Journal of Mammalogy 
Published Quarterly by the American Society of Mammal ogists 
VoL. 3 NOVEMBER, 1922 No. 4 
CLOSE OF THE AGE OF MAMMALS 
By Henry Fairfield Osborn and H. E. Anthony^ 
The Age of Reptiles is estimated at 10,000,000 years by the thick- 
ness of sedimentary rocks. When the Age of Reptiles closed, land, 
air, and sea were left free for the rise of the mammals. The five sur- 
viving orders of reptiles were confined to the temperate and tropical 
zones and were only to a limited degree mammal destroyers. 
The beginning of the Age of Mammals is estimated at 3,000,000 yeai's 
ago. Out of very small and primitive progenitors, monotreme, mar- 
supial, and placental, there evolved over the entire globe — land, sea, 
and air — a teeming mammalian life. Mammalian perfection reached 
its climax at the close of Pliocene time, about 400,000 years ago. The 
palppontologist follows this marvelous creation with wonder and ad- 
miration, as he traces the rise and adaptive radiation of twenty-four 
orders, one hundred sixty-six families, three thousand genera, fifteen 
thousand species, and its varieties of races of mammals. 
The first effort at mammalian adaptation to land conditions in early 
Tertiary times was a failure; it was followed by natural extinction of 
five orders of mammals. The second mammalian adaptation culmi- 
nated in the Upper Pliocene world and included man. This perfected 
Mammal World gradually diminished during the First, Second, and 
Third Glaciation, and suffered a terrible blow during the Fourth Glacia- 
tion, which led to the elimination of many superb races of mammals, 
and one or more of the inferior races of man. This was an extreme 
climatic change in which the Holarctic reindeer was driven down to 
1 This paper has been a year in preparation under the direction of the Senior 
author, who also contributes the introduction. The Junior author contributes 
the body of the paper on the fur trade, after a year of search and correspondence. 
219 
JOURNAL OF MAMMALOGY, VOL. 3 , NO. 4 
