556 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



supposed female skull referred to B. curtum (Am. 

 Mus. 1005). The permanent incisors were protruded 

 very early and have weak implacements in the crowded 

 premaxillae. Hence they are sometimes shed in old 

 animals. 



Ancestral brontotJieres in the lower Titanoiherium 

 zone. — The Eocene ancestry of this subfamily is still 

 in some doubt; its earliest known members may prove 

 to be the species Diplacodon elatus Marsh and Eoti- 

 tanotherium osborni Peterson of Uinta C (upper 

 Eocene). 



Relatively small and short-horned ancestors of the 

 brontotheres have fortunately been determined by 

 the writer as occurring according to Hatcher's geologic 

 record in the very base of the lower Titanotherium zone, 

 definitely proving that the Brontotherium phylum early 

 separated from the Megacerops phylum and at a much 

 earlier datef r om the Brontops- Menodus phylum. These 

 primitive brontotheres are comparatively rare in the 

 lower beds. They include two species as follows: 



Brontotherium leidyi Osborn is a comparatively 

 small animal with short horns, which is determined as 

 a brontothere, however, by two very distinctive 

 characters — first, the transversely oval section of the 

 summit of the horn; second, the very progressive 

 condition of the premolar grinding teeth, in which the 

 tetartocones are well developed. This animal is of 

 extraordinary interest by comparison with B. platy- 

 ceras in showing the extreme transformation in the 

 horn region which took place in the course of the 

 deposition of 200 feet of sediment. It is represented 

 by a number of admirably preserved specimens. 



Brontotherium hypoceras (Cope). — This is an animal 

 first named Symhorodon hypoceras by Cope from the 

 tip of a horn and some other fragments of the skull. 

 He was struck by the transversely oval section of 

 this horn tip. This very imperfect type (PL CLXVI, 

 fig. 174) was fortunately compared with the skull in 

 the National Museum and determined by Osborn as 

 a most interesting transitional stage between B. 

 leidyi and the species of the higher levels. Like B. 

 leidyi it has rounded grinding teeth with low cusps, 

 well-developed tetartocones, and obtuse canines. 

 There is no question as to its phyletic position. 



Brontotheres of the middle Titanotherium zone.— 

 Brontotherium- hatcheri Osborn, from the middle Titano- 

 therium zone (Chadron B) includes animals of inter- 

 mediate size, named in honor of J. B. Hatcher, the 

 chief explorer of the Titanotherium-he&Ting beds, 

 which connect B. leidyi and B. hypoceras of the lower 

 zone with B. gigas and its successors of the upper zone. 



Brontotherium tichoceras (Scott and Osborn) possibly 

 belongs geologically to the upper zone, but both its 

 geologic and its phyletic position are somewhat un- 

 certain, although there is no doubt whatever as to its 

 general affinity to Brontotherium. 



Brontotheres of the upper Titanotherium zone. — 

 Brontotherium gigas Marsh, a giant form, was probably 

 characteristic of the lower part of the upper zone, 

 namely, Chadron C 1. The evolution of the special 



characters of the phylum, the horns, and the zygo- 

 matic arches now appears to be accelerated. As shown 

 in the accompanying table six succeeding stages, 

 species, subspecies, or mutations can be distinguished, 

 to which specific names may here be given. While 

 the evolution of the dominant characters of the horns, 

 connecting crests, and buccal processes of the zygomata 

 is progressive, and while the premolars follow the 

 universal law of increasing complication by the devel- 

 opment of the tetartocones, the series of grinding teeth 

 as a whole is partly arrested and in some respects re- 

 trogressive in development. On account of the 

 shortening of the face the premolar series is shorter 

 than in Menodus, but the premolars have the most 

 advanced tetartocones known in any genus except 

 Megacerops, and both molars and premolars are ex- 

 tremely broad, so that in basal view the dentition 

 appears enormous. It is true that in B. platyceras 

 (Field Mus. 12161) the premolar-molar series is 

 slightly shorter than in B. gigas elatum (Am. Mus. 

 492). The incisors, on the contrary, in the males at 

 least, are remarkably persistent, and one of the 

 readiest means of distinguishing Brontotherium is by 

 its reduced cingula on the superior incisors, which are 

 quite different from the smooth, rounded incisors of 

 Megacerops. 



Stratigraphic horizons of Brontotheres 



