CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



815 



they are gradually pushed forward into younger and 

 younger ontogenetic stages until finally they appear 

 on the skull before birth. 



5. The horn rudiments, through phylogenetic 

 acceleration or retardation, appear at earlier or later 

 geologic stages in certain phyla than in other phyla 

 (fig. 734). 



sh ape of the horn (fig. 733) is like that seen in the 

 ontogeny of the horns of cattle. 



Ontogenesis of the horns of cattle. — The ontogenesis 

 (development) of the bony horns of domestic cattle (PI. 

 XLV) appears to be closely similar to the phylogene- 

 sis (evolution) of the horn rectigradations in titano- 

 theres. The comparison is useful. The horn biochar- 



FiGURE 733. — -Rectigradations and allometrons in the skulls, teeth, and feet of titanotheres 



Eectigradations are shown on the cuspules of the lower second premolar teeth; allometrons are shown in the proportions of the head (brachy- 

 cephalic, mesaticephalic, dolichocephalic); allometrons are shown in the proportions of the median metacarpal (dolichopodal, mesatipodal, 

 brachypodal). I, Eoiitanops, an ancestral lower Eocene mesaticephalic, dolichopodal titanothere; n, Palaeosyops, a broad-headed (brachy- 

 cephalio), broad-footed (brachypodal) upper middle Eocene titanothere; UI, Telmaiherium, a medium-headed (mesaticephalic) upper Eocene 

 titanothere; IV, Manteoceras, a medium-headed (mesaticephalic), medium-footed (mesatipodal) middle Eocene titanothere; V, DolicliorMnus, 

 a long-headed (dolichocephalic) short-footed (brachypodal) upper Eocene titanothere. II-V represent four independent phyla of Eocene 

 titanotheres which are widely divergent in the allometric evolution of the head and of the feet but are convergent in the independent evolu- 

 tion of similar cusp rectigradations on the teeth and similar horn rudiments (H) on the skull. 



6. At the time of their first appearance horns seem 

 to be equally developed in both sexes, but gradually 

 they become much larger and more formidable in the 

 males than in the females. 



7. In the titanotheres the horn swelling rises at the 

 junction of the nasals and the frontals (black shading 

 in fig. 733). It is borne chiefly on the nasals in 

 dolichocephalic skulls, chiefly on the frontals in 

 brachycephalic skulls. The original low, rounded 



acter in cattle is known to involve a large number of 

 coordinated and correlated biocharacters, all of which 

 are germinal in origin, namely: (1) A psychic predis- 

 position to use the horn, which is manifested at or soon 

 after birth; (2) a thickening of the epidermal cells 

 above the bony horn rudiment, which in ontogeny 

 appears earlier than the bony rudiment itself; (3) 

 early agglutination of the hair to form the rudiments of 

 the horny sheath; (4) appearance of the bony swelling, 



