EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



543 



Sequence qf species. — The following summary affords 

 the principal characters by which the chief species 

 so far recognized may be distinguished from one 

 another: 



1. Megacerops coZoradensis Leidy is the genotype. The type 

 is lost and is known only from the figure of the coossified nasals 

 and horns, which indicates an animal about the size of M. bucco. 

 Titanotherium zone, Colorado. 



2. Megacerops (" Symborodon") bucco (Cope) is a more pro- 

 gressive species. It is decidedly brachycephalic. It still retains, 

 however, the long thin nasals. The horns have shifted to a more 

 anterior position. The buccal processes of the zygomata now 

 become very prominent, as in the brontotheres. The occipital 

 pillars begin to expand, the parietal crest is broader, and as a 

 distinctive, progressive character, the tetartocones of the pre- 

 molars are more developed. It is important to note that this 

 extreme type probably belongs to the upper beds. Colorado 

 and South Dakota. Titanotherium zone, level Chadron B. 



3. Megacerops acer Cope is represented by the male type 

 skull and by a female skull which Cope made the type of his 

 species M. altirostris. It is also mesaticephalic and differs from 



M. copei in the thick, short nasals, in the divergence of the 

 horns, in the somewhat more anterior position of the horns, 

 while it resembles M. copei in the small size of the tetartocones 

 of the premolars. The occiput (PI. CLVII, C; fig. 451) is 

 readily distinguished from that of any brontothere by its slender 

 characters, indicating that the muscles of the neck were not 

 so robustly developed in these animals. Colorado and South 

 Dakota. Titanotherium zone, upper beds. 



4. Megacerops copei (Osborn), named in honor of Professor 

 Cope, appears to belong to the middle Titanotherium zone (Chad- 

 ron B). In proportions the skull is mesaticephalic; the horns 

 are vertical, elongate as seen from the front, and their form 

 suggested the unique name "rabbit's ears," which is applied 

 to this skull. They are placed typically directly above the 

 orbits, yet the nasals are very thin, and, as shown in the section 

 (fig. 450), there is no connecting crest. Colorado and South 

 Dakota. Titanotherium zone, upper beds (Chadron C). 



5. Megacerops assiniboiensis Lambe is an animal of small 

 size, known only by the jaw. Saskatchewan, Swift Current 

 Creek. Titanotherium zone. 



6. Megacerops riggsi Osborn is distinguished by its especially 

 short massive jaw. Colorado. Titanotherium zone. 



Synopsis of specific cJiaracters qf Jour species of Megacerops 



M. copei, Nat. Mus. 4711 



M. altirostris, Am. Mus. 6350 ' 

 (type) 



Skull 



Nasals 



Horns 



Horns, malar ridge 



Horns, top section 



Horns, connecting crest 



Horns, position 



Internal flange of buccal 

 process. 



Occiput and pillars 



Occiput pits 



Occiput parietal crest 



Premolars 



Mesostyle p' 



Mesaticephalic 



Thin 



Vertical 



Prominent 



Strongly oval 



None 



Over orbits 



Not prominent 



Narrow 



Absent? 



Narrow 



Tetartocones smaller. 



Absent 



Extremely brachy- 

 cephalic. 

 Thin, cf long, 9 medium 



Divergent 



Absent 



Rounded to oval 



None 



c? anterior to orbits 



9 ?intermediate 



Prominent 



Expanding 



Absent? 



Broad 



Tetartocones more de- 

 veloped. 

 Present 



Mesaticephalic? 



Thick, short 



Divergent 



Intermediate 



Oval 



Well-defined 



[Anterior to orbits 



(cT? 



( 9 not prominent 



Narrow 



Pronounced 



Narrow 



Tetartocones smaller. 



Mesaticephalic? 



Thick, short. 

 Divergent. 

 Faint or absent. 

 Rounded to oval. 

 None. 



Intermediate. 



Narrow. 

 Absent?. 

 Intermediate. 

 Tetartocones smaller. 



Present. 



o A small female. 



Jaw cTiaracters. — The jaws of Megacerops are read- 

 ily distinguished by the abbreviation of the anterior 

 portion corresponding with the extreme abbreviation 

 or brachyopy of the facial region, with which the ab- 

 breviation of the lower premolar series is also corre- 

 lated. A second distinction is the narrowness of the 

 chin and the small size and approximation of the 



*• Regarded as the female of M. acer. 



canines, correlated with the reduction of the lower 

 incisor teeth. The anterior part of the face in Mega- 

 cerops is contracted and the lips were probably narrow 

 and pointed in contrast with the broad, massive lips 

 of Brontotherium. 



Details of the contrasts in the character of the jaw 

 are as follows: 



Jaw characters in Megacerops and Brontotherium 



