14 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



which the greater number of genera and species of 

 titanotheres have been described; and the Linnaean 

 generic and specific names can not be replaced unless 

 two systematic names have been given to the same 

 animal. Rather than introduce a new terminology 

 we attempt to place each Linnaean species in its proper 

 phyletic position — that is, in its true phylum — and to 

 connect it with other species by intermediate or transi- 

 tion stages, which are termed mutations, the "ascend- 

 ing mutations" of Waagen as distinguished from the 

 contemporaneous "mutations" of De Vries. 



LINNAEAN METHODS OF DEFINING SPECIES, GENERA, 

 AND PHYLA OF TITANOTHERES 



Between 1847 and 1902 as many as 29 genera and 

 67 species of Eocene and Oligocene titanotheres were 

 defined, but of all the definitions given hardly a single 



Proceeding along these lines Marsh and Cope defined 

 a number of genera of titanotheres, certain of which 

 have since proved to be closely successive members of 

 the same phylum and consequently members of the same 

 genus. Osborn went to the opposite extreme in attempt- 

 ing to reduce all the titanotheres to a single genus. In 

 his paper of 1896, entitled "The cranial evolution of Ti- 

 tanotJierium" (Osborn, 1896.110),hereached the wholly 

 erroneous conclusion that there had been only a single 

 distinct and definable genus of titanotheres — the origiaal 

 Titanotherium of Leidy — and that all the variations 

 among the titanotheres were of the rank of species, rep- 

 resenting different stages of development. This has 

 proved to be a greater error than that of Marsh, because 

 it was based on the hypothesis that the titanotheres 

 belonged to a single — monophyletic — line of descent. 



APPEARANCE AND EXTINCTION OF MAMMAL ORDERS IN NORTH AMERICA 



Archaic J\fa7n7na2s - soled, hlacTo. Mode?-nLze<t .MajnTnaZs - outiine 



Figure 12. — Diagram showing the gradual extinction of orders of archaic mammals (solid black) of earliest 

 Eocene time and their gradual replacement during later Eocene time by the ancestors of modernized orders 

 of mammals (outline), including related forms that are now extinct 



one has proved to be distinctive and valid. The 

 main characters utilized in the old classifications by 

 the chief contributors to the history of the Oligocene 

 titanotheres — that is, by Leidy, Marsh, Cope, Scott, 

 and Osborn — were the following: 



1. The presence or the absence and the number of incisor 

 teeth (Cope and Marsh, in generic definition). 



2. The number of premolar teeth (Marsh, in generic definition). 



3. The development of the cingulum on the premolar teeth 

 (Cope andMarsh, in generic definition). 



4. The presence of a second cone on the last superior molar 

 (Marsh, in generic definition). 



5. The length and shape of the nasal bones (Cope, Marsh, 

 Scott, and Osborn, in generic definition). 



6. The length and shape of the fronto-nasal horns (Cope 

 and Marsh, in generic definition). 



7. The presence or absence of a trapezium in the carpus 

 (Hatcher, in phyletic definition). 



RECOGNITION OF MANY LINES OF DESCENT) POLYPHYLY 

 THE KEY TO INTERPRETATION OF THE FAMILY 



In January, 1901, a few months after the studies 

 for this monograph were begun, all the data, observa- 

 tions, skull sections, and measurements were assembled, 

 and by July of the same year it was demonstrated by 

 Osborn that at least four lines of separate descent 

 are to be found among the lower Oligocene titanotheres, 

 and this number has since been increased to five or 

 eight. 



In 1902 Osborn established the fact that throughout 

 lower Oligocene time, when the Titanotherium-he&vmg 

 beds were being deposited, as many as eight more or 

 less different phyla, or series, were independently 

 evolving in the same region. Certain of these phyla 



