CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



817 



horns in a certain way, but in each this predisposition 

 manifests itself at different times. According to the 

 Lamarckian principle, the germinal psychic predisposi- 

 tion to use the horn should appear first, and this initia- 

 tion should be followed after a certain period by the 

 ontogenetic bony horn rudiment due to habit, but 

 there is no evidence for this sequence. The horn rec- 

 tigradations, like the cusp rectigradations, seem to 

 be initiated as a germinal phenomenon; it would appear 

 that the psychic tendency and the horn rudiment ap- 

 pear simultaneously, but as yet there is no positive 

 evidence on this important matter of initiation. 



DENTAL RECTIGRADATIONS 



The grinding teeth of the titanotheres are 

 sluggish in evolution as compared with those 

 in other perissodactyls, especially the horseS) 

 in which the grinding teeth are rapidly accel- 

 erated in evolution. Nevertheless they ex- 

 emplify even more perfectly than the horns 

 the modes of rectigradation. Interest in the 

 cusp evolution is enhanced by the fact that 

 the cusp rectigradations in the lower teeth 

 are mechanically coordinated and timed with 

 the cusp rectigradations in the upper teeth. 

 The causes of this constant and perfect 

 mechanical adjustment of the rectigradations 

 and allometry of the upper and lower grind- 

 ing teeth of titanotheres and other mammals 

 presents one of the most difficult problems in 

 germinal evolution, the various explanations 

 of which wiU be discussed in the third part of 

 this section, under the heading "Theoretic 

 causes of evolution." Here it may be sufficient to 

 point out the modes of rectigradation without making 

 reference to the theory of the underlying causes. 



The adaptive cenotelic (Gregory, 1910.1, 1914.1) 

 principle in the rectigradations and allometrons of the 

 titanotheres, as in all other perissodactyl quadrupeds, 

 is to molarize the premolar teeth — that is, to convert 

 the premolars into teeth of the molar pattern, adap- 

 tively because a herbivorous animal can more effec- 

 tually crush and grind its food with sLx complete grind- 

 ers (molars and molariform premolars) than with three 

 complete grinders (molars) and three incomplete 



grinders (premolars). As shown in detail in Chap- 

 ter V this molarization takes the same course in all the- 

 Eocene titanotheres, namely: 



1. Molarization generally, but not invariably, is first 

 introduced in the fourth premolar (p|), which is usu- 

 ally the most progressive tooth of the premolar series. 

 Thus, these teeth are the first to acquire a partial molar 

 pattern. (For an exception, note Palaeosyops copei.) 



2. The molarization in p| and the other premolar 

 teeth of the titanotheres never attains the same degree 

 of mechanical perfection that it does in the horses; 

 the premolar teeth never become exactly like the 

 molars — they are not perfectly molarized. 



Figure 735. — Cusp rectigradations in Telmaiherium 



The cusp rectigradations in the lower teeth are coordinated and timed with the cusp rectigradations- 

 in the upper teeth, as shown here in the occlusion ot the jaws of Telmaiherium cuUrideJis, 

 Princeton Mus. 10027 (type); one-half natural size. After W. K. Gregory. A, Crown view^ 

 lower teeth (heavy lines) superposed on the upper teeth (Ught lines); B, inner side view. 



3. The molarization advances at a uniform rate, 

 respectively and successively, in pf, p|, pf, in the 

 order named, but affects only slightly p|. This phylo- 

 genetic acceleration of the premolar rectigradations is 

 more rapid in pf and less rapid in pf . 



4. It follows that the fourth upper and lower pre- 

 molars (pf) pass through a series of stages of recti- 

 gradation, which are taken up successively and passed 

 through by p|, pf , whereas pf , with its simple mechan- 

 ical crown, retains its remote ancestral condition. 



These four stages of molarization are illustrated in 

 Figures 732, 735, and the accompanying analytical table 



