EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



297 



SECTION 4. THE MIDDLE AND UPPER EOCENE 

 TITANOTHERES 



PHYIA DISTINGUISHED 

 Some of the middle Eocene titanotheres represented 

 in the lower Bridger beds may have been evolved from 

 forms related to the Eotitanopinae of the Wind River 

 formation. A geologic interval covering a long period 

 (including Huerfano B= Bridger A) separates the 

 titanotheres of the Wind River B from those of 

 Bridger B, and during this period there was a marked 

 transformation in the proportions of the head, for in 

 the titanotheres of Wind River B the face is longer 



than the cranium (dolichopic), whereas in those of 

 Bridger B and succeeding subdivisions the cranium is 

 longer than the face (brachyopic). 



In the titanotheres now to be described this change 

 in faciocranial proportions probably occurred during 

 the deposition of Bridger A and Huerfano B. In the 

 10 or 12 genera of titanotheres of the middle and upper 

 Eocene the cranium is longer than the face. These 

 animals fall broadly into two large groups, which are 

 more or less theoretically subdivided (1917) into two 

 groups and into six chief phyla or lines of descent as 

 shown in the accompanying table. 



Characteristic features of groups of titanotheres 



[Compare flg. 219, p. 265] 



Palaeosyopine group: Palaeosyops, Limnohyops, Telmatherium, Sthenodectes 



Manteoceras-Dolichorhinus group: Manteoceras, MesatirUnus, Dolichorhinus, 

 Metarhinus, Ehadinorlimus, Diplacodon 



Skull brachycephalic to mesaticephalic. 

 Horn rudiments retarded in evolution. 

 Occiput rounded or high. 

 Zygomata deepened vertically. 

 Canines more pointed, erect. 

 Third superior incisor oaniniform. 



1. Subfamily Palaeosyopinae "(Limnohyops, Palaeosyops), ex- 



tremely brachycephalic. 



2. Subfamily Telmatheriinae (Telmatherium), mesaticephalic 



to brachycephalic. 



Skull mesaticephalic to dolichocephalic. 

 Horn rudiments precocious in evolution. 

 Occiput primitively low and broad. 

 Zygomata shallow vertically. 

 Canines more obtuse, recurved. 

 Third superior incisor incisiform. 



3. SubfamilyManteoceratinae = Brontopinae (Manteoceras, Pro- 



titanotherium, Brontops), progressively brachycephalic. 



4. Subfamily Dolichorhininae (Mesatirhinus, Metarhinus, Doli- 



chorhinus) , mesaticephalic to dolichocephalic, facial region 

 downturned. 

 ?5. Subfamily Megaceropinae=?Rhadinorhininae (?Rhadino- 

 rhinus, Megacerops), mesaticephalic, facial region up- 

 turned. 

 6. Subfamily Brontotheriinae=?Diplacodontinae (Diplaco- 

 don, Brontotherium) , horns precociously developed. 



The extreme forms of the two groups — namely, 

 Palaeosyops and Dolichorhinus — also contrast widely 

 in the detailed characters of the skull, as shown in 

 longitudinal and cross sections in Figure 254. 



The subfamilies 1-6, according to the Osborn system 

 (see Chap. I), correspond with the phyla, or vertical 

 lines of descent, which have been established among 

 the Eocene titanotheres, also between the Eocene 

 and Oligocene titanotheres. Thus it is now Icnown 

 that Manteoceras and Protitanotherium are related to 

 Brontops of the Oligocene. Diplacodon of the upper 

 Eocene is of uncertain affinities with the lower Oligo- 

 cene genera. It is possible but by no means demon- 

 strated that Rhadinorhinus is related to the Oligocene 

 brontotheres and Megacerops, as suggested by Gregory. 



SPECIES OF PALAEOSyOPINAE AND DOLICHORHININAE 

 FROM THE UPPER HUERFANO (TROGOSUS ZONE) 



The discovery of two very distinct phyla of true 

 titanotheres in the lower Eocene confirms the theo- 

 retic separation of the titanotheres into subfamilies 

 as occurring in lower Eocene time. In the Huerfano 

 formation we have evidence of two subfamilies, as 

 follows : 



Palaeosyopinae (perhaps derived from 

 the Eotitanopinae) 



Hornless. 



Slender nasals. 



Subbrachycephalic. 

 More robust proportions. 



Dolichorhininae (Manteoceras-Me tarhi 

 nuS'MesaUrkinus-VolichOThinus grouTp) 



Osseous horn rudiments at 



nasofrontal junction. 

 Nasals very stout, laterally 



decurved. 

 Mesaticephalic. 

 Smaller proportions. 



The first subfamily is represented by numerous 

 specimens of Palaeosyops fontinalis Cope; the second 

 group is represented by the single type specimen of 

 the new genus Eometarhinus {E. huerfanensis) . 



SYSTEMATIC DESCRIPTIONS OF THE MIDDLE AND UPPER 

 EOCENE TITANOTHERES 



THE PALAEOSYOPINE GROUP (PALAEOSYOPS, LIMNO- 

 HYOPS, TELMATHERIUM, STHENODECTES) 

 DISTINCTIVE FEATURES AND GEOLOGIC HORIZONS 



Osborn finally included the genus Telmatherium 

 within the palaeosyopine group, although there are 

 some grounds for placing it closer to the Manteo- 

 ceratinae. The telmatheres appear to have had a 

 long and independent evolution of their own (see fig. 

 697) and thus constitute the distinct subfamily Tel- 

 matheriinae. 



