386 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



pattern (RJiadinorJiinus) . Yet so far as we know 

 these long-headed animals of the Mesatirhinus- 

 DolichorMnus phylum were not destined to survive and 

 give rise to any Oligocene titanothere but rather to 

 terminate in an excessively specialized type. 



CONVERGENCE OR KINSHIP CONTROL 



These dolichorhines afford an illuminating illustra- 

 tion of kinship control in the simultaneous evolution 

 of different character groups. Some of these character 

 groups are predetermined or controlled by ordinal, 

 others by family, others by generic affinities with 

 related titanotheres. The general expression of kin- 

 ship control may be summarized as follows: 



1. Perissodactyl or ordinal kinship is shown, among 

 other characters, in the independent progressive com- 

 plication of the premolar-molar teeth, three of the 

 premolars tending to acquire the pattern of the molars, 

 although this pattern is less perfectly developed in 

 other Perissodactyla. 



2. Titanothere family kinship is indicated in the 

 independent progression of the development of naso- 

 frontal horns, the flattening of the top of the cranium. 



FiGUBE 325. — Leidy's cotypes of Palaeosyops 

 ( = Mesaiirhinus) Junius 



Natural size. Crown view of premolar and molar in the 

 museum of the Acad. Nat. Sci. Philadelphia. Bridger (?)B; 

 level doubtful. A, Eight fourth lower premolar; B, posterior 

 part of third lower molar. 



the obliteration of the sutures on the top of the 

 cranium, and the elongation of the middle portion of 

 the cranium. 



3. The subfamily kinship to the Manteoceratinae is 

 indicated in the persistent subtriangular shape of the 

 horns, in the development of an infraorbital shelf, and 

 in the contracted posterior nares and broadening 

 nasals. 



In their progressive dolichocephaly, a tendency 

 that strongly affects the middle region of the skull 

 between the orbits and the auditory meatus, these 

 animals (Mesatirhinus-Dolichorhinus) are partly inde- 

 pendent of their subfamily, family, or ordinal relations 

 and follow an extreme adaptive direction of their own 

 in the elongation of the midcranial region and of the 

 teeth. 



In this special adaptation to their partial grazing 

 habits the dolichorhines further parallel certain of the 

 Equidae and other grazing animals, such as the cattle, 

 in their cyptocephaly. (See figs. 213, 214.) The 

 incisor teeth further acquire deep posterior pits, or 

 pockets, analogous to the pits that are developed in 

 the incisors of the upper Oligocene Equidae and that 

 tend to become typical cropping teeth. 



DIVERGENT OR INDEPENDENT EVOLUTION OF CHARACTER 

 GROUPS IN THE DOLICHORHINES 



The independent evolution of these four or five 

 groups of characters as observed in the skull alone 

 obviously affords only a partial picture of the play 

 and interaction between the vast number of contem- 

 poraneous processes that are involved in the evolution 

 of the members of this phylum. If we could similarly 

 compare all parts of the vertebral column and of the 

 limbs, we should probably discover many additional 

 illustrations of this law of the evolution of groups of 

 characters under the influence partly of kinship and 

 partly of independent adaptation. 



The principle of independence or divergence is well 

 illustrated in the skull. In Figure 302 the skulls of 

 Manteoceras and the MesatirMnus group are compared 

 as seen from the side. The palatal view of Manteoceras, 

 Metarhinus, MesatirMnus, and DolichorJiinus (flg. 324) 

 brings out the resemblances and contrasts between 

 these four forms. The superior view (fig. 323) also 

 brings out the wide progressive divergences between 

 these undoubtedly related forms. 



We may also compare superior views (PI. LXXX) 

 of the skulls of Metarhinus and Rhadinorhinus, show- 

 ing how the latter departs from the other members of 

 this dolichorhine group in the abbreviation and point- 

 ing of the nasals and in the reduction of the infraorbital 

 processes. 



PROGRESSIVE DOLICHOCEPHALY IN MESATIRHINUS- 

 DOLICHORHINUS 



The dolichocephaly, which is the chief progressive 

 character of the MesatirTiinus-Dolichorhinus phylum, 

 is beautifully illustrated in the accompanying series 

 of illustrations (figs. 339, 340). 



A very important fact (see the following table) 

 is brought out by the cranial indices and ratios in this 

 series of species — namely, that while the skulls lengthen 

 and become relatively narrower, the facial portion is 

 not relatively abbreviated as it is in the Manteoceras 

 phylum, because the faciocephalic index is the same 

 in the terminal member of the series, D. JiyognatJius, 

 as in M. megarJiinus, the most primitive member of the 

 series. In other words, in Manteoceras the face is 

 abbreviated; in the dolichorhines it is not. 



Cranial indices of MesatirMnus and Dolichorhinus 



M. megarhinus 



M. petersoni 



D. superior 



D. longioeps 



D. intermedins 



D. hyognathus ( = cornutus) 

 D. fluminalis 



56-59 

 51-52 



52 



41-46 



43-46 



45 



48 



49 



49 



51-53 



41 



39 



38 



35-37 



38 

 36 



