EOCENE AND OLIGOCENE TITANOTHERES OF MONGOLIA 



909 



edges of the fiat-lying strata are exposed. Tliese 

 strata are formed of ratlier loose and slightly indurated 

 sandstone and clayey sand of considerable variety of 

 texture, quality, and color. In the upper members 

 of the Ardyn Obo formation fossils were obtained, 

 including several fine specimens of rhinocerids and 

 numerous fragments of turtles. Although it is judged 

 to be of mid-Tertiary age, the Ardyn Obo can not be 

 correlated with either the Hsanda Gol or the Houldjin 

 formation. 



Morris and Granger (notes, 1925) report as follows: 

 On top of the Ardyn Obo beds lies a heavy sand- 

 stone capping, which has preserved at its present 

 level this great mesa. The beds are predominantly 



Fauna. — Matthew and Granger (December 18, 1923, 

 pp. 1-5) described the small collection of mammals 

 obtained in 1922 at "Promontory Bluff," on the Sair 

 Usu-Kalgan trail, about 150 miles from Sair Usu and 

 350 from Kalgan. They concluded that this appears 

 to be an Oligocene fauna; Cadurcotherium, Schizo- 

 therium, and Cynodictis are generically characteristic 

 of the Phosphorites of Quercy; a giant tortoise 

 (Testudo insolitus) is apparently in a rather primitive 

 stage. The nearest zoogeographic affinities of this 

 fauna are with western Europe rather than with the 

 western United States. 



Osborn (1925) added the important titanothere 

 genus and species Bronfops gohiensis, which relates 



Figure 769. — Ardyn Obo formation (lower Oligocene) of Mongolia, where the types of Brontops gohiensis and Cadurcothe- 

 rium ardynense were discovered 

 American Museum camp of 1923 in the foreground. After Am. Mus. negative No. 251603. 



sandy but include some layers of shale. The fossils 

 were collected along the face of the great Ardyn Obo 

 bluff and at its base, about 300 feet from the summit 

 sandstone capping; the remaining 200 feet lies out in 

 the basin and is not well exposed. The Cadurco- 

 therium ardynense quarry, opened in 1922 and again 

 worked ia 1923, was withia 100 feet of the top of the 

 mesa, and most of the teeth and bones of this small 

 aquatic rhinoceros came from about this level. The 

 numerous artiodactyl jaws, accompanied by an occa- 

 sional rodent or carnivore, were found on knolls at 

 the base of the great bluff, 150 and 200 feet below its 

 top. The skull of the titanothere Brontops gohiensis 

 was found at the lowest collecting level, about 300 

 feet below the top of the sandstone capping. 



this formation to the lower Oligocene of South 

 Dakota. 



Brontops gohiensis life zone. — The name "Brontops 

 gohiensis life zone" has been assigned by Osborn 

 to the Ardyn Obo formation to emphasize the fact 

 that Brontops gohiensis is strikingly close in its evolu- 

 tion to Brontops hrachycephalus and to Teleodus avus, 

 discovered in the lower Titanofherium life zone 

 (Chadron formation) in the great Badlands of South 

 Dakota. This robust, broad-headed little titano- 

 there was discovered some 300 feet below the top 

 of the Ardyn Obo formation, which is altogether 

 over 500 feet in thickness as measured by Morris. 

 We may confidently assign a lower Oligocene age 

 to this formation. A single lower molar represents 



