538 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Measurements of Menodus monianus 



Millimeters 



Canine alveoli, anteroposterior diameter 31 



Canine, transverse diameter 19 



Nasals, transverse 96 



Nasals, longitudinal (so far as preserved) 87 



Second superior molar, transverse 88 



Second superior molar, anteroposterior 76 



Molar series, superior 218 



Affinities to Menodus. — As shown in the above de- 

 tailed description this animal agrees with M. "ingens," 

 and especially with M. varians, in the vestigial char- 



FiGURE 447. — Teeth and nasals of Menodus montanus 



valid species, although more perfect material may 

 relate it still more closely or even specifically to M. 



giganteus. 



SECTION 6. THE BRONTOTHERIINE GROUP 



We now consider the brontotheriine group as dis- 

 tinguished from the menodontine group. It contains 

 two phyla, closely affiliated in their Eocene origin 

 but widely diverging in their Oligocene evolution — ■ 

 namely, the Megaceropinae and Brontotheriinae. 



GROUP CHARACTERS 



Lower Oligocene titanotheres of me- 

 dium to larger size. Horns progres- 

 sively elongating in the males until 

 they attain great size at the expense of 

 the nasals, which are atrophied. Face 

 abbreviated, or brachyopic. Skulls with 

 broadly spreading zygomatic arches and 

 brachycephalic characters in the audi- 

 tory region, in the occiput, in the broad 

 proportions of the upper grinding teeth, 

 and in the arching of the opposite series 

 of grinding teeth. All grinding teeth 

 devoid of external cingula in males. 

 Premolar grinding teeth precociously 

 becoming more molariform than in 

 other groups. Orbits small. 



This group includes all the known 

 long-horned titanotheres, both the 

 smaller megaceropines and the larger 

 brontotheres. Of the two the mega- 

 ceropines are less formidable animals, 

 without incisor teeth, and with a less 

 powerful action of the horns. The 

 brontotheres are hj far the most formid- 

 able of all the titanotheres known, ani- 

 mals of gigantic size, with powerful 

 horn action, hence appropriately termed 

 by Marsh "thunder beasts." The ana- 

 tomical resemblances between these 

 animals are by no means confined to 

 the superficial similarities but extend to 

 all parts of the skull and teeth, as we 

 have seen in the introduction of this 



Yale Mus. 10022 (type of Diconodon monianus Marsh). A, Third right upper molar. The elongate pro- 

 portions and hypocone surrounded by a cingulum are seen also in certain other specimens of Menodus 



(of. Am. Mus. 1067). B, Fourth upper premolar and first and second true molars of the left side. The chapter, and pomt tO a COmmOU anCCStry 



elongate proportions and sharp internal and external cingula are seen as in J/enoii«5. C, Alveoli of the . -p„„p„„ time 



upper incisors and canines. The upper incisors were vestigial, as in Menodus; the canines were large. -^^ -^"*- . ' t .-i u 



implying male se.x. Di, Top view of distal portion of nasals. The paired projections are seen as in The phylctlC characters 01 the brOUtO- 



Menodus. Di, Front view of nasals. One-half natural size. 



acter of the incisors, the bifanged premolar, the pres- 

 ence of a distinct hypocone on the last superior molar, 

 the marked external cingula on both premolars and 

 molars, and the strong internal cingula on the pre- 

 molars. It apparently differs from Menodus in the 

 narrow nasals and the more transversely extended 

 proportions of the molar teeth. 



It seems probable that this fragmentary specimen 

 should be considered provisionally as the type of a 



theriine group common to Megacerops 

 and Brontotherium are indicated below: 



1 . There is a general increase in size of the skull and 

 skeleton. 



2. The postorbital region of the skull shows rapid 

 elongation, the preorbital region rapid abbreviation, 

 occiput widely prolonged behind the zygomata, or- 

 bits small, and never a prominent postorbital process. 

 In uncrushed skulls the orbits do not appear as if 

 closed posteriorly. 



