12 



THE AGE OF MAMMALS 



The enlarged teeth are often so similar as to be very misleading in regard 

 to relationship or affinity. 



As regards the elongation of the teeth the parallel with that of the feet 

 is very close indeed, for we distinguish the following kinds of teeth: 



Brachyodont, primitive short-crowned teeth, with simple roots and 

 simple cusps, and usually with simple conic, crescentic, or crested 

 cusps, as of the pig, deer, and mastodon. 



Hypsodont, elongate grinding teeth, as of the ox, horse, and elephant. 



Just as the power of an herbivorous animal to move long distances or 

 to take wide excursions in search of food or to move rapidly in escaping 

 its enemies is brought about through changes in the number of digits, and 

 in the form and proportions of the feet, so the power of an animal to live a 



long period of time and to 

 assimilate the harder kinds of 

 food is increased through 

 changes of form and propor- 

 tion in the teeth. The hypso- 

 dont horse attains over thirty 

 years of age; the hypsodont 

 elephant lives about a hundred 

 years. 



Elongate teeth may be far 

 more highly perfected me- 

 chanically and have more 

 complicated crowns, consist- 

 ing of three different dental 

 tissues of three degrees of 

 density, namely, enamel, den- 

 tine, and cement, thus afford- 

 ing three degrees of resistance, 

 and always presenting a rough 

 or uneven grinding surface. 

 The passage from short- 

 crowned to long-crowned teeth also marks the passage from browsing 

 forms, living on softer kinds of food, to the grazing forms, living on the 

 harder kinds of food, as well as from more short-lived animals to more 

 long-lived animals. Here again we see that the elongation of the teeth like 

 the elongation of the feet is eminently adaptive. For example, where 

 physiographic changes reduce the softer herbage and increase the harder 

 grasses, and separate the favorable feeding grounds as well as the drinking 

 pools, the change of proportion is principally in the direction of elongation 

 of the feet, the teeth, and the skull respectively, or dolichopody (feet), 

 hypsodonty (teeth), and dolichocephaly (skull). 



D 



Fig. 4. — Tooth proportions, or brachyodonty and 

 hypsodonty. B. Brachyodont, short-toothed, Masto- 

 don americanus. A. Hypsodont, long-toothed, £'ZepAas 

 meridionalis. D. Brachyodont, short-toothed, Anchi- 

 therium aurelianense. C. Hypsodont, long-toothed, 

 Equus caballus. 



