148 



TITAJSfOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Hatcher was less fortunate in his observations on the 

 evolutionary changes in the dentition, stating that 

 "the number of incisors, though probably never 

 constant, even in the same species, shows a tendency 

 to decrease in skulls found near the summit of the 

 beds," and concluding that "the number of incisors 

 can hardly be considered as of either generic or specific 

 importance in the Titanotheridae, where they are no 

 longer functional and vary with individuals in the 

 same species and with age in the same individual. 

 The same may be said of the presence or absence of 

 the first premolar." 



After noting certain other changes rightly believed 

 by him to be progressive, such as (1) the loss of the 

 trapezium, (2) the development of a postero-internal 

 cone on the third upper molar, and (3) the flattening of 

 the horns, Hatcher concludes his paper by giving a 

 tabular paleontologic section of the " Titanotlierium 

 beds," with a general description of the forms char- 

 acterizing the three ascending divisions. As to the 

 number of genera, he gives the impression that he 

 •regards all the various species ascribed by Marsh and 

 Cope to different genera (except Teleodus avus Marsh) 

 as referable to the single highly variable genus 

 Titanotherium Leidy. 



FIRST EUEOPEAn NOTICE (TOTJIA, 1892) 



The next important event is the discovery of a 

 titanothere of Oligocene type in Europe, described as 

 Menodus rumelicus by Toula (1892.1). This dis- 

 covery, in connection with that of the Transylvanian 

 BracJiydiastematJierium, described by Bockh and 

 Maty in 1876 (1876.1), extended the known range of 

 the titanotheres to the Old World. 



DISTINCTIONS OF SEX (OSBOEN AND WOETMAN, 1895) 



In 1895 Osborn and J. L. Wortman (1895.105) 

 published a corrected restoration of Titanotlierium 

 based upon the fine skeletal material secured by the 

 American Museum field parties in 1892 and 1894. They 

 ventured the conclusion that "it is probable that 

 certain wide differences in the development of the 

 horns, which have been assigned a generic value, 

 are merely sexual characters. " 



MONOPHYIETIC INTEEPEETATION (OSBOEN, 1898) 



The extreme development of the erroneous theor}' 

 that all the various species of Oligocene titanotheres 

 belonged to the single genus Titanotherium and were 

 practically monophyletic is worked out in a very 

 elaborate way in Osborn's paper "The cranial evolu- 

 tion of Titanotlierium," published in 1896. This 

 'was the most comprehensive review of the subject 

 that had hitherto appeared and was illustrated by 

 numerous text figures and several folded plates. 

 The direct observations were based chiefly on the 

 large collection of titanotheres in the American 

 Museum and to a less extent upon figures and descrip- 



tions previously published. Part I, the systematic 

 introduction, includes a chronologic list of generic 

 and specific terms, with references and a brief history 

 of the progressive complication of the nomenclature, 

 after which the author says (Osborn, 1896.110, p. 

 162): 



It is obvious that the only method of clearing up this hetero- 

 geneous list [of nominal genera and species] is first to establish 

 certain laws of cranial development, and second to apply 

 these laws to the distinction of genera and species in chrono- 

 logical order. Examined in this way, the vast array of genera 

 and species is resolved into one or possibly two genera and 

 about fourteen definable species. 



Accordingly in Part II, "Principles of cranial and 

 dental evolution," we find a study of the differences 

 in size of skull, shape of horns, nasals, zygomatic 

 arches, auditory meatus, cingula on grinding teeth, 

 incisors, canines, second internal cone of last molar, 

 etc., aU considered as indicating either specific or 

 sexual or individual differences within the limits 

 of a single genus, Titanotlierium. This is followed by 

 the "Revision and definition of species," in which 

 some 27 species, including the new T. ramosum, are 

 discussed. The known species from the lower, middle, 

 and upper beds are arranged in a single or monophy- 

 letic series, beginning with the T. heloceras-trigonoceras 

 ingens series, continuing with torvum, rohusfum, 

 doliclioceras, elatum, amplum, acer, and culminating 

 with ramosum and platyceras. 



This analysis, although wholly wrong in treating 

 all the species as members of a monophyletic series, 

 not only laid the foundation for the present evolution- 

 ary and phylogenetic treatment of the group but 

 established, as it were, the technique of investigation 



POIYPHYIETIC INTEEPEETATION (OSBOEN, 1902-1919) 



The reaction against the monophyletic theory was 

 felt by the same author as a result of more extended 

 research. In his paper of 1902 on "The four phyla 

 of Oligocene titanotheres, " after acknowledging the 

 services of the late Professor Marsh and admitting the 

 incorrectness of the monophyletic theory, Osborn 

 says (1902.208, p. 91): 



This second review is an abstract of a portion of the results 

 obtained for the United States Geological Survey monograph 

 "The titanotheres," now in preparation. It covers practically 

 aU the type material in the Yale, National, American, and 

 Harvard Museums, and advantage has been taken of the 

 invaluable field observations by Hatcher of the levels on which 

 the different skulls in the National Museum collection were 

 discovered. The section method also has been very greatly 

 extended and, taken in connection with the teeth and the 

 detailed structure of the skull, has proved to be a sure criterion 

 of specific and phjdetic character. 



Four important considerations had led Osborn to 

 give up the monophyletic theory: First, from his 

 phylogenetic studies on the rhinoceroses of Europe 

 and America (Osborn, 1898.143; 1900.192) he had 

 concluded that, contrary to earlier opinions, this 



