EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



267 



Telmatherium cultridens there appears to have been 

 a distinct nasofrontal protuberance, but in the geologi- 

 cally later Telmatherium ultimum only the faintest 

 indication of its presence is found; it is possible that 

 the horn retrogressed in this phylum. In Manteo- 

 ceras manfeoceras of the upper part of the Bridger 

 formation (middle Eocene) the protuberance, although 

 small, is perfectly distinct and fully characteristic in 

 form. In ProtitanotJierium of the upper Eocene the 

 horns (figs. 317-319) consist of oval protuberances 



about 20 millimeters high and 90 millimeters long. 

 In nearly all the lower Oligocene titanotheres the 

 horns are of large size and finally become the domi- 

 nant feature of the whole skull, affording generic and 

 specific characters. 



The horns are believed to have evolved concomi- 

 tantly with the fighting habits of these animals and 

 with the general increase in size and body. The 

 conditions of the horns in the titanotheres may be 

 summarized as follows: 



Summary of character or condition of the horns in Eocene and Oligocene titanotheres 



Subfamily or genus 



Character or condition of horns 



Lambdotheriinae _ 



Eotitanopinae 



Palaeosyopinae, . . 



Lambdotherium zone (Wind River B) . 

 do 



Telmatheriinae _ 



Sthenodectes 



Manteoceratinae 



Dolichorhininae : 



Mesatirhinus-Doliohorhinus_ 



Metarhinus 



Rhadinorhininae : 



Rhadinorhinus 



Manteoceratinae : 



Protitanotherium _ 



Eotitanotherium. _ 

 Brontopinae 



Lower horizons of Bridger Basin to lower 

 horizons of Washakie Basin, inclusive. 



Upper horizon of Bridger Basin to Uinta 

 C, inclusive. 



Uinta B 1 only 



Upper horizons of Bridger Basin to Uinta C. 



Upper horizons of Bridger Basin to Uinta 



B 2. 

 Lower horizons of Washakie Basin to 



Uinta B 1, inclusive. 



Uinta B 1 only_ 

 Uinta C onlv_- 



Uinta B 2 



Chadron A to C, inclusive. 



Menodontinae. . 

 Megaceropinae. 



.do. 



Brontotheriinae. 



.do_ 

 _do_ 



Hornless. 

 Do. 



Hornless, or nasofrontal horn swelling barely 

 perceptible. 

 Do. 



Horn swelling small. 



Horn swelling small but distinct. 



Horn swelling more pronounced; on nasals 



only. 

 Horn swelling small; chiefly on nasals. 



Horn swelling small. 



Nasofrontal horn swelling pronounced and 



progressive. 

 Do. 

 Nasofrontal horn swelling at first small, 



slowly Isecoming progressively larger. 

 Do. 

 Nasofrontal horn swelling of medium to 



large size. 

 Nasofrontal horn swelling at first small, 



rapidly becoming progressively larger. 



PROPORTION AND RECTIGRADATION IN THE GRINDING 

 TEETH OF EOCENE TITANOTHERES 



The chief characters of the grinding teeth in the 

 Eocene titanotheres were evolved from the bunose- 

 lenodont pattern (see fig. 221) and were modified by 

 changes of proportion and rectigradation, under eight 

 principles, as follows : 



1. The primitive grinders seen in Lambdotherium 

 and Eotitanops are extremely low crowned, or brachyo- 

 dont. The numerous phyla can be distinguished 

 chiefly by the different degrees and rates of elonga- 

 tion of the crown, which shows progressive hypso- 

 dontism. 



2. The sLx main grinding teeth in the upper and 

 lower jaws, p^-m^, p^-nis, are closely crowded together, 

 and this crowding causes the crowns of the grinders to 

 be closely proportioned to the brachycephaly or 

 dolichocephaly of the skull. In brachycephalic titano- 

 theres the transverse diameters of the grinding 



teeth generally exceed the anteroposterior diameters, 

 whereas in dolichocephalic skulls the reverse is true. 

 Thus we shall speak of the grinders as of the "brachy- 

 cephalic" or of the "dolichocephalic" type. 



3. The general tendency of the grinders in titano- 

 theres is to become macrodont, because the pattern 

 of the grinding teeth is mechanically imperfect, and 

 the grinders compensate in size, in some degree, for 

 what they lack in mechanical perfection, 



4. The transformation of the "cone and crescent" 

 or bunoselenodont pattern of the upper and lower 

 grinding teeth in the titanotheres can be best under- 

 stood by comparing that pattern with that seen in the 

 other bunoselenodonts — the primitive chalicotheres, 

 horses, and paleotheres — a pattern similarly derived 

 from the same primitive type of upper and lower 

 grinding teeth (fig. 222), which presents four main 

 cones above and four main cones below, known as the 

 "primary molar cones." 



