830 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



In 1904, by a study of the progressive percent- 

 ages of increments in the skulls of Palaeosyops and 

 Bolicliorliinus for this monograph, Gregory was able 

 to establish the principle of difl'erential as opposed 

 to harmonic dolichocephaly. In 1907 Osborn, in 

 considering the nearness of affinity of the subdoli- 

 chocephalic MesaiirMnus to the relatively broad- 

 skulled Manteoceras, concluded that broadening and 

 lengthening were "quantitative" characters, which 

 were largely independent of remote hereditary control. 

 In 1908 Gregoiy's studies indicated that the doli- 

 chocephalic Menodus and the brachycephalic Brontops, 

 together with the intermediate genus Allops, con- 



Figure 744. — Differences in the proportions of premo- 

 lars and molars corresponding to differences in the 

 proportions of the skull 



Progressive brachycephaly: Graph I shows numerical excess of trans- 

 verse over anteroposterior diameters of the upper premolars and molars 

 in successive stages of the progressively brachycephalic Brontops 

 phylum. 



Contrast of dolichocephaly and brachycephaly: Graph II shows contrasts 

 in the proportions of the upper premolars and molars between 

 dolichocephalic (.^) and brachycephalic titanotheres (B, C, D). The 

 dolichocephalic form (A) has relatively narrow premolars and posi- 

 tively narrow molars, in which the transverse becomes less than the 

 anteroposterior diameter. 



stituted a separate group, the menodontine, in con- 

 trast to the brontotheriine group, including the rela- 

 tively long-skulled Brontotherium and the broad- 

 skulled Megacerops. 



In a study of reversal in the proportions of the feet, 

 Gregory in 1910 (1910.1) adduced evidence to show 

 that the broad-footed genera Palaeosyops, Manteo- 

 ceras, Megacerops, Brontotherium had been derived 

 from narrow-footed stem forms and that their broad 

 magnum had been derived from a narrow magnum 

 having a wedge-shaped lower end, much like that of 

 Eotitanops. 



By a close study of the Eocene titanotheres in the 

 spring of 1914 Osborn and Gregory discovered a 

 principle that may afford a key to the phylogeny of 

 the titanotheres: Differential lengthening and broad- 

 ening of all parts of the skull and feet have acted 

 either simultaneously or in combination at different 

 rates in the several phyla. 



APPLICATION OF THE PROPORTIONAL REVERSAL 

 PRINCIPLE TO THE TITANOTHERES 



Eotitanops. — Eotitanops horealis is dolichocephalic. 

 The details of the dentition and infraorbital, malar, 

 and other characters of the skull show that it is 

 structurally ancestral, or nearly so, to the progres- 

 sively brachycephalic LimnoJiyops and Palaeosyops. 

 This fact favors the view that the narrow-footed 

 Eotitanops ^' gave rise to the broad-footed Palaeosyops. 



Telmatherium. — Telmatherium unquestionably re- 

 sembles Manteoceras in some characters, the Palaeo- 

 syopinae in others. A close study of the skull and 

 dentition of T. cultridens shows no feature inconsistent 

 with remote derivation from a mesaticephalic species 

 allied generically to Eotitanops horealis. Telmathe- 

 rium ultimum conserved the mesaticephalic features 

 of the basicranial region, but the middle part of the 

 skull, like that in all Uinta Basin forms, had already 

 undei'gone considerable elongation. The premolar 

 series, p^-p*, as shown by percentage ratios, is rela- 

 tively longer in T. cultridens than in Eotitanops; but 

 in T. ultimum the molars have lengthened so much 

 that the premolar series is again relatively short. In 

 this species, however, the molars have also broadened 

 decidedly, so that they are very large and somewhat 

 broad. Here, then, was a combination of differential 

 broadening and lengthening. 



Manteoceras. — As compared with the very primitive 

 and ancient titanothere Eotitanops, the hypothetical 

 ancestors of Manteoceras in the Wind River were also 

 mesaticephalic and probably allied to the forms that 

 are ancestral to Telmatherium and to Palaeosyops. 

 The Bridger Manteoceras shows a broadening of all 

 parts, as well as a lengthening of the middle portion 

 of the skull; the grinding series also shows a slight 

 broadening and a relative increase in size. But in 

 the Uinta C stage (M. uintensis Douglass) a marked 

 dolichocephalic tendency has supervened, affecting 

 especially the postcanine diastema and the length of 

 the premolars and of the first and second molars. In 

 the feet Manteoceras manteoceras shows a marked 

 secondary broadening of the slender-footed type- and 

 is clearly allied to the narrow-footed Mesatirhinus. 



Protitanotherium. — Protitanotherium, to judge from 

 the resemblances in the dentition and skull to 

 Manteoceras uintensis, was possibly a derivative of 

 some earlier species of Manteoceras. In common with 

 all other upper Eocene and Oligocene titanotheres 

 Protitanotherium had probably suffered an elongation 

 of the middle portion of the skull, but in addition to 

 and superseding this, a strong brachycephalic tendency 

 had set in which, to judge from the front part of the 

 skull and dentition, had brought Protitanotherium 

 near to the stem of Brontops. 



" The fifth digit of E. iorealis is perhaps not reduced past hope of subsequent 

 increase. It is broken off in the type. 



