CHAPTER VII 



EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



SECTION 1. METHODS BY WHICH THE TITANOTHERE 

 SKELETON HAS BEEN STUDIED 



PRINCIPIES OF THE EVOLUTION OF THE IIMBS OF 

 HOOFED ANIMALS 



The postcranial skeleton of the titanotheres, al- 

 though less fully known than the skull, is no less 

 significant in its bearing on our knowledge of the 

 evolution of these animals and of that ancient West 

 which was their habitation or the scene of their migra- 

 tions. To understand that ancient West we must try 

 to bring its animals and plants back to life. The 

 attempt to restore the titanotheres as living and 

 migrating animals has led to the establishment of four 

 new principles in the evolution of the limbs of the 

 hoofed mammals generally, principles that have been 

 worked out by the author in cooperation with Dr. 

 William K. Gregory, who has published (Gregory, 

 1912.1, pp. 267-294) a preliminary study which 

 includes many independent observations of his own 

 on the mechanics and adaptations of limb movement. 

 These principles were discovered through comparison 

 of the skeleton and musculature of all the perissodac- 

 tyls and of the proportions of the upper and lower 

 limb segments in a large number of ungulates, in- 

 cluding perissodactyls, artiodactyls, amblypods, and 

 proboscideans. These four principles of limb evolu- 

 tion, which had been only partly or incompletely 

 recognized previously and which are fully described 

 in Chapter IX, are briefly as follows: 



1. The relative length and the angulation of the 

 upper and lower segments of the limbs and the planes 

 of the articular facets furnish a means of elucidating 

 the adaptations to speed and to weight in all the 

 hoofed mammals, living and extinct. Thus by de- 

 termining the relative lengths and proportions of the 

 limb segments among living forms in which the 

 speed, weight, and general limb movements are loiown, 

 we may estimate the adaptations to similar functions 

 and habits in the titanotheres and other extinct forms. 



2. Apart from their ancestral paleotelic adapta- 

 tions, all ungulates, in their bony and muscular 

 systems, show secondary cenotelic adaptations to 

 similar mechanics of speed and weight, which form 

 closely analogous or convergent groups and are exhib- 

 ited in the form and the proportions of the limbs and 

 of the shoulder and pelvic girdles. 



3. Within each of the nine families of perissodactyls 

 that are more or less closely related to the titanotheres 

 analogous or convergent adaptation produces closely 

 similar limb and shoulder-girdle forms from more or 

 less dissimilar ancestral forms. 



4. Between the primitive, light-limbed, subcursorial 

 Lambdoiherium type and the ponderous Brontotherium 

 type, the titanotheres pass through four stages of limb 

 types (figs. 685, 686). From a light body and limb 

 type {Lambdotherium) , analogous to that of the primi- 

 tive cursorial horses, they enter a medium limb stage 

 (Eotitanops) like that of the tapirs, then pass through 

 something near a primitive rhinoceros stage {Mesa- 

 tirJiinus), and ultimately attain the final titanothere 

 stage {Brontotherium), which is in some respects 

 similar to that seen in the elephants. 



The titanotheres and other hoofed mammals that 

 exhibit these four stages in the development of the 

 limbs are broadly designated as follows: 



1. Subcursorial digitigrades, partly perfected in 

 swift limb movements, including PJienacodus (condy- 

 larth), EoMppus (horse) , Lamhdotherium (titanothere), 

 primitive types of ungulates of lower Eocene time, in 

 which limb proportions are inherited from ancestral 

 unguiculates and show evidence of remote ambulatory 

 and even of still more remote arboreal adaptation 

 (Matthew, Gregory). In these animals the radius 

 and tibia are relatively long; the metapodials, typified 

 by Mts III and Mtc III, are relatively short. 



2. Mediportal digitigrades, of medium weight and 

 speed, with moderately heavy body and limbs and 

 clumsy motion (digitigrade), such as Tapirus (tapir), 

 MesatirMnus (titanothere). Most middle-sized quad- 

 rupeds of middle Eocene time have limb proportions 

 intermediate between the cursorial and graviportal 

 extremes. These proportion ratios survive in the 

 existing Tapirus, which, however, in its body and 

 limb proportions is more cursorial than the Eocene 

 titanotheres. 



3. Subgraviportal digitigrades, partly transformed 

 into or prophetic of the weight-bearing (graviportal) 

 type, such as Palaeosyops (titanothere). 



4. Graviportal digitigrades, of perfected weight- 

 bearing type, with angulate limbs capable of more or 

 less rapid movement according to the length and the 

 angulation of the limbs, such as Opsiceros (black 

 rhinoceros), Brontotherium (titanothere). These ani- 

 mals are all digitigrades — that is, the feet rest partly 

 on terminal hoofs, partly on pads beneath the phal- 

 anges. They are heavy or bulky forms, such as the 

 rhinoceroses and titanotheres, which retain the 

 digitigrade type of foot, although some (Menodus) 

 have straight hind limbs. 



Besides the four types of limbs or limb movement 

 that are exemplified in the titanotheres there are three 

 other types, which are not yet known in animals of 

 the titanothere group, as follows: 



583 



