608 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Measurements oj pelves of Eocene titanoiheres compared, luitTi tapirs, in millimeters 



A comparison of these figures and measurements 

 demonstrates that the main features of the progressive 

 evolution of the titanothere pelvis in graviportal 

 adaptation are the following: 



1. Relative expansion of the iliac crest. 



2. Reduction of the thin crista iliaca and expansion of 



the rugose superior borders of the ilium. 



3. Relative abbreviation of the os innominatum. 



4. Uniform rugose convexity of the superior crest of the 



ilium. 



The above are all progressive graviportal characters. 

 It is demonstrated that even as far back as middle 

 Eocene time the early weight-bearing or subgraviportal 

 type of ilium was well established among the titano- 

 theres, and that from the evidence afforded by the 

 ilium alone these animals were heavier bodied and 

 slower moving of limb than the modern tapirs. Our 

 analysis (see below) of the graviportal adaptation in 

 the ilium makes entirely clear the general functional or 

 adaptive stages through which the pelvis of these 

 Eocene titanotheres is passing. We observe that the 

 relatively elongate ilium of Limnohyops is in the first 

 stage, that Palaeosyops with its shorter ilium and 

 heavier body has passed beyond this, and that 

 Manteoceras possesses the fully developed graviportal 

 type of ilium. 



The principal family characters of the pelvis appear 

 to be as follows: (1) The metapophyses of the posterior 

 lumbar vertebrae articulate with the anterior border 

 of the ilium, as in Equus; (2) in LimnoTiyops the 

 first and second and half of the third sacral vertebrae 

 expand to articulate with the Uium, whereas in 

 Tapirus only one and half of another unite with the 

 ilium; similarly in Manteoceras three sacrals enter 

 into the iliac union; (3) the ilium is much longer than 

 the ischium; (4) the superior border of the ilium is 

 slightly indented (Limnohyops) or uniformly convex 

 {Palaeosyops, Manteoceras); (5) the peduncle of the 

 ilium is broad and short, its dorsal border presenting 

 a sharp ridge; (6) the ischia are not separated or cleft 



posteriorly as in the Amynodontidae, Rhinocerotidae, 

 and other Perissodactyla; (7) the suprasacral area 

 (tuber sacrale) progressively expands; (8) the rugose 

 borders (tuber sacrale and tuber coxae) expand and 

 unite to obliterate the thin intermediate crista iUaca. 



ILIUM 



The ilium of Eocene titanotheres exhibits a con- 

 siderable range of generic characters, as follows: 



UIUM OF THE PAIAEOSYOPINE GROUP 



Limnohyops. — Ilia moderately expanded, with in- 

 dented superior border. 



Palaeosyops. — Os innominatum elongate, iliac crest 

 uniformly convex, expanding into a broad border in 

 Bridger C or D stages. 



UIUM OF THE MANTEOCEEAS-DOLICHORHINUS GROUP 



Mesatirhinus. — The fragmentary specimen Am. 

 Mus. 1571 exhibits a slender peduncle of the ischium. 

 The ilium is unknown. 



Manteoceras. — The pelvis provisionally associated 

 with this genus exhibits a very broad, uniformly 

 convex superior border of the ilium; os innominatum 

 relatively broadening and abbreviated. 



Dolichorhinus. — Superior borders of ilium mod- 

 erately expanded or narrower than in the supposed 

 Manteoceras; os innominatum moderately elongate. 



The detailed description of these various types of 

 pelves is given in the succeeding section of this memoir. 



Materials. — A very large number of femora are 

 preserved, many of which have the tibiae associated. 

 On these it is possible to establish the femorotibial 

 type of the titanotheres, which from the beginning 

 to the end of their evolution is very distinctive. 

 The femur of even the ancestral Eotitanops (fig. 500) 

 displays some of the characteristic titanothere fea- 

 tures, although it retains the flexed knee, distal 



