EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 



505 



v^omer, and there is a very prominent rugosity at the 

 junction of the basisphenoid and basioccipital, as seen 

 m Brontops and Menodus. 



Dentition. — Since the animal is of advanced age, we 

 find just within the canine a large alveolus which cer- 

 tainly contained a large incisor tooth. In early life 

 there were probably two incisors, separated in the 

 median line by a diastema. The canines are long 

 (40 mm.) and pointed, with a postero-internal 

 cingulum. The first premolar is a relatively stout, 

 persistent, bifanged tooth, thrust closely against the 

 canine, partly by crushing; the tetartocone of p^ occu- 

 pies a very small part of the inner face of the crown, as 

 in B. dispar. Illustrating the mechanical ineffective- 

 ness of the internal cones of the molar teeth is the fact 

 that whUe the ectolophs are worn out of proportion in 

 m'~^, the internal cones of m^, m^ are still unaffected 

 by wear. The hypocone of m' is quite prominent and 

 still connected with the cingulum. 



The abbreviation of the premolar-molar series is a 

 retrogressive character which this phylum apparently 

 shares with the Brontotherium phylum. The molar 

 series, measuring 302 millimeters, is actually shorter 

 (index 45) than that in the much less specialized 

 form D. hicornutus, in which the teeth measure 340 

 millimeters with an index of 50. Similarly, in the 

 great skulls of Brontotherium the tooth row is actually 

 shorter in the highly specialized B. platyceras than in 

 the ancestral species B. gigas. 



Supposed jemale skull (Nat. Mus. 4710). — The 

 result of searching comparison and measurements is 

 the reference of this skull as a female of Diplodonus 

 amplus. The morphologic difference is about as great 

 as between the type of D. hicornutus and that of D. 

 tyleri. 



The affinities to Brontops in this female skuU are 

 shown in the foUowing points: (1) Small lateral in- 

 cisors persistent; (2) canines of rounded form; (3) 

 premolars with retarded tetartocones, rounded in- 

 ternal and fainter external cingula; (4) horns of small 

 size, with long, flat external face and rounded top 

 section; (5) orbits with broad postorbital processes; 

 (6) a broad zygomatic shelf, and downward extension 

 of the occiput as in B. dispar; (7) rugosity on the 

 basisphenoid, with the vomerine bridge carried well 

 back. 



The special resemblances of this supposed female to 

 D. amplus are (1) 'the general similarity of the horns 

 in respect to their position, basal sections, and con- 

 necting crests; (2) the smaller zygomatic section; (3) 

 the sharp preorbital malar bridge. 



The chief points of difference between this specimen 

 (Nat. Mus. 4710) and the Yale type of D. amplus, 

 which is a male, are (1) absence of hornlets, perhaps 

 a sexual distinction; (2) greater width, flatness, and 

 slenderness of the nasals, which may be attributed 

 in part to the nondevelopment and lack of forward 



advancement of the horns; (3) smaller buccal processes 

 of the zygomata, which, however, in section suggest 

 those of D. amplus on the inner and inferior faces 

 especially. (See fig. 427.) 



The reference of this skull to D. amplus is therefore 

 still provisional. 



Subfamily MENODONTINAE 



Titanotheres chiefly of lower Oligocene age, reaching 

 their climax in the upper levels of the Titanotherium 

 zone (Chadron formation). Distinguished by narrow 

 heads (mesaticephalic) diverging into phyla with long 

 heads (dolichocephalic) and broad heads (brachyce- 

 phalic). Horns short, slightly shifting forward, broadly 

 trihedral in basal section and widely divergent at 

 the summits. Nasals typically elongate, broad and 

 square distally, secondarily abbreviate. Incisor teeth 

 reduced or vestigial. Grinding teeth with prominent 

 cingula. Premolars with accelerated tetartocones. 



This is the second branch of the short-horned ti- 

 tanotheres, which in many respects is closely related 

 to the first branch, the Brontopinae; in others it 

 appears to present original characteristics of its own. 

 It is typified by the genus Menodus, also known as 

 Titanotherium, and includes the related genus Allops. 

 Between the two are transitional forms of subgeneric 

 rank described as Menops and Anisacodon by Marsh. 

 The resemblances of the original species of these two 

 main phyla of Menodus and Allops, namely, Menodus 

 heloceras and Allops walcotti, both occurring in the 

 very base of the Titanotherium zone, are relatively 

 close to Brontops hrachycephalus. Yet it would appear 

 that the Menodontinae diverged from the Brontopinae 

 before the end of Eocene time. While the Menodus 

 phylum maintains its mesaticephalic and dolicho- 

 cephalic form throughout, members of the Allops 

 phylum become brachycephalic and converge toward 

 members of the Brontops phylum. Thus Allops crassi- 

 cornis resembles Brontops rohustus in its proportions. 



The ancestors of Menodus and of Allops, although 

 not readily distinguishable in the base of the Titano- 

 therium zone (Chadron A) , indicate that the divergence 

 took place in late Eocene time, when the following 

 main distinctions were established: 

 Menodus phylum 



Primitively mesaticephalic, progressively dolichocephalic. 



Incisors §;§, extremely vestigial, buried beneath the gums. 



Canines very prominent, rounded, or compressed transversely. 



Nasals typically broad and elongate. 



Skeleton tall. 



Feet stilted, of mediportal type. 



Allops phylum 

 Primitively mesaticephalic, progressively brachycephalic. 



Incisors reduced, 



with rounded summits. 



Canines prominent, compressed anteroposteriorly, expanded 

 transversely. 



Nasals progressively abbreviating. 

 Skeleton little known. 

 Feet broader. 



