INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



17. Titanotherium-Mesohippus zone (Brontops robustus zone, 

 fauna; Chadron C fauna; Brontops dispar zone, Chadron B 

 and Brontops brachycephalus zone, Chadron A fauna) . 



16. Theoretic zone (Uinta C 2). 



15. Diplacodon-Protitanotherium-Epihippus zone (Uinta C 1 

 fauna) . 



14. Eobasileus-DoKchorhinus zone (Uinta B 2 and Washakie B 2 

 faunas) . 



1.3. Metarhinus zone (Uinta B 1 and Washakie B 1 faunas). 



12. Uintatherium-Manteoceras-Mesatirhinus zone (Bridger C 

 and D and Washakie A faunas). 



11. Paleosyops paludosus-Orohippus zone (Bridger B 

 fauna) . 



10. Eornetarhinus-Trogosus-Palaeosyops fontinalis zone 

 (Bridger A and Huerfano B faunas). 

 9. Lambdotherium-E otitanops-C oryphodon zone 

 (Wind River B and Huerfano A faunas) . 



Estimated duration of tJie titanothere epoch. — 

 The duration of the titanothere epoch, from 

 the time of the earhest known member of 

 the family {Larnbdotherium) to that of the 

 last product of titanothere evolution {Bron- 

 totherium) is estimated as 600,000 years. 

 This estimate is based on the calculation of 

 Walcott, made from measurements of the 

 rate of geologic sedimentation, that the 

 entire Tertiary period covered not more than 

 3,000,000 years. If estimates made by Bar- 

 rell (1917.1, p. 892), based on radioactivity, 

 can be verified the duration of Tertiary time 

 should be extended to 54,000,000 years. If 

 this estimate is accepted the duration of 

 the titanothere epoch alone would extend to 

 11,000,000 years. Though the geologic esti- 

 mate of 600,000 years for titanothere evolu- 

 tion seems to be too small, the physical esti- 

 mate seems to be too great, and for the 

 present we may regard the estimate based on 

 geologic data as ranging between 600,000 and 

 1,000,000 years. 



HAYDEN'S SUBDIVISIONS OF THE EOCENE AND THE 

 OLIGOCENE 



The geologic formations in which titano- 

 there remains occur and the life zones into 

 which these formations are subdivided have 

 been discovered and described during the last 

 56 years, the first report on them being that 

 of Meek and Hayden (1862.1), in which the 

 entire Tertiary geologic column is represented 

 in a "General section of the Tertiary rocks 

 of Nebraska," reproduced here in facsimile. 



There is little doubt that when Hayden described 

 the White River group as "1,000 feet or more" in 

 thickness, as including the "Bad Lands of White 

 River; under the Loup River beds, on the Niobrara, 

 and across the country to the Platte," and as com- 

 posed of "white and light-drab clays, with some beds 

 sandstone, and local layers limestone," he had in mind 

 the area extending from Cheyenne River of South 

 Dakota to the region south of North Platte River, 



displayed in the accompanying map and panoramic 

 section. This section includes at its base the Titano- 

 therium and Oreodon zones (Chadron and Brule for- 

 mations), from which Hayden listed certain char- 

 acteristic forms of animal life, such as TitanotJierium 

 {=Menodus), Choeropotamus {=Ancodus, Hyopota- 

 mus), "Rhinoceros" {=Caenopus), AncJdtherium 

 {= Mesohippus)' , Hyaenonodon {= Hyaenodon) , Ma- 

 chair odus { = Dinictis). 



Gemral Section of the Tertiary rocks of Nebraska. 



Figure 6. — The Meek and Hayden Tertiary section of 1862 



The deposits named are now known to include the following: 



"Loup River beds." The lower Pleistocene fauna listed is found in an area that includes 

 deposits of the Pliocene and upper Miocene (Ogalalla formation of Darton) . 



"White River group," including lower Miocene (Arikaree formation of Darton) and Oligo- 

 cene (Brule and Chadron formations of Darton). The " Choeropotamus" is Ancodus 

 ameTicanus, the ancodont of the Chadron formation (Titanotlierium zone). 



"Wind River deposits" (summit of the lower Eocene). 



"Fort Union or Great Lignite group" (basal Eocene). 



These Titanotherium and Oreodon zones are now 

 regarded as lower and middle Oligocene, respectively, 

 and above them have been discovered the Protoceras 

 and Leptauchenia zones, which embrace the highest 

 sediments assigned to the Oligocene. The combined 

 thickness of the Oligocene at this point is 600 to 650 

 feet. Above it, to the east, are "light-drab clays," 

 having a total thickness of 500 feet, and these, when 

 combined (1,150 feet), correspond to the "1,000 feet 



