550 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Megacerops riggsi (Osborn) 

 Plates CLVIII-CLX; text figures 208, 455_ 

 I For original description and type references see p. 242] 



Type locality and geologic horizon. — Horsetail Creek, 

 northeastern Colorado ; Titanotherium zone. 



Specific characters. — Of small size, smaller than any 

 Imown individual of Megacerops or Brontotherium . A 



Figure 455. — Lower jaws of Megacerops assiniboiensis and 

 M. riggsi 



A, Megacerops riggsi, Am. Mus. 6364 (type). A small titanothere having a massive 

 short jaw, a short chin, and a swollen ramus. (See PI. CLVIII, A.) The cheek 

 teeth lack cingula. B, if. assiniboiensis, Ottawa Mus. (type). A small short 

 jaw provisionally referred to Megacerops. The grinding teeth lack e.xtemal 

 cingula. One-flfth natural size. 



very massive jaw with a small coronoid process and 

 a very short symphysis. Premolar series greatly 

 abbreviated (85 mm.). Premolars and molars with 

 reduced external cingula. 



The type of this species, named in honor of Mr. 

 E. S. Riggs, of the Field Museum of Natural History, 

 is a jaw in the Cope collection (Am. Mus. 6364) which 

 was wrongly referred by Cope to his species "Symhoro- 

 don" acer. It represents a highly specialized and 

 small form of Megacerops. 



SUBFAMILY BRONTOTHERIINAE 



Titanotheres attaining the largest size, chiefly of 

 the upper Titanotherium zone, although known from 

 the lower beds upward. The horns progressively 

 longest, most broadly oval, and flattened, shifting 

 forward with the extreme reduction of the free portion 

 of the nasals; very prominent connecting crest, pro- 

 gressively increasing size. Two pairs of persistent 

 superior incisor teeth in males; canines large, obtuse. 

 Brachycephaly expressed in the great horizontal 



expansion of the buccal processes, in the proportions 

 and arching of the grinding teeth, and in the inclosure 

 of the auditory meatus in the males. Occiput much 

 produced behind the zygomatic arches. Vertex of 

 skull in superior view elongate. 



The ancestry of this great Oligocene phylum may 

 possibly be found in the upper Eocene, perhaps in 

 species of Diplacodon or of Eotitanotherium, described 

 on pages 439, 441. 



In these huge animals the titanothere family reached 

 a climax. The generic name "thunder beast," based 

 on the genotype species Brontotherium gigas, is highly 

 appropriate because it applies to the most robust and 

 most massively horned not only of the titanotheres 

 but of all the known Perissodactyla. 

 Marsh mistakenly associ- 

 ated with the type jaw of B. 

 gigas the skull of Menodus, "B. 

 ingens," to define the generic 

 characters of Brontotherium, 

 and he assigned the actual 

 skuU of B. gigas to a different 

 genus, naming it Titanops 

 elatus; but we have found that 

 the lower jaw of B. gigas is in 

 a stage slightly antecedent to 

 that of Titanops elatus Marsh. 

 We have also discovered that 

 this great animal Brontothe- 

 rium gigas {elatum) is a central 

 form, whose ancestors {B. leidyi) 

 extend down to the base of the 

 Titanotherium zone and whose 

 successors {B. platyceras) extend 

 up to the very summit of Chadron 

 C (the upper Titanotherium zone). 

 Thus in the present memoir 

 Brontotherium is shown to embrace 

 a most remarkable and nearly 

 monophyletic series or succession 

 of species, eight or possibly nine 

 of which are now known from the 

 lower to the higher levels, present- 

 ing certain common generic char- 

 acters throughout. Modified by a 

 progressive increase in size and by a series of remarkable 

 stages in the evolution of the horns, in the recession of 

 the nasals, and in the expansion of the buccal processes 

 of the zygomata, they culminate in a unique type of 

 perissodactyl skull, that of B. platyceras, which appears 

 to be the last of its great race. This species is cer- 

 tainly in the last stage of evolution along its line. 



Affinities. — The nearest allies of Brontotherium are 

 members of the genus Megacerops, but the true 

 brontotheres (the males at least) are readily dis- 

 tinguished by the presence of two pairs of upper and 

 lower incisor teeth, by their robust canines, the 



Figure 456. — Sec- 

 tions and contours 

 of skull of Mega- 

 cerops? syceras 



Ottawa Mus. (type), provi- 

 sionally referred to Mega- 

 cerops partly because the 

 basal section of the horns 

 is roundly quadrate and 

 has a flat external face. 

 Nasals of moderate length, 

 connecting crest low. 

 One-seventh natural size. 



