212 



THE AGE OF MAMMALS 



at the very base of the Ohgocene, which rest directly upon the irregularly 

 eroded surfaces of the Upper Cretaceous. As shown in the accompanying 

 map, this Chadron Formation (black lines) was widely distributed in 

 South Dakota, Nebraska, and Wyoming, and extends up into British 

 Columbia to the Swift Current Creek Formation. Again in Montana we 

 find the Pipestone Creek, first explored by Douglass,' which yields the 

 mammals of smaller size, or microfauna.- Since the Titanotherium beds 

 of the Big Badlands are mostly coarse and largely fluviatile, our knowledge 

 of the American mammals of this stage is still rather limited except as 

 regards the titanotheres, which are magnificently represented and undergo 

 their entire final evolution and extinction in this short period of two hun- 

 dred feet of deposition. 



The first to thoroughly explore this zone was Hatcher,^ while searching 

 for titanothere skulls and skeletons. In 1893 he divided the zone into 



^^y 



B 



Fig. 100. — Heads of Lower Oligocene titanotheres. Representing four contemporaneous 

 phyla, or lines of descent of {A) Megacerops, (B) Titanotherium, (O Symhorodon, (D) Bron- 

 totherium. After originals by Charles R. Knight in the American Museum of Natural 

 History. 



three levels: a lower, characterized by titanotheres of very small size, 

 with small horns; a middle, by titanotheres with horns of intermediate 

 size; and an upper, by giant titanotheres, some of which exhibit magnificent 

 horns. Osborn * subsequently showed that these dominant mammals rep- 

 resent four phyla or grand divisions, namely: 



Short-horned 



Long-horned 



[ Titanotherium, long-headed, slender-limbed, lacking incisor teeth. 

 I Megacerops, short-headed, stout-limbed, with incisor teeth. 



f Syjnborodon, smaller, lacking incisor teeth. 

 1. Brontotheriimi, larger, with incisor teeth. 



This polyphyletic character, or adaptive radiation of the titanotheres, 

 affords us a hint as to varied local conditions which are also reflected in 



' Douglass, Fossil Mammalia of the White River beds of Montana. Trans. Amer. Philos. 

 Soc, n.s., Vol. CC, 1901, pp. 1-42. 



* Ibid., New Vertebrates from the Montana Tertiary. Ann. Carneg. Mus., Pittsburg, 

 Vol. II, no. 2, 1903, pp. 145-200. 



3 Hatcher, J. B., The Titanotherium Beds. Amer. Natural., March 1, 1893, pp. 204-221. 



* Osborn, H. F., The Four Phyla of Oligocene Titanotheres. Bull. Amer. Mus. Nat. 

 Hist., Vol. XVI, Art vii, Feb. 18, 1902, pp. 91-109. 



