28 



TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



2. Motion and locomotion; migration in search of 

 food and to escape enemies; adaptation to perform 

 the act of reproduction and to protect the young. 

 These purposes involve all the mechanical changes of 

 the structure of limb and body. 



The operation of the principle that, under the domi- 

 nance of these modes of mechanical adaptation each 

 organ, structure, and character is adaptively evolved 

 for some special service to the organism is not invariably 

 evident in respect to all changes in the proportion of 

 characters. Certain characters of proportion, such as 

 extreme broad-headedness or extreme long-headedness, 

 seem to interfere with adaptation; they appear to be 

 carried so far in one direction as to render the animal 

 less adapted to survive than its less specialized ances- 

 tral forms. In other words, certain tendencies of 

 evolution may carry a phylum beyond its require- 

 ments in adaptation. 



Aside from this question of the different degrees of 

 survival or actual elimination value of certain tend- 

 encies of evolution, there can be little doubt that in 

 its origin and development each character, sooner or 

 later, responds and reacts independently to the con- 

 ditions of the environment, quite apart from the 

 question as to the causes of such response. The teeth 

 react to the kinds of food; the feet and limbs to the 

 kinds of soil. 



The principles of the divergence of quadrupeds 

 from each other in their independent adaptations in 

 the skull, teeth, limbs, and feet are fully discussed 

 elsewhere (see p. 123) in the treatment of the principle 

 of adaptive radiation. Though they may have lived 

 apparently in the same region and have been fossilized 

 side by side in the same sediments, all distinct species 

 of quadrupeds have locally different habits and habi- 

 tats. The structure of the skull, jaws, and teeth re- 

 sponds to their habits and tastes ; the structure of the 

 feet and limbs responds to their habitats — the nature 

 of the ground, etc. 



PHYLETIC DIVERGENCE IN THE EVOIUTION OF NEW 

 PROPORTIONS IN HORSES AND IN TITANOTHERES 



All the families of an order of Perissodactyla start 

 their career from a similarly proportioned ancestral 

 stem form such as that described in Chapter X (p. 760) 

 as the stem perissodactyl. Starting with the same 

 complement of characters, divergence in proportions 

 separates the families of perissodactyls more and more 

 widely from one another. In the Equidae (horses), 

 for example, the head form of the earliest known 

 ancestor (EoMppus) is very similar to that of the 

 earliest known ancestor (Eotitanops) of the family 

 Brontotheridae. In both these primitive skulls the 

 orbit is near the center of the head, and in the later 

 forms it apparently moves backward or forward, but 

 what really happens is that the skull is elongated in 

 front of the orbit in the horse and is elongated behind 

 the orbit in the titanothere. (See fig. 21.) 



A comparison of the forms shown in Figure 21 

 with those shown in the following figures will demon- 

 strate the marked similarity of the lower Eocene 

 forms and the very wide divergence of the modern 

 forms. The skulls of the ancestral tapir, horse, and 

 titanothere {Systemodon, Eohippus, and Eotitanops) 

 are in many ways much alike, the chief differences 

 consisting in (1) the details of the characteristics of 

 the dentition, (2) the relative position of the orbits, 

 (3) the depth of the head through the back part 

 of the lower jaw, and (4) the size of the muzzle. 

 The primitive titanothere prophetically suggests the 

 titanothere characters in the relatively heavy muzzle 

 and stout lower jaw. The primitive horse Eohippus 

 prophetically suggests the modern horse in the taper- 

 ing form of the slender lower jaw and in the general 

 contour of the skull, except that the eye is placed near 

 the middle of the head, as in other primitive perisso- 

 dactyls. The primitive perissodactyl Systemodon, 

 regarded by Osborn as an ancestral tapiroid, had a 

 somewhat longer, more pointed muzzle but was 

 otherwise very similar to the contemporary horse 

 Eohippus. 



These differences of proportion between the facial 

 region in front of the orbit and the cranial region 

 behind the orbit are partly correlated in adaptation 

 to the elongation (hypsodonty) of the crowns of the 

 grinding teeth. In the horse and in most of the rumi- 

 nant artiodactyls the face is elongated to accommodate 

 the vertically elongated (hypsodont) grinding teeth. 

 In the titanotheres, which are browsing animals, 

 and in the browsing rhinoceroses of India and of 

 Africa the orbit is directly above the grinding teeth 

 and the cranium is slightly elongated, as shown in 

 Figure 22. Thus it may be stated as a general prin- 

 ciple of skull evolution that in browsing ungulates 

 the cranium tends to be elongated and the face tends 

 to be abbreviated, whereas in grazing ungulates, 

 like the white rhinoceros of Africa, in which the grind- 

 ing teeth are elongated, the face is elongated, and 

 the cranium is abbreviated. 



It follows that these respective proportions of the 

 region in front and back of the eyes are adaptive; 

 they are part of the general correlation of skull 

 proportions with the functions of the grinding teeth 

 employed in the prehension of food, as provided for 

 chiefly in the shape of the upper and lower lips, 

 which are obtrusible and flexible both in the browsing 

 rhinoceroses and in the grazing horse, which occasion- 

 ally browses. When the horse is browsing it extends 

 its lips very much in the manner of the browsing 

 rhinoceros, except that in the rhinoceros the independ- 

 ent motion and the pointing of the upper lip are more 

 extreme. In the grazing white rhinoceros the upper 

 lip is extremely broad and square. The animal 

 subsists largely on grasses, which it crops with its 

 square lips, exactly in the manner that the horse 



