788 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



It is yerj important to note that the indices given 

 in the table are zygomatic (buccal) (brachycephalic 

 and dolichocephalic) as distinguished from true cranial 

 (head) indices. The cranium proper is extremely 

 elongate. For example, in BrontotJierium curtum it 

 extends backward, overhanging the occiput, and for- 

 ward, overhanging the orbits — in fact, the elongation 

 of the craniofacial summit of the skull is strictly 

 dolichocephalic in the original sense of the word as 

 defined by Ketzius. 



DIFFEEENTIAI EVOIUTION IN SKUH PROPORTIONS IN 

 THE SEVERAL PHYIA OF TITANOTHERES 



During the Eocene the dominant tendency in the 

 Manteoceras and Dolichorhinus phyla was increase in 

 the length of the middle portion of the cranium. This 

 became extreme in DolichorMnus. In Palaeosyops and 

 Limnohyops the middle portion of the skull was not 

 elongated. During the upper Eocene and lower Oligo- 

 cene this increase of the middle portion of the skull was 

 surpassed in most phyla by the broadening of the skull, 

 and especially of the zygomatic arches. During lower 

 Oligocene time (Chadron A to C) the three main phyla 

 exhibit approximately equal increments in total 

 cranial length, Menodus being only slightly in advance 

 of the others in its percentage increase, as shown in 

 the accompanying diagram (fig. 708). 



During the same period the zygomatic breadth, 

 already very great in male skulls of Brontops hrachy- 

 cepJialus, shows a percentage of increase to the stage 

 of Brontops robustus relatively less than in the Meno- 

 dus phylum. The reason Menodus during the same 

 geologic period acquired greater zygomatic increase is 

 that the primitive form Menodus heloceras was mark- 

 edly mesaticephalic, lacking the zygomatic expansions 

 entirely, whereas in the largest male, M. giganteus, 

 these expansions caused the skull to attain a width 

 index of 55.3. Similarly, the horns appear to increase 

 in length very rapidly in Menodus, because of the wide 

 range from the extremely short horns of the lower 

 Oligocene M. heloceras to those of the great Field 

 Museum skull of M. ingens, in which they have an 

 outside length of 290 millimeters. 



In the Menodus phylum all the parts except the 

 horns enlarge more uniformly and harmonioiisly than 

 in the BrontotJierium phylum. It should be noted 

 that although the total length of the basilar axis of 

 the skull exhibits a similar percentage of increase in 

 the three main phyla, this increase is differently 

 divided between the different parts of the skull. 

 In the Menodus phylum, for example, the facial 

 region elongates naore rapidly than in the Bronto- 

 therium phylum, in which the midcranial region 

 elongates rapidly and the facial region is abbreviated. 



PALATE AND SHIFTING POSTERIOR NARES 

 In general, there is a broadening of the palate and 

 gradual shifting backward of the posterior nares. In 

 the more primitive forms the posterior nares opened 

 more anteriorly between the second pair of grinding 

 teeth, m-. In Palaeosyops and Limnohyops they 

 frequently open opposite the middle of m^ In other 

 middle Eocene genera the posterior nares open either 

 opposite the posterior part of m^ or between m^ and 

 m^ In the Oligocene genera the opening of the 

 posterior nares is variable, generally opposite m' and 

 sometimes behind m'. 



RUDIMENTS OF HORNS ARISING INDEPENDENTLY IN 

 EOCENE PHYLA 



Development of liorns and teeth. — The independent 

 origin of the horn rudiments at the junction of the 

 frontal and nasal bones in members of four sub- 

 families of Eocene titanotheres is one of the most 

 significant facts discovered in the course of the 

 researches for this monograph. In the accompanying 

 table the more or less rapid development of the 

 incipient horns is contrasted with the more or less 

 rapid development of the incipient cusps on the 

 premolar grinding teeth. 



As is shown very clearly in Chapter V (pp. 266-267) 

 and Chapter XI (pp. 810-811) the horn rudiments 

 appear geologically early and are very prominent in cer- 

 tain phyla ( Manteoceras, Mesatirhinus) at a time when 

 in other contemporaneous phyla (Palaeosyops, Telma- 

 therium) the skulls are still smooth, entirely hornless. 

 It is only in the late stages of Palaeosyops and in geo- 

 logically late stages of Telmatherium that the horn 

 rudiments appear at all. These facts have a double 

 significance, which is fully discussed in Chapter XI 

 (pp. 883-884). First, it would seem that there is a pre- 

 disposition for the evolution of horns in a certain 

 region of the skull; second, that this predisposition 

 manifests itself not uniformly but in some phyla 

 earlier than in others. There are two deductions from 

 these phenomena: 



First, the origin of frontonasal horns is a titanothere 

 family characteristic. 



Second, this origin at earlier or later geologic stages 

 is a subfamily or generic characteristic. 



Comparison with other perissodactyls. — In the Peris- 

 sodactyla, as in all other ungulates, there are three 

 more or less concomitant steps in the origin of horns, 

 namely : 



1 . Psychic predisposition to use the horn as a means 

 of offense and defense, correlated with other offensive 

 and defensive psychoses, linked also with male and 

 female sex glands. 



2. Dermal and epidermal thickening, derm pads, 

 and epidermal horny sheaths, protecting layer. 



3. Osseous swellings, exostoses, bony swellings be- 

 neath the dermal pads or horny sheaths. 



