CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



881 



the elongate or hypsodont grazing type or into the 

 lophodont or crested type. In the absence of trans- 

 verse crests between the inner cones and the outer 

 crescents elongation would produce a number of 

 separate columns and crescents. 



It is interesting to observe, in the evolution of the 

 titanotheres, the attempt of nature to create a hypso- 

 dont crown out of this cone and crescent type by the 

 elongation of the crescents on the outer side of the 

 superior grinding teeth. The inner cones do not 

 share in this elongation; the result is a tooth half 

 hypsodont and half brachyodont, obviously a very 

 poor mechanism. 



The bunoselenodont tooth was therefore incapable 

 of further evolution in any direction; no mechanical 

 progress or perfection was possible. 



Against the theory that the form of the teeth alone 

 caused the extinction of these great animals we must 

 record the fact that the giant elotheres of lower 

 Oligocene time, which had teeth that were still less 

 effective (of an all-cone pattern), survived to a later 

 geologic period than the titanotheres. If we knew 

 all about the life of these elotheres we should probably 

 find that they had some compensating advantage in 

 local habitat, perhaps recourse to river-border life. 



The hypothesis that the bunoselenodont tooth 

 pattern was, if not the sole, at least a potent cause of 

 extermination is supported by strong collateral 

 evidence, which is presented in the accompanying 

 diagram (PI. XLVIII). It appears that all buno- 

 selenodont quadrupeds, whether belonging to the 

 Artiodactyla or the Perissodactyla, disappeared during 

 the Oligocene epoch or early in the Pliocene. The 

 only bunoselenodont mammals that survived are the 

 chalicotheres, which persisted in the Northern Hemi- 

 sphere until the end of Pliocene time, probably in a 

 forest habitat. The defective tooth structure in 

 these animals was probably compensated for by the 

 development of giant claws on the feet, which gave 

 these strange quadrupeds certain advantages similar 

 to those that were enjoj^ed by the giant sloths. 



Elongation (hypsodonty) of the molar teeth in relation 

 to longevity and reproductive power. — The elongation 

 of the molar teeth is obviously a direct advantage in 

 the promotion of longevity and fertility, an advantage 

 that might lead to selection of fluctuations in length. 

 The elongate crowns of the teeth of the elephants 

 and the horses enable individuals to live many years 

 and to produce a large number of young. Elephants, 

 which are provided with successive long-crowned 

 teeth, live, according to Darwin, between 90 and 

 100 years and, although slow-breeding animals, pro- 

 duce a large number of young. Horses, which have 

 long-crowned teeth, live to the age of 25 years and, 

 foaling every year, multiply with great rapidity. 



In contrast, an animal like the titanothere Palaeo- 

 syops, with its short-crowned teeth, would live a com- 



paratively short time and produce comparatively 

 few young. Through long periods of geologic time 

 this relation of longevity to reproduction would 

 tend in the same habitat to replace races that had 

 short-crowned teeth and that were therefore short 

 lived with races that had long-crowned teeth and 

 that were therefore long lived. 



As a matter of fact, this theoretical condition is 

 modified by change of habitat, because we observe 

 that the short-crowned Cervidae of browsing habits 

 hold their own in the forests and that the generally 

 long-crowned Antilopidae are perfectly adapted to life 

 on the plains. 



Inadaptation of small hrain. — The chief advantages 

 of brain capacity undoubtedly appear in relation to 

 adaptability of habit and resourcefulness in times of 

 exposure, to alertness in avoiding new dangers to 

 which the young may be exposed, and to enterprise in 

 seeking new habitats, qualities that are especially 

 valuable at times of climatic change and of severe 

 competition. 



Modern quadrupeds differ widely in regard to 

 resourcefulness under adverse conditions of environ- 

 ment, as illustrated on the western plains during the 

 great winter storms. When sheep, cattle, and horses 

 meet the im expected conditions incident to a blizzard, 

 the sheep disappear first, the cattle second, the horses 

 last — that is, the horses, largely owing to instincts 

 which they have inherited from northern ancestral 

 strains, meet new conditions of life in most extraor- 

 dinary ways, whereas cattle of southern wild ancestral 

 strains are far less resourceful. 



The paleontologist knows nothing of the psychic 

 qualities of an extinct animal; he can judge its 

 brain power only by examining its intracranial cast, 

 which often reproduces the size and external form 

 of the brain with exact fidelity. Lartet, following 

 Cuvier, was among the first to allude to the law 

 of progressive cerebral development of certain of the 

 Tertiary mammals. He says (1868.1, pp. 1120-1122) : 



II rfeulterait en effet d'un certain nombre d'observations 

 relevees a divers (Stages de la stratigraphie tertiaire, que, plus 

 les mammiferes remontent dans I'anoiennetil des temps geolo- 

 giques, plus le volume de leur cerveau se r^duit par rapport 

 au volume de leur tete et aux dimensions totales de leur 

 corps. * * * On a dit que les plus grands mammiferes 

 sont ceus qui vivent le plus longtemps; ce qui serait plus pres 

 de la verity, c'est que la longevite normale parait s'aocroitre 

 en raison directe du volume absolu du cerveau. 



Marsh in 1884 considered brain size in relation to 

 the final extinction of thQ uintatheres. He observes 

 (1884.1, p. 190): 



The small brain, highly specialized characters, and huge bulk 

 rendered them incapable of adapting themselves to new con- 

 ditions, and a change of surroundings brought extinction. 

 * * * The Dinocerata, with their very diminutive brain, 

 fixed characters, and massive frames, flourished as long as the 

 conditions were especially favorable, but with the first geo- 

 logical change they perished and left no descendants 



