832 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



very pronounced in the most advanced members of 

 MesatirJiiniis petersoni and culminated not only in 

 the long-skulled Dolichorhinus but even more in the 

 allied Sphenocoelus. The feet of the Mesatirhinus 

 were derived from those of Eotitanops by a slight 

 relative broadening of the carpals, tarsals, and 

 metapodials. Dolichorhinus, contrary to former views, 

 was not dolichopodal but so far as the evidence shows 

 was subbrachypodal, the foot proportions being 



markedly hypsodont and had very flat ectolophs. 

 Certain specimens of Metarhinus fluviatilis had a lower 

 jaw like that of a minute Megacerops. Not all 

 metarhines were small; a lower jaw (Am. Mus. 1859) 

 shows, in fact, clear indications of a species of Meta- 

 rhinus larger than B. diploconus. 



Rhadinorliinus. — As elsewhere fully shown, Rhadino- 

 rhinus in many characters foreshadows the stem form 

 of the Brontotheriinae; R. diploconus especially is 



Figure 746. — Models of heads of Oligocene titanotheres showing general proportions, especially form of lip and horns 

 A, Brontops (brachy cephalic); -B, Menodus (dolichocephalic); C, Megacerops (hyperbrachycephalic); D, Brontotherium (brachycephalic). 



derived by moderate broadening and little or no 

 lengthening from those of forms like Mesatirhinus. 



Metarhinus. — Metarhinus is undoubtedly a near 

 ally of Mesatirhinus and even parallels Dolichorhinus 

 very closely in many characters of the dentition and 

 skull; but in Metarhinus the basicranial and mid- 

 cranial regions, after becoming subdolichocephalic, 



iyonyh 



Figure 747. — Separability and imperfect blending of allometric biocharac- 

 ters in the facial bones of the horse {Equus caballus, 2), ass {E. asinus, 

 (?), and mule 

 Left column: Bones of the side of the face, preorbital region, c, The bump on the forehead of the 



horse and mule not observed in the ass. 5, The point at which the section of the nasals is taken. 

 Middle column: Nasal bones of the horse, ass, and mule viewed from above, showing that the nasal 



bones of the mule closely approximate in proportions those of the horse, s. Point at which the 



transverse section is taken. 

 Right column: Transverse section of the nasal bones of the horse, ass, and mule at the point 



indicated by s, showing the shallow nasal bones of the ass, a pure monophyletie type; the vari- 



abihty ( F'- V) in the depth of the nasals in the horse, which is due to the fact that the domestic 



JE. caballus is a type derived by interbreeding of several distinct races, and the intermediate 



depth of the nasals in the mule. 



ceased to elongate, and the elongation of the face was 

 retarded at an early period. The molars became 



distinguished by the short, upturned face, the narrow 

 preorbital region, the rounded malar-lacrimal bar, the 

 saddle-shaped skull top, the narrow, pointed nasals, and 

 many other brontotheriine characters. The mingling 

 of dolichocephalic and brachycephalic characters in 

 R. diploconus foreshadows a similar mingling in Bronto- 

 therium, which has an elongate skull and a short face. 

 Possible bearings of the facts stated on phy- 

 logeny. — Assuming provisionally the correct- 

 ness of this view, the history of the bronto- 

 theriine skull may have been as follows: 



1. A mesaticephalic member of Eotitanops 

 gave rise to the subdolichocephalic Rhadino- 

 rhinus, in which, however, the face was not 

 lengthened. The middle and basicranial parts 

 of the skull continued to lengthen, but per- 

 haps in late upper Eocene time the tendency 

 toward brachycephaly gathered momentum 

 and resulted in the broad zygomata, the broad 

 frontals, and the widely spreading horns of 

 Brontotherium. The molars in the Rhadino- 

 rhinus stage were relatively long anteroposte- 

 riorly, but later they widened rapidly with the 

 skull. The premolars also widened, and the 

 incipient tetartocones were carried to an ex- 

 treme in Brontotherium. 



2. In the allied Megacerops the broadening 

 tendency early attained predominance, result- 

 ing in a broad, short-faced skull. 



3. The changing proportions of the feet in 

 the same phylum were hypothetically as fol- 

 lows: The narrow foot of Eotitanops gave 

 rise, chiefly by an increase in size but also by 



some degree of broadening, to the narrow foot of 

 Rhadinorhinus ; the narrow foot of Rhadinorhinus may 



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Horse 



