448 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



deeply indented occiput of Brontoiherium gigas elatum 

 (fig. 378, F). _ 



Inclosed auditory meatus. — The progressive union of 

 the postglenoid and post-tympanic processes in the 

 titanotheres parallels that which we observe in a 

 comparison of the rhinoceroses, tapirs, and horses 

 (fig. 379). For example, in the skull of the primitive 

 Menodus Jieloceras (fig. 378, A) the external auditory 

 meatus is widely open below, paralleling the condition 

 which we observe in the Sumatran rhinoceros, R. 

 (Dicerorhinus) sumatrensis (fig. 379, C). In Bronto- 

 therium platyceras the auditory meatus is reduced to 

 a small foramen inclosed by a solid wall of bone, 

 paralleling the condition observed in the Javan 

 rhinoceros, R. sondaicus (fig. 379, D). 



SEXUAL CHARACTERS COMMON TO All PHYLA 



The following sexual characters, though common to 

 all phyla, are most conspicuous in titanotheres from 

 the higher geologic levels : 



Male 



1. Skulls larger. 



2. Canines larger and more 



robust. 



3. Incisors larger, more con- 



stant. 



4. Nasals broader, more ro- 



bust at tips. 



5. Horns more powerful and 



robust. 



6. Connecting crest very 



prominent. 



7. Arches more widely ex- 



panded. 



8. Zygomatic-cephalic indices 



higher, more brachy- 

 cephalic. 



9. Occiput more robust, ex- 



panding, and rugose. 

 10. Closure of cranial sutures 

 accelerated. 



1. Skulls smaller. 



2. Canines smaller, more 



pointed. 



3. Incisors smaller and more 



variable. 



4. Nasals narrower, less ro- 



bust at tips. 



5. Horns shorter, more point- 



ed, less completely ossi- 

 fied at tips. 



6. Connecting crest less 



prominent. 



7. Zygomatic arches less 



widely expanded. 



8. Zygomatic-cephalic indices 



lower, more mesatice- 

 phalic. 



9. Occiput less robust. 



10. Closure of cranial sutures 

 retarded. 



The incisors are apparently more variable and are 

 more likely to drop out of place in the females than 

 in the males, especially in individuals of BrontotJierium 

 (PL XIX). The smaller canines are among the most 

 persistent characteristics of the female (PI. XX). 

 Our observations do not confirm Hatcher's remark 

 that "a feeble internal cingulum" upon the premolars 

 is a female character. The entire grinding series ap- 

 pears to be relatively as large and as vigorously devel- 

 oped in females as in males. Between the females 

 and the males in the ascending series of BrontotJierium 

 there is a very marked and rather puzzling disparity 

 in the size of the skull. 



TEETH: DISTINCTIVE FEATURES AND EVOLUTION 



Incisors, superior and inferior, considered as phyletic 

 characters. — The strong or the feeble development of 

 the incisors and the presence or the absence of certain 

 members of the incisor series are two characters that 

 are distinctive of the phyla, genera, and species. 



In contrast to the Eocene titanotheres, all the known 

 Oligocene titanotheres, except Teleodus, had only two 

 pairs of incisor teeth. As early as upper Eocene time 

 the reduction and the loss of incisors is foreshadowed 

 in ProtitanotJierium and Diplacodon by the hypertrophy 

 of certain pairs of incisors and the atrophy of others. 

 This liypertrophy, atrophy, and disappearance of the 

 incisors is graphically presented below: 



0. 0. 

 Megacerops: „ „ „ 



, . , 0. 12. 13 , 0. 0. 

 Menodus: ~. — ^- to — 5 — 



i^ i' i' 



Brontoiherium: -■ — ■ — 7; to „'.,-, 



ii. 12. 0. 12. 



0. i^. i^ 0. 0. i' 



Broniops brachycephalus: . ' . ' ^ to 



ii. ij. 0. 12. 



Teleodus avus: 



ProHlanotherium : 



In the Broniops phylum the third lower incisor 

 (is) is apparently the first to disappear in the lower 

 jaw, and the third upper incisor (i^) the first to dis- 

 appear in the upper jaw. The jaw of Teleodus avus 

 (PL XIX, D) contains six incisors. The outermost 

 pair (is) have very short roots and insecure tenure, 

 so that further evolution in the same direction would 

 probably result in the crowding out of is. By far 

 the largest teeth with the longest roots are the second 

 incisors (12). Intermediate in size are the first in- 

 cisors (ii). 



In the upper jaw of Brontoiherium (PL XIX, A) the 

 median pair (i') have apparently been lost; the per- 

 sistent teeth represent the second incisors (i^) and the 

 greatly enlarged third incisors (i^). In the lower jaws 

 of Brontoiherium a third pair of incisors (is) have 

 apparently been lost. The first pair (ii) have spread 

 apart, leaving a diastema in the midline; the second 

 pair (i2) remain large and usurp the position of the 

 third (is). 



The form of the crown of the incisor teeth is also 

 highly distinctive. In members of the menodontine 

 group the incisor crowns are smoothly rounded or 

 conic, often laterally compressed, as in Teleodus 

 (PL XIX, D). In members of the brontotheriine 

 group the incisors are cingulate posteriorly. 



Canines, superior and inferior, considered as sexual 

 characters. — The canines (PL XX) are highly dis- 

 tinctive of each phylum and of each genus, as they 

 differ widely in form and in function. The shape is 

 the same in both sexes, but those of the male are 

 always larger and much more powerful than those of 

 the female. For example, in Menodus giganteus 

 (Am. Mus. 505) the male tusks measure 62 by 34 

 millimeters, whereas the female tusks (Am. Mus. 506) 

 measure 40 by 21 millimeters. In Brontoiherium 

 elatum also, as shown by a comparison of five skulls, 

 the female tusks are about two-thirds the size of the 

 male tusks. 



