EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



337 



of L. prisons, the posterior crest being well defined 

 and the entoconid ridge being somewhat more decided. 



A larger and more progressive jaw, also of doubtful 

 specific reference, is Am. Mus. 12201 (fig. 266, C), 

 from Bridger level C 4. In this the measurement 

 from the condyle to the incisive border is 365 milli- 

 meters; p2-m3, 171. The paraconids are distinctly de- 

 fined on p2 and pa. The teeth are otherwise very 

 simple, although the posterior crescent (hypolophid) 

 is deepened and slightly broadened. In p4 a distinct 

 entoconid is seen. The true molars measure 111 milli- 

 meters; the ramus behind ma, 94 millimeters. 



A jaw (Am. Mus. 12198) from Henrys Fork (level 

 D 2) is in a slightly more advanced stage of evolution 

 on the evidence presented in its inferior premolar 

 teeth. 



Palaeosyops copei is the last known species of this 

 palaeosyopine race. Its specialized condition and its 

 reduction in size may indicate that it was declining 

 and on the point of extinction. On the other hand, it 

 may have migrated from this region. 



SUBFAMILY TEIMATHEEHNAE OSBORN 



Middle and upper Eocene titanotheres of larger size. 

 Skull with elongated cranial and abbreviated facial 

 region, mesaticephalic {Telmatherium) or sub-brachy- 

 cephalic {Sthenodectes) . Basicranium abbreviated. 

 Nasofrontal horns retarded in development. Denti- 

 tion of macrodont type; large cingula; incisors heavy, 

 sublanceolate; premolars progressively molariform; 

 molars large, progressively subhypsodont. 



Geologic liorizon and geographic distriiution. — The 

 genera are Telmatherium, mesaticephalic, of levels 

 Bridger C 3 to Uinta C 1, and Sthenodectes, mesati- 

 cephalic to brachycephalic, of level Uinta B 2. 



These are the least known of the middle and upper 

 Eocene titanotheres. Remains are infrequently found, 

 and the parts preserved are incomplete. The Bridger 

 region was probably beyond the center of their favorite 

 habitat. The telmatheres appear to have been the 

 most elegant and graceful as well as the most progres- 

 sive and active of the middle Eocene titanotheres; in 

 this respect they correspond with the Menodontinae 

 of the lower Oligocene. As shown in Figure 257 they 

 appear suddenly in the upper Bridger (levels C and 

 D) and extend up into Uinta C 1, which probably rep- 

 resents the end of upper Eocene time. No known 

 telmathere is directly related to the Oligocene Menodus, 

 yet certain telmatherines may have given rise to the 

 Oligocene offshoots known as the Menodontinae. The 

 resemblances and differences between Telmatherium 

 and Menodus are set forth below. 



Resemllances to contemporary titanotheres. — The tel- 

 matheres may be regarded as intermediate in anatomy 

 between the Limnohyops-Palaeosyops type and the 

 Manteoceras type. The earliest species known, T. 

 cultridens, exhibits certain resemblances to Manteo- 

 ceras, others to Limnohyops. The skull as a whole is 



long; the basicranial region is short; the zygomata 

 are moderately arched. A distinctive feature of the 

 face is that the frontonasal horn swelling is feeble or 

 wanting, perhaps because the horns were compensated 

 for by the large, powerful canine tusks. 



These relatively dolichocephalic, subhypsodont, 

 supposedly subdolichopodal, and subcursorial charac- 

 ters doubtless indicate that the telmatheres frequented 

 firmer ground and made longer excursions for harder 

 kinds of food than did members of the Palaeosyops 

 phylum. They were also probably more intelligent 

 and alert. Since the rise of Telmatherium, Manteo- 

 ceras, and other phyla possessing relatively long- 

 crowned molars occurred simultaneously with the 

 decline of the earlier group of Palaeosyops and 

 Limnohyops, the general replacement of the latter by 

 the former may be attributed to the mechanical 

 superiority of their grinding teeth as well as to physio- 

 graphic changes from forests and lakes to more open 

 flood-plain country. 



Compensation for small horns hy large tusTcs. — The 

 pronounced development of the canine tusks in the 

 telmatheres indicates that they were probably com- 

 bative and vigorous fighters; another respect in which 

 they resemble the Oligocene menodonts. This 

 development of the tusks may have compensated for 

 the absence or retarded development of the osseous 

 nasofrontal horns. In the earliest known species, 

 T. cultridens, this horn rudiment was evidently 

 represented by a slight nasofrontal convexity. The 

 condition of the horns in T. validum is unlcnown, but 

 even in a male of T. ultvmum the horn rudiment is only 

 slightly developed (PL XVI), while the canine tusks 

 and lateral incisors are both enlarged and tusldike 

 (PI. LV). 



Two suhphyla. — There is evidence of an early 

 division of the Telmatherium phylum into Telmathe- 

 rium (mesaticephalic to dolichocephalic, index 60, 

 incisors moderately large) and Sthenodectes (mesati- 

 cephalic, index 63-65, incisors greatly enlarged). 



Geologic succession. — The telmatheres appear in 

 Bridger C 3 and extend into Uinta C 1, through a 

 vertical thickness of over 1,500 feet. Their span of 

 life thus covers a very long period of geologic time. 



History of discovery. — As stated above, the Tel- 

 matherium phylum is comparatively little known, 

 probably because the known areas of deposition did not 

 present a habitat favorable to these animals; they are 

 very rare in the upper Bridger deposits; they are as 

 yet unknown in the deposits of Washakie Basin; and 

 only a few specimens have been found in the deposits 

 of the Uinta Basin. 



The first remains of a member of the group to be dis- 

 covered were the maxilla and superior teeth that 

 Marsh described as Telmatherium validum in 1872. 

 His type description was brief and was published 

 without illustration; the geologic entry is simply the 



