782 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



evolution in all the other perissodactyls. This is 

 clearly brought out in Chapter IX, sections 1-2. 



Second, the evolution of the grinding teeth is de- 

 cidedly inferior to that of all the other families of 

 perissodactyls. The titanothere molar, though capa- 

 ble of indefinite enlargement, is not capable of trans- 

 formation into a hypsodont tooth. 



Third, the transformation of the skull, on the con- 

 trary, is a far more extreme specialization in the titano- 

 theres than in the tapirs or even in the horses; it is, 

 on the whole, more extreme than in the rhinoceroses. 

 Consequently, cranial evolution and transformation 

 are among the most distinctive characteristics of the 

 titanothere family. In many characters, as pointed 

 out in Chapter I (pp. 28-32), the cranial transformation 

 in the titanotheres is analogous to that of the rhinocer- 

 oses; in other characters it is unique. 



In brief, although the feet and teeth of the titano- 

 theres are conservative, retaining more or less their 

 ancestral form, the skull is extremely progressive, 

 presenting greater variety of form and proportion than 

 that of any other perissodactyl. In every subfamily 

 the skull is so plastic as to be in a continuous state 

 of transformation, radiating into great extremes of 

 structure, which show the widest possible differences of 

 proportion and in which all resemblance to the ances- 

 tral type is completely lost. The horns finally become 

 the dominant feature of the skull and appear to condi- 

 tion the evolution of all the other parts. 



The transformation of the Eotitanops type of skull 

 into the Brontotherium type, which is epitomized in 

 Figure 709, as compared with the transformation in 

 other perissodactyls, shows the following principal 

 features: (1) Loss or reduction of parts is relatively 

 infrequent; (2) few rectigradations (new parts) arise, 

 the only absolutely new features being the horns and 

 additional cusps on the premolar teeth; (3) increase in 

 bulk is enormous and with one exception is continuous 

 and progressive in every branch; (4) changes of pro- 

 portion are great in all parts of the skeleton, especially 

 in the skull. 



Loss or reduction of parts. — The known losses and 

 numerical reductions of parts may be summarized as 

 follows: (1) Incisor teeth (f) retained in some phyla 

 (Brontotherium, Brontops) in more or less functional 

 condition; incisors entirely lost (%) in other phyla 

 (Menodus, Mega cer ops); (2) intermediate conules on 

 the superior grinding teeth lost in all phyla, a loss 

 that was the final cause of the mechanical imperfection 

 of the teeth and of the extinction of the family, in 

 contrast with the horses, in which the conules save 

 the race; (3) trapezium, inner bone of the second row 

 of the carpus, variable or absent. 



Rectigradations. — New cusps and cuspules appear 

 on the upper and lower premolar teeth, being inde- 

 pendently developed in each of the titanothere phyla 

 at more or less rapid rates of evolution, and osseous 



horns appear at the junction of the nasal and frontal 

 bones, being independently developed in five distinct 

 phyla and more or less rapidly evolving. 



Harmonic increase in size. — During the period of 

 time represented by the Wind River, Bridger, Washa- 

 kie, true Uinta (C), and Chadron formations there 

 was a great increase in bulk or mass of body in the 

 titanotheres. In the smallest known true titanothere, 

 Eotitanops gregoryi, the skull was smaller than that of 

 a wolf, and the bodj' therefore probably weighed less 

 than 150 pounds. On the other hand, the largest 

 titanothere certainly weighed much more than an 

 adult black rhinoceros, whose body weight is estimated 

 in Brehm's Tierleben as 1,600 kilograms, or 3,500 

 pounds. It would weigh less, however, than a large 

 African elephant, whose estimated weight is 4,000 

 kilograms, or 8,800 pounds. Therefore, if we assign 

 a weight of 2,800 kilograms, or about 6,000 pounds, 

 to the largest titanothere its weight would have been 

 about 40 times as great as that of its diminutive 

 ancestor Eotitanops gregoryi. 



Changes in proportions (allometrons) . — Each phylum 

 of the titanotheres has its distinctive rate of increase 

 of the grinding area of the teeth in relation to the 

 length of the skull, as shown in the accompanying 

 table. Of the Oligocene phyla, Menodus has the 

 relatively largest grinding area; Megacerops and Bron- 

 totherium have the relatively smallest. 



Measurements , in millimeters, showing progressive increase in 

 length of true molar series as compared with total length of 

 skull 



Eotitanops borealis 



Limnohyops laticeps 



Palaeosyops major, Am. Mus. 12182 



Palaeosyops leidyi, Am. Mus. 1544 



Palaeosyops leidyi, Am. Mus. 1516 



Manteoceras manteoceras. Am. Mus. 



1569 



Manteoceras manteoceras, Am. Mus. 



1545, now in Nat. Mus 



Manteoceras washakiensis. Am. Mus. 



13165 



Telmatherium ultimum, Am. Mus. 2060_ 

 Brontotherium leidyi, Carnegie Mus. 93_ 



Brontotlierium gigas 



Menodus giganteus. Field Mus. 5927 



Menodus giganteus, Am. Mus. 505 



Basilar 

 length 

 of skull 



313 

 "410 

 389 

 415 



414 



492 



523 



-490 

 510 

 665 

 830 

 825 

 777 



54 



90 



94 



100 



102 



103 



118 



116 

 129 

 190 

 241 

 270 

 250 



Ratio of 

 m'-ms to 

 basilar 

 length 

 (per 

 cent) 



17 

 21 

 21 

 24 

 24 



20 



23 

 25 

 28 

 29 

 32 

 32 



Comparison of the percentage increase of the 

 body as a whole and of the grinding teeth as a whole 

 shows that the increase of the grinding teeth in 

 bulk approximately kept pace with the increase in 



