EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



383 



very progressive premolars; and broad molars. The 

 skull was probably brachycephalic — that is, it had 

 broad zygomata. 



subfamhy doiichoehininae riggs 



PHYLA AND RELATIONS 



These Eocene titanotheres branch from the same an- 

 cestral stock as that of Manteoceras. Some are doli- 

 chocephalic, and some are mesaticephalic. The horn 

 swellings are developed chiefly on the nasals, partly 

 on the frontals. Facial region laterally compressed; 

 elongate symphyseal union of premaxillaries; orbits 

 prominent; infraorbital processes more or less promi- 

 nent. First occurring (Mesatirhinus) on upper levels 

 of the Bridger Basin (C and D) and lower level of the 

 Washakie Basin (A), reaching a dolichocephalic cli- 

 max {DolichorMnus) in Washakie B and Uinta B 1 

 or terminating in dwarfed mesaticephalic forms 

 (Metarhinus) and other collateral branches. 



Following Eometarhinus of Bridger A, Huer- 

 fano B, the subfamily Dolichorhininae contains 

 the following phyla or separate series: 



1. Mesatirhinus; probably ancestral to Dolichorhinus. 



2. Dolichorhinus; extremely dolichocephalic, becoming 



extinct. 



3. Metarhinus; aberrant, small to dwarfed; broad nasals. 



4. Sphenocoelus; little known, perhaps a branch of 



Mesatirhinus. 



A closely related subfamily, Rhadinorhininae, 

 contains 



5. Bhadinorhinus; nasals short, pointed; possibly ances- 



tral to Megaceropinae. 



In their common ancestral characters these 

 animals exhibit closer affinities to the Manteoceras 

 phylum than to either the Palaeosyops-Limno- 

 hyops phylum or the Telmatherium phylum. 

 They possess in common small canine tusks 

 and rudimentary but progressively developing horns 

 and thus do not appear to have been vigorous 

 fighters, their relation to other animals doubt- 

 less being defensive rather than aggressive. They 

 possess long and rather straight rows of grinding 

 teeth, usually narrow, and the zygomatic arches are 

 slender and not widely projected. They are thus 

 readily distinguished from their broad-headed con- 

 temporaries, such as Manteoceras. 



The known members of the series geologically are 

 the two species of Mesatirhinus of Bridger C and D, 

 each of which gives rise more or less directly to the 

 extraordinarily large, long-headed Dolichorhinus of 

 the upper beds of the Washakie Basin and the middle 

 beds of the Uinta Basin. The known species of Meta- 

 rhinus are confined to the sandstone of the fluviatile 

 deposits of the Uinta Basin, a fact which suggests that 

 they may have been small aquatic animals. At the 

 other extreme stand the species of Bhadinorhinus, 

 readily distinguished by short, pointed nasals and the 



absence of infraorbital shelves and exceptionally in- 

 teresting because of their apparent resemblances to 

 the great Megacerops phylum of the Oligocene. 



A simple key to the skeletal characters of these mid- 

 dle and upper Eocene animals is as follows: 



A. Nasals elongate, spreading laterally, decurved; prominent 



infraorbital processes (Dolichorhininae); face bent down- 

 ward: 



1. Mesatirhinus; primitive, dolichocephalic, horns rudi- 



mentary, feet elongate. 



2. Dolichorhinus; progressive, hyperdolichocephalic, horns . 



prominent, feet abbreviate. 



3. Metarhinus; mesaticephalic to subdolichocephalic, nasals 



expanding, dwarfed in size. 



4. Sphenocoelus; hyperdolichocephalic, basieranial pits. 



B. Nasals abbreviate, pointed, no infraorbital shelves (Rhadino- 



rhininae): 



5. Rhadinorhinus; dolichocephalic, facial region upturned. 



Figure 322. — Phjdogenetic relations of the species of Metarhinus, 

 Mesatirhinus, Dolichorhinus, and Rhadinorhinus 



The author's theories (1919) as to the phylogenetic 

 relations of these five phyla are expressed in Figure 

 322. 



HISTORY OF •DISCOVERY AND CLASSIFICATION 



The separation of these five distinct phyla has been 

 an extremely long and difficult undertaking, beginning 

 with the work of Cope in 1872 and ending in 1919. 

 Even now the position of Sphenocoelus and the relation- 

 ship of Bhadinorhinus and Megacerops await elucida- 

 tion. The chronologic record follows: 



1872. Cope describes, under the name " Palaeosyops 

 vallidens," fragmentary upper and lower jaws from the 

 Washakie Basin. This little-known species, probably 

 from Washakie B, is now provisionally regarded as a 

 primitive stage of Dolichorhinus, namely, D. vallidens. 



1889. Scott and Osborn describe as "Palaeosyops 

 hyognathus" a large lower jaw from Washakie B. 

 This now proves to be Dolichorhinus hyognathus. 



1891. Earle describes as "Palaeosyops megarhinus" 

 a small skull from Washakie A, recognizing, however, 



