INTEODXJCTION TO MAMMALIAN PALEONTOLOGY 



19 



proportion may be followed through long periods of 

 time from their incipient to their final stages, in 

 which various climaxes of change of proportion are 

 reached, such as extreme length or breadth of head or 

 extreme length or shortening of the feet. 



PROPORTIONS OF THE SKULL IN BEARS AND IN 

 TITANOTHERES 



In comparing the Eocene and Oligocene titanotheres 

 with the modern bears {Ursus), for example, as 

 studied by C. Hart Merriam (1918.1), we may note 

 certain parallelisms. The members of each of the 

 eleven subfamilies of titanotheres are distinguished by 

 certain proportions of the skull — that is, they are 

 broad-headed, round-headed, or long-headed — by 

 the shape of the horns and the acceleration or retarda- 

 tion in their development, by the presence or 

 absence of cutting (incisor) teeth, by certain 

 proportions of limb, according as they are 

 swift-footed (cursorial), slow-footed (medi- 

 portal), or heavy-footed (graviportal), and 

 by other minor features. The methods ap- 

 plied to the study of the existing bears may 

 be applied to the study of the skull or other 

 hard parts of the titanotheres. In the titano- 

 theres, however, we may observe all these 

 changes of proportion actually in progress 

 from stage to stage as revealed by paleontol- 

 ogy, whereas in the bears we can observe only 

 certain structural forms, which, so far as our 

 observation goes, appear to be fixed or com- 

 pleted, although they undoubtedly represent 

 stages in a state of actual progression. 



B. Bridger and succeeding titanotheres — Continued. 



6. Manteoceratinae; mesatioephalic to brachycephalic; 



accelerated development of the horns; mediportal 



{Manteoceras, Prolitanotherium) . 

 5. Diplacodontinae; dolichocephalic; accelerated molar- 



ization of the premolars; imperfectly known 



(Diplacodon) . 

 4. Telmatheriinae; mesaticephalic to dolichocephalic 



{Telmatherium, Sthenodecles) . 

 3. Palaeosyopinae; brachycephalic; short-limbed {Palae- 



osyops, Limnohyops) . 

 A. Wind River titanotheres ; face longer than cranium : 



2. Eotitanopinae; medium-limbed, mediportal (Eoti- 



1. Lambdotheriinae; light-limbed, cursorial {Lambdo- 

 thenum) . 



The above scheme presents the eleven subfamilies 

 of titanotheres as they were distinguished in 1914. 



GEOGRAPHIC DISTRIBUTION 

 AT PRESENT TIME 



True R 



ZcoUfiical WlPES 



-GEOGRAPHIC DISTRIBUTION 

 IN PAST TIME 



FEATURES 



DISTINGUISHING 

 TITANOTHERES 



PHYLA OF 



The first application of changes of propor- 

 tion to the arrangement of the subfamilies of 

 titanotheres is the following synopsis, pre- 

 pared in 1914: 



Proportions of skull and limbs; presence and absence of characters 

 distinguishing the subfamilies {main phyla) of titanotheres 

 lOsborn, 1914.409] 



B. Bridger and succeeding titanotheres; cranium longer than 

 face: 

 11. Brontotheriinae; mesaticephalic to brachycephalic; 

 horns long, transversely flattened, and divergent 

 (Brontolherium) . 

 10. Megaceropinae; mesaticephalic to extreme brachy- 

 cephalic; horns long, vertically placed; no incisor 

 teeth (Megacerops (—Symborodon)). 

 9. Brontopinae; brachycephalic; horns short, rounded, 

 or oval; incisors persistent (Brontops {=Mega- 

 ceratops), Diploclonus) . 

 8. Menodontinae; mesaticephalic to dolichocephaUc; 

 short triangular horns; incisor teeth reduced or 

 wanting; feet and limbs long {Menodus {=Titano- 

 therium), Allops). 

 7. Dolichorhininae; mesaticephaUc to doUchocephalic; 

 limbs, so far as known, short (Dolichorhinus, 

 Mesatirhinus, Sphenocoelus, Metarhinus, Rhadi- 

 norhinus) . 



Figure 16. — Theoretic descent of existing members of the dog family 

 (Canidae) from a common ancestor 



A represents the ancestral type. Dots represent intergradations indicated by paleontologic 

 observations (vertical lines) covering five periods of geologic time. A', B, B', C, and C rep- 

 resent existing forms, and dots represent a few existing intergradations demonstrated by zoo- 

 logic observations (horizontal lines). Heavy lines and the adjacent dots represent the phyla; 

 also the past and present distribution of geographic (ontogenetic and environmental) sub- 

 species, races, and intergrades. 



Since that time certain phyla have been condensed by 

 the discovery of titanotheres that link together some 

 of these subfamilies, and others have been expanded 

 by the discovery of new subfamilies, such as the 

 Rhadinorhininae. 



MUTATIONS OF WAAGEN 



Where the fossil material is abundant the genera 

 and species are found to be connected by a series of 

 intergradations. These intergradations, though con- 

 tinuous, are measurable, and therefore a species is 

 subdivisible into a series of intergrading forms. The 

 monophyletic, systematic, or taxonomic unit division 

 of these species is the mutation of Waagen, which is a 

 subspecific stage in the development of one or more 

 characters. Such an actual sequence of mutations of 

 Waagen may be illustrated in the genus BrontotJierium, 

 as indicated on the following page. 



