794 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Uatio of weight of brain to weight of body in certain ungulates 



PERISSODACTYLA 



Tapirus indicus L 



Tapirus americanus L. 



Tapirus americanus L_ 

 Tapirus americanus L_ 

 Equus zebra L 



PHOBOSriDEA 



Katio of 



brain to 



body 



100 



947 

 140 

 247 



758 0. 13 



[.00 



. 10 

 .71 

 .40 



1 month 4 days 



old. 

 Fatlier of above. 

 Young animal. 

 Prettv thin. 



The weight of the brains of the early Tertiary 

 ungulates has been considered by Lartet (1868.1), 

 by Bruce (1883.1), by Marsh (1884.1) by Cope, and 

 by Osborn. Cope refers to Lartet's anticipation of 

 Marsh's laws of brain weight, which are as follows: 



1. All Tertiary mammals had small brains. 



2. The size of the brain gradually increased during 

 the Tertiary period. 



3. This increase was confined mainly to the cerebral 

 hemispheres, or higher part of the brain. 



4. In some groups the convolutions of the brain 

 have gradually become more complex. 



5. In some groups the cerebellum and the olfactory 

 lobes have even diminished in size. 



6. There is some evidence that the same general 

 law of brain growth holds good for birds and reptiles 

 from the Cretaceous to the present time. 



Marsh enunciated also two additional laws: 



1. The brain of a mammal belonging to a vigorous 

 race, fitted for a long survival, is larger than the aver- 

 age brain of that period in the same group. 



2. The brain of a mammal of a declining race is 

 smaller than the average brain of its contemporaries 

 of the same group. 



DENTAL MECHANISM 



The evolution of the upper grinding teeth of the 

 titanotheres, as shown in Figure 718, A-E, proceeds 

 from a purely brachj^odont, low-crowned browsing 

 type to a semihypsodont browsing and grazing type. 

 In the earlier stages of evolution {Eotitanops, Lim- 

 nohyops, Palaeosyops) the mode of mastication and the 

 action of the mandible indicate that the principal 

 motion of the jaws was vertical, like that of the 

 omnivorous Carnivora. 



In this chopping action of the jaws the pointed in- 

 cisors, the relatively long, pointed canines, the rela- 

 tively simple premolars, and the low-crowned molars 

 were correlated with seven changes as follows: (1) 

 The unique feature of the dental evolution is the semi- 

 hypsodonty (elongation) of the outer side of the crowns 

 of the upper grinding teeth and persistent brachyo- 

 donty of the inner side; (2) with this change came the 

 inclination of the wearing plane of the tooth row 

 toward the midline of the tooth, (3) the complication 

 of the premolars by the addition of internal cusps, 

 (4) the ratchet and cog like relations of the upper and 

 lower molars, (5) the changes in the proportions and 

 the arrangement of the masticating muscles, (6) the 

 increasing importance of the oblique shearing effect in 

 the swing of the mandible, together with (7) its pro- 

 nounced fore-and-aft rocking motion (W. K. Gregory). 

 These seven coadapted changes resulted in a dental 

 mechanism which combined cutting and triturating 

 functions in a very complex manner and which may 

 have been capable of masticating as wide a range of 

 vegetable food as that of the modern Rhinoceros 

 (Opsiceros) Mcornis. 



The titanotheres, like other perissodactyls, doubtless 

 had a simple stomach and were incapable of ruminat- 

 ing. Thus the digestive apparatus as a whole was 

 inferior to that of the contemporary camels (Lep- 

 taucJienia), of the numerous oreodonts with crescentic 

 teeth, and of the ancestors of the modern selenodont 

 artiodactyls. Consequently the titanotheres may 

 have derived less nutriment from a similar amount of 

 food to that consumed by the contemporary artio- 

 dactyls or may virtually have been debarred from 

 certain kinds of food and certain feeding ranges. The 

 wide and increasing prevalence of ruminant artio- 

 dactyls after the decadence of the titanotheres seems 

 to imply a superior digestive apparatus if not a supe- 

 rior dental mechanism, or else a profound, change of 

 herbage. 



After the extinction of the titanotheres the only suc- 

 cessful competitors of the artiodactyls were certain 

 perissodactyls, like the horses and rhinoceroses, espe- 

 cially the great Siberian rhinoceroses {Elasmotherium) ■ 

 I The Equidae, starting with the same type of molar 



