10 THE AGE OF MAMMALS 



Subsequent research has shown that, considering the great difficulties under 

 which, they labored, Hayden and Leidy formed a surprisingly accurate 

 conception of the geologic or time succession of mammalian forms in Oligo- 

 cene, Miocene, and Pliocene times. Hayden's explorations of 1869-1870 

 in the interior of the Rocky Mountain region led to the discovery of the 

 Bridger deposits of Middle Eocene Age, the rich extinct mammalian life of 

 which was monographed by Leidy in 1873. 



The complete geologic succession of the vast ancient life of the Amer- 

 ican continent was destined to demonstrate the evolution law. The Vjrief 

 narrative of this succession, as known to-day, forms one of the chief sub- 

 jects of the present volume. Between 1871 and 1873 Marsh and Cope 

 took up the labors of Leidy in the exploration of the Rocky Mountain ba- 

 sins, rich in fossil life. The first ten years of their work not only modified 

 our ideas of the descent of the mammals, but brought together data for 

 a number of important generalizations: for Marsh's demonstration ^ that 

 the size of the brain was an important factor in survival, that most small- 

 and smooth-brained mammals early became extinct; for Cope's proof 

 that the hoofed animals sprang from types with simple, five-toed feet, rest- 

 ing largely upon the sole, and with conic cusps on the grinding teeth. 

 Finally between 1879 and 1883 came Cope's demonstration 

 that the grinding teeth of most of the mammals had 

 passed through a triangular form in the early Eocene 

 period, in which three tubercles, or a tritubercular struc- 

 ture, is apparent. 



Primitive and -progressive forms of teeth. — This dis- 

 covery by Cope ^ of primitive and widespread trituberculy 

 in the molar or grinding teeth was a great step forward. 

 We find in the previous descriptive works or odontog- 

 Primi- raphies of Cuvier and of Owen not even a suspicion of 



tive stage in tooth ,, . . ~ ..... , . ,. 



development. Typ- ^^^ existence oi a common or primitive type ot grinding 



icai tritubercular, tooth from which the highly specialized "cheek teeth" of 



mesonychid °creo- *^^ different mammals have evolved. At the present time 



dont, Triisodon of we may Compare the molars of the clawed and the hoofed 



the Basal Eocene, i -ji i ^i i ^i r 



Puerco Formation, mammals With each other because they sprang trom a 

 After Osborn, 1907. common tritubercular type, just as we compare the hand 

 or foot of man, of the horse, the cat, because we know 

 they sprang from a common five-fingered type. Nearly all kinds of 

 mammals, hoofed quadrupeds, monkeys, carnivores, insectivores, rodents, 

 marsupials, are found building up their grinding teeth on the basis of this 

 primitive tritubercular ancestral form.^ We thus have a key to the com- 



' Marsh, O. C, Dinocerata. A Monograph of an Extinct Order of Gigantic Mammals. U.S. 

 Geol. Stirv., Vol. X, Washington, 1884, p. 58 fol.; see also Marsh, '74 and '85 in Bibliography. 



- Cope, E. D., On the Trituberculate Type of Molar Tooth in the Mammalia. Pal. Bull. 

 no. 37, Proc. Amer. Philos. Soc, Vol. XXI, Dec. 7, 188.3, pp. 324-326. 



^ See Osborn, H. F., Evolution of Mammalian Molar Teeth, 8vo. New York, 1907. 



