DISCOVERY OF THE TtTANOTHERES AND ORIGINAL DESCRIPTIONS 



149 



group was in a high degree polyphletic, embracing 

 many parallel phyla and having a wide adaptive 

 radiation; second, the principle of dolichocephaly and 

 brachycephaly (Osborn, 1902.207), as interpreted by 

 him in the rhinoceroses and other groups, raised the 

 presumption that similar differences would be found 

 to distinguish genera and phyla among the titano- 

 theres; third, he had learned to realize that the extent 

 to which parallel and convergent evolution had oper- 

 ated in many allied phyla had been but little appre- 

 ciated by earlier writers, who had largely failed also 

 to distinguish between persistent, progressive, and 

 retrogressive characters; fourth, an examination of 

 the titanothere skulls collected by Hatcher and now 

 in the National Museum, which Hatcher had recorded 

 exactly as to level, enabled him, with the aid of prin- 

 ciples just stated, to distinguish several distinct phyla 

 and to foUow them from the lower part through the 

 middle and into the very top of the " Titanotherium 

 beds." The characters of these phyla were summar- 

 ized by Osborn as follows (1902.208, pp. 92-94): 



THE FOUR GENERA 



Titanotherium Leidy applies to .long-limbed animals with long 

 skulls, persistently long and broad nasals, short triangular 

 horns placed slightly in front of the e3'es, vestigial incisors ^o' 

 large canine teeth. Known from the base to the summit of the 

 [lower] Oligocene. 



Megacerops Leidy applies to titanotheres with broad skulls, 

 nasals progressively shortening, short horns rounded or oval 

 in section, shifting anteriorly, one or two pairs of incisor teeth, 

 .|r}, medium-sized canine teeth. Known from the base to the 

 summit of the [lower] Oligocene. 



Probably related to this are the subgenera of the t3'pes named 

 Allops and Diploclonus by Marsh, differing from the above in 

 horn characters. Known chiefly from the upper beds. 



Symborodon Cope includes titanotheres with skulls of varying 

 proportion, nasals slender and progressively shortening, horns 

 elongate and peculiar in being placed above the eyes instead of 

 shifting forward, incisors vestigial l^, canines small, approx- 

 imated. Known only from the middle and upper beds. 



Brontotherium iVlarsh embraces the largest titanotheres, with 

 very broad zygomatic arches, nasals shortening while horns 

 elongate and shift forward; incisors persistent, f in the males, 

 canines stout and obtuse. 



Representatives of Titanotherium and Megacerops can be now 

 continuously traced from the base to the summit of the [lower] 

 Oligocene. Primitive species of Brontotherium also appear at 

 the base, although the phyletio sequence through the middle 

 to the upper beds is not so clear. Symborodon suddenly appears 

 in the middle beds. 



Viewed in the light of the foregoing principles, the 

 variations in the horns, nasals, incisors, cingula, etc., 

 took on new meanings — biologic, phylogenetic, and 

 systematic; so that, after more than half a centm-y of 

 research (1846-1902) the systematic problem presented 

 by the Oligocene titanotheres appeared in its main 

 features to be solved. Subsequent research, however, 

 has led to certain regrettable but apparently necessary 

 changes in nomenclature: (a) The name " Megacerops" 

 Leidy, as defined above, has been set aside for Brontops 

 IVIarsh, for the reasons given below; (b) the name 



Titanotherium Leidy has been abandoned for the prior 

 name Menodus Pomel; (c) the name Symhorodon Cope 

 has been replaced by the prior name Megacerops 

 Leidy. 



RECENT DISCOVERIES BY LULI, lAMBE, AND OTHERS 



There remain to be recorded the following contri- 

 butions: (1) The description of Megacerops tyleri by 

 Lull (1905.1), based upon a fine skull and lower jaws 

 with associated limbs, discovered by the Amherst 

 CoUege paleontologic expedition of 1903; (2) the 

 description of Brontotherium hatcheri and Symhorodon 

 copei by Osborn in 1908 (1908.318), based on skulls 

 in the National ]Museum; (3) the description of Mega- 

 cerops primitivus and M. assinihoiensis, based on frag- 

 ments obtained from Saskatchewan, Canada, by 

 Lambe in 1908 (1908.1); (4) W. K. Gregory observed 

 (a) that there is an alliance between Brontops, Allops, 

 and Menodus as these terms are now used by Osborn, 

 indicated by certain intermediate forms between the 

 extremely brachycephalic Brontops roiustus and the 

 dolichocephalic Menodus giganteus, (b) that there is 

 also an alliance between Brontotherium and Megacerops 

 (Symhorodon) in spite of the differences in the incisors. 

 Hence the former group — Brontops, Allops, Menodus — ■ 

 has been called the menodontine group, and the latter 

 group — Brontotherium, Megacerops — has been called 

 the brontotheriine group. 



Possibly the most valuable general result of the 

 study of the titanotheres has been the fact that it 

 has made possible the close examination of an extensive 

 evolutionary history, stretching from the lower Eocene 

 to the summit of the lower Oligocene. IVTany observa- 

 tions have been made on the precise modes of evolu- 

 tion, especially with regard to the way in which char- 

 acters first appear and subsequently develop. The 

 results of this evolutionary study are set forth in 

 Chapters V, VI, VH, and XI of the present work. 



THE EOCENE TITANOTHERES 



PIONEER DISCOVERIES 



WORK IN THE BRIDGER, WASHAKIE, AND UINTA BASINS BY lEIDY, 

 MARSH, COPE, SCOTT, OSBORN, AND OTHERS (1870-1889) 



Prof. F. V. Hayden, in the course of his historic 

 explorations in the fossiliferous beds of the Kocky 

 IMountains and Great Plains, obtained at Church 

 Buttes, near Fort Bridger, Wyo., a number of isolated 

 teeth, which were described by Leidy (1870.2) under 

 the name Palaeosyops paludosus. This was the first 

 Eocene titanothere made laiown to science, 24 years 

 after the discovery of Prout's "gigantic Palaeotherium" 

 (Titanotherium) in South Dakota. Although Leidy 

 noted that the lower molar of Palaeosyops "resembles 

 in its constitution those of Palaeotherium, (Jlialicothe- 

 rium, and Titanotherium," he did not classify the new 

 genus with the titanotheres, for the reason that at 

 that time he thought Titanotherium and Chalicotherium 

 were allied to the Artiodactyla. (See p. 247.) Soon 



