radial are much less reliable than the metapodial 

 indices. 



