THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



761 



Protitajiotheriunv eTTtoT-c/inaiuTn 



Upper Eocene (upperUinfa) 



3fa7iteoc£ras TTzaTiteoceras 



MiddleEocene (upperBridger) 



Jjimnohyops priscus 



MiddleEocene (lowerBridger) 



EotUfxnops horecdis 



Lower Eocene(Wind River) 



Figure 690. — Progressive stages of structural evolution in the skull and molar teeth of titanotheres 

 In Eotitanops the facial part of the skull is longer than the brain case (cranium). In Brontotherium the face is very short and the brain 

 case very long. The horn swellings (H) first appear in Manteoceras and become very prominent in the succeeding stages. The top of 

 the skull becomes deeply concave. The outer wall and the V-shaped cusps of the upper molar teeth (paracone, metacone) become 

 very deep, and the inner cusps (protocone, hypocone) retain their low, conical form. All the lower molars retain the W-shaped 

 crown, which increases considerably in depth. 



