ENVIRONMENT OF THE TITANOTHERES 



113 



FAUNAL DIVISIONS IN THE CHADRON FORMATION 

 THEEE FAUNISTIC LEVELS DETERMINED 



In the series of sediments that were deposited on 

 the uneven surfaces of the Pierre shale and that con- 

 sist of fine clays, which were traversed and at many 

 places secondarily eroded by river channels and which 

 were very gradually accumulated during an extremely 

 long period, we should not expect to find anything re- 

 sembling clearly defined stratification or horizontal 

 and vertical succession of species and genera. Never- 

 theless, we owe to the genius and the untiring explora- 

 tion of Hatcher a division of the Chadron formation 

 into lower, middle, and upper levels, which we shall 

 designate Chadron A, Chadron B, and Chadron C, 

 and which correspond to similar divisions of the deposits 

 of the Rocky Mountain basins. 



In his paper of 1893 (1893.1), "The Titanoiherium 

 beds," Hatcher remarked that these beds were so 

 named by Meek and Hayden in 1857 from the genus 

 TitanotJierium, established by Leidy in 1852. Al- 

 though we are obliged to replace the generic name 

 TitanotJierium by Menodus it seems best to retain 

 Titanotherium as the historic zonal name for these sig- 

 nificant beds. 



The thickness of the "Titanotherium beds" at dif- 

 ferent localities in Wyoming, Colorado, the Dakotas, 

 and Saskatchewan, as recorded above, varies, having 

 a maximum of 500 feet and a minimum of 30 feet. 



Hatcher, accepting a total of 180 feet as the maxi- 

 mum thickness of these beds in the Big Badlands of 

 South Dakota, assigned 50 feet to the lower level, 100 

 feet to the middle, and 30 feet to the upper (1893.1, 

 p. 210). During the field seasons of 1886, 1887, and 

 1888 Hatcher collected for the present monograph 

 material including 105 nearly complete Titanotherium 

 {Menodus) skulls and parts of numerous skeletons, as 

 well as disarticulated bones, besides remains of many 

 other associated animals. Early in the season of 

 1886 it became apparent that certain forms of skulls 

 were characteristic of certain horizons in the "Titano- 

 therium beds." This fact indicated the desirability of 

 keeping, so far as possible, an exact record of the 

 horizon from which each skull or skeleton was taken. 

 From actual measurement the vertical range of the 

 titanotheres in the Big Badlands was found to be 



about 180 feet. For convenience in keeping a record 

 of horizons the beds were divided into three divisions 

 of 60 feet each, and each of these three divisions was 

 subdivided into three divisions of 20 feet each. As 

 each skull or skeleton was dug out a separate letter or 

 number was given to it and it was assigned to that 

 subdivision of the beds from which it was taken. 



STRATIGEAPHIC DISTRIBUTION OF SPECIES OF OLIGOCENE 

 TITANOTHERES 



In 1888 Hatcher drew up a manuscript table for 

 Professor Marsh in which the lower, middle, and 

 upper divisions of the "Titanotherium beds" were 

 each subdivided into three levels, and in which he 

 placed the letters assigned to many of the skulls 

 found by him. In 1901 Hatcher revised this table for 

 Osborn for use in the present monograph. In the 

 summer of 1902 the United States Geological Survey 

 sent Messrs. N. H. Darton and J. B. Hatcher to the 

 Big Badlands of South Dakota for a resurvey of the 

 localities where some of the skulls were found by 

 Hatcher in order to determine precisely the elevation 

 of these localities above the Pierre shale, at the 

 base of the beds. Prof. Eberhard Fraas, of Stuttgart, 

 accompanied the party and made some interesting 

 observations on the mode of deposition of these beds. 

 (Fraas, 1901.1.) This experienced geologist con- 

 cluded that the "Titanotherium beds" consisted of 

 river and flood-plain deposits whose surfaces were 

 exposed during the dry seasons of the year; that 

 parts of the overlying Brule clay — the beds in the mid- 

 dle Oreodon zone — were deposited in shallow lakes, the 

 dissolved materials, of varying concentration, giving 

 rise to the banded layers; and that the reddish strata 

 of the Oreodon zone (Brule) were formed of eolian 

 loess. 



In the following table the results of records and ob- 

 servations made by Hatcher, indicated by the abbre- 

 viation J. B. H., are supplemented by the results of a 

 few observations made by N. H. Darton of the United 

 States Geological Survey, E. S. Riggs of the Field 

 Museum, W. H. Reed of the University of Wyoming, 

 and Walter Granger of the American Museum. The 

 species are arranged in the five generic phyla deter- 

 mined by Osborn, namely, Brontops, Allops, Menodus 

 {= Titanotherium) , Megacerops { = Symborodon) , Bron- 

 totherium. 



