EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



473 



of transition stages, almost unique in mammalian 

 paleontology, which connect the diminutive B. Ira- 

 chycepTialus from level A, the base of the Titanotherium 

 zone, with the large animals of level B known as B. 

 dispar, a form distinguished not only by its larger 

 size throughout but by rounded horns, shorter nasal 

 bones, more robust buccal processes of the zygomatic 

 arches, and somewhat more complicated premolar 

 grinding teeth. It appears that B. dispar is highly 

 characteristic and distinctive of the B levels, or middle 

 beds, but that a form of B. dispar passes up into the 

 base of the upper Titanotherium zone. 



Diploclomis phylum. — The existence of a 

 second phylum is indicated by specimens that 

 are probably from the upper parts of the lower 

 and from the middle Titanotherium zone. 

 The stratigraphic records in regard to them 

 are incomplete. This is the species Diploclonus 

 bicornutus (Osborn), which is remotely related 

 to B. dispar and is distinguished, as the name 

 indicates, by a duplication of the horns on the 

 inner sides but especially by its very narrow, 

 elongate nasals and the straight outer contour 

 of the horns, as seen from the front. Nasals 

 of somewhat similar type are observed in the 

 animal from Assiniboia, Canada, which was 

 named Menodus selwynianus by Cope. 



The most interesting biologic conclusion to 

 be drawn from this assemblage of varied types 

 is that in the lower and middle Titanotherium 

 beds there were many kinds of small and'J'mid- 

 dle-sized titanotheres more or less closely 

 related to Brontops. The group is certainly 

 diphyletic, possibly polyphyletic, and is there- 

 fore all the more difficult of analysis. 



The type Brontops in the upper Titanotherium 

 zone. — On the lower levels of Chadron C, the 

 upper Titanotherium zone, are found the great 

 animals to which Marsh gave the name Sronfops 

 robustus, as well as the surviving members of 

 the Brontops dispar series. At first this animal appears 

 to be widely separated from Brontops brachycephalus 

 and B. dispar, but like B. dispar, which Marsh also 

 placed in the genus Brontops, B. robustus is represented 

 by a large number of skulls in different stages of growth, 

 and there can be no question that certain character- 

 istics of age, growth, and sex of B. robustus are close to 

 some of the variations in B. dispar, but no direct 

 ancestor to the type stages is known. The more 

 important ascending mutations will be expounded in 

 the more precise study of the phylum which follows. 



Special distinctions from Menodus. — As already 

 stated, these animals belong to the same group as 



the Menodontinae and are more closely related to 

 Menodus than they are to either Megacerops or 

 Brontotherium. It is therefore desirable to summarize 

 their distinctive characters: (1) The males show 

 brachycephaly, having a zygomatic index that ranges 

 in general from 79 to 91 (mesaticephaly) ; the females 

 have a zygomatic index that ranges from 64 to 69; 

 (2) all retain one or two pairs of upper and lower 

 incisors, which are full sized and round crowned; (3) 

 the canines in males are progressively shortened in 

 the ascending series, whereas in Menodus they are 



Figure 404.- 



-Lower jaws of Melarhinus fluviatilis and Brontotherium 

 hatcheri 

 , M. fluviatilis, Am. Mus. 2059; White River, Uinta Basin, Utah; Uinta B 1; two-ninths 

 natural size. B, B. hatcheri, Am. Mus. 1070; Hat Creek, Nebr.; Chadron formation; one-sixth 

 natural size. 



persistently long and pointed; (4) the tetartocones 

 of the premolars are retarded in evolution, whereas in 

 Menodus they evolve rapidly; (5) as the horns shift 

 forward the nasals are progressively reduced in 

 length — they are not quadrate but broadly expanded 

 distally at their extremities; (6) the horns are typi- 

 cally suboval or cylindrical in basal section and have 

 rounded rather than trihedral tops as in Menodus; 

 (7) the face progressively shortens, whereas in Meno- 

 dus it remains persistently long; (8) in correlation 

 with the shortening of the face the tooth row becomes 

 bent upward anteriorly; (9) the zygomata expand 

 progressively. 



