254 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



1. Loss of old characters (paleomorphs) : Absolute 

 loss of character is rare in the titanotheres. The tra- 

 pezium is the only bone known to be lost duriag the 

 recorded history of the family, whereas in the horses 

 many bones disappear. The incisor teeth disappear in 

 several phyla. The grinding teeth lose certain cusps. 



2. Appearance of new characters (neomorphs), in- 

 cluding rectigradations : A large number of new cusps 

 appear on the premolar grinding teeth, and a few new 

 features appear on the molar grinding teeth. A pair 

 of new hornlets appear on the frontonasal region of 

 the skull. 



3. Increase or diminution of size as a whole (hetero- 

 morphs) : Increase of size is the prevailing tendency 

 among the titanotheres, but in some phyla size is per- 

 sistent or is even arrested, as in the dwarf Metarhinus; 

 or it is reduced, as in the species Palaeosyops copei. 



4. Change in proportions of different regions of the 

 skeleton (heteromorphs, including allometrons) : Such 

 changes are expressed in ratios — for example, 



length of tibia 

 length of femur 



These ratios are extremely significant. Thus one phy- 

 lum may become short limbed or brachymelic, another 

 long limbed or dolichomelic ; one short footed or 

 brachypodal, another long footed or dolichopodal; one 

 phylum may become large headed or macrocephalic, 

 another small headed or microcephalic. 



5. Change in proportions of single parts of the 

 cranial skeleton (allometrons) : Such changes are best 

 expressed in indices, such as 



width of skull 

 length of skull 

 Some series become long headed or dolichocephalic, 

 others broad headed or brachy cephalic; some become 

 long faced or dolichopic, others become short faced or 

 brachyopic. 



The manner in which these changes of proportion 

 (allometrons) and the successive addition of rectigra- 

 dations serve to distinguish the genera of Eocene 

 titanotheres from one another is clearly shown in the 

 following descriptive characterizations of ten Eocene 

 genera : 

 Lambdotherium: Small, long headed, long limbed, without 



horns. 

 Eotitanops: Larger, long faced, limbs somewhat heavier, horn- 

 less. 

 Limnohyops: Still larger, broad headed, short faced, light 



limbed, broad footed, hornless. 

 Palaeosyops: Massive, broad headed, short faced, heavy limbed, 



short footed, rudiments of horns (rectigradations). 

 Telmatherium: Large, long headed, short faced, light limbed. 



rudiments of horns (rectigradations). 

 Manteoceras: Large, medium headed, short faced, medium 



limbed, short footed, small, distinct horns (rectigradations). 

 Mesatirhinus: Of medium size, narrow headed, short faced, light 



limbed, long footed, small horns quite distinct. 

 Metarhinus: Very small, medium headed, short faced, light 



limbed, horns not very distinct. 

 Dolichorhinus: Large, extremely long headed, short limbed, short 



footed, horn rudiments very prominent. 



Rhadinorhinus: Medium size, medium headed, light limbed, 

 horn rudiments indistinct. 



The degrees of change among the "species" con- 

 stituting each of these "genera" are exemplified in 

 the "standard measurement tables" that accompany 

 the description of every genus in Chapters V and VI. 

 In these tables it is demonstrated, first, that the 

 Linnaean lines of division between species do not exist ; 

 second, that occasionally the type and paratype spec- 

 imens of a single species selected by the pioneer pale- 

 ontologists belong to separate stages because they 

 were found at different geologic levels. Some of these 

 ancient specific names have historic value and are 

 retained for convenience, although some that were 

 applied to forms on the border line between two specific 

 stages are very inconvenient. 



PROPORTIONS AND FLEXURES OF THE SKULL 



The skull is the chief center of evolution movement 

 in the titanotheres, both in the transformation of its pro- 

 portions and in the development of horns, and with the 

 teeth it furnishes a complete key to the evolution, 

 relationship, and ascent of these mammals, although 

 the proportions of the skeleton and the feet also 

 furnish valuable indications. The forms of the feet, 

 which are evolving so rapidly and are so significant in 

 the horses, are relatively stationary in the titanotheres. 



The chief principles in the transformation of the 

 skull through changes of proportion are the following: 



1. Elongation and narrowing of the skull as a whole — that 

 is, dolichocephaly. 



2. Abbreviation of the cranium and elongation of the face — 

 that is, proopic dolichocephaly or dohchopy, as in Equus. 



3. Abbreviation of the face and elongation of the cranium. — 

 that is, postopic dolichocephaly or brachyopy, as in all Oligocene 

 titanotheres. 



4. Abbreviation and broadening of the skull as a whole — 

 that is, brachycephaly. 



5. Flexure of the facial upon the cranial region — that is, 

 cyptocephaly. 



The principal measurements of the skull in the 

 titanotheres differ somewhat from those employed in 

 the craniometry of the Equidae. (Osborn, 1912.382.) 

 They are listed below and are illustrated in Figure 211. 



Direct measurements of skull 



1. Cephalic or basilar length from incisive border to occipital 

 condyles, inclusive. 



2. Facial length from postorbital process to incisive border, 

 inclusive (projected on the basilar Une). 



3. Cranial length from postorbital processes to occipital 

 condyles, inclusive (projected on the basilar line). 



4. Facial breadth, or frontal width, as measured in the 

 horses across the postorbital processes to establish the cephalic 

 index. This measurement is not practicable in titanotheres. 



5. Zygomatic breadth across the widest part of the zygomatic 

 arches. 



6. Premolar-molar length, or superior grinding series (p' to 

 m^ if the first premolar is retained, otherwise p^ to m^). 



7. Molar length, anteroposterior measurement along middle 

 of crowns (m' to m') . 



