INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



15 



embrace one or more of the genera originally proposed 

 by Pomel, Leidy, Cope, and Marsh. Other phyla 

 correspond with certain genera — for example, Menodus 

 Pomel (syn. Titanotherium Leidy), Brontops Marsh, 

 Allops Marsh, Megacerops Leidy, BrontotheriumMarsh. 

 These five generic names correspond to members of 

 five phyla that persisted throughout a very long period 

 of geologic time. The remaining phyla of titanotheres 

 are branches that persisted only for a short time, so 

 far as we know at present — for example, Diploclonus. 

 As shown in the accompanying diagram (fig. 15) 

 these generic phyla are branches of the family tree of 



has adopted in expressing the relationships and 

 descent of the rhinoceroses, animals whose evolution 

 presents in many respects analogies to the evolution 

 of the titanotheres, especially in the modes of the 

 evolution of horns, in the loss or retention of cutting 

 teeth (incisors), and in the adaptations of limb struc- 

 ture to swift and slow movement. 



RELATION OF THE PHYLOGENETIC CLASSIFICATION TO 

 THE LINNAEAN CLASSIFICATION 



Linnaeus described one or more species of mammals 

 geographically distributed in space (see table on p. 16), 



FiGUEE 13. — Phenacodus (A) and Coryphodon (B) drawn to the same scale 

 Restorations made by Charles R. Knight under the author's direction. 



the titanotheres. When two of these branches run 

 close together they may for convenience be united into 

 a single subfamily. Thus, for purposes of description 

 the graphic presentation of the titanothere family tree 

 in the accompanying diagrams may be supplemented 

 by the systematic subdivision of these animals into 

 12 subfamilies and 24 genera, as shown on a subse- 

 quent page. 



The free use of subfamily divisions to distinguish 

 the branches of Eocene and Oligocene generic phyla 

 from one another is similar to that which the author 



whereas the phylogenetic classification of the titano- 

 theres covers species extending over both space and 

 time. The geographic range of the existing red deer 

 (Cervus) and of the extinct titanotheres lies within 

 the same zoogeographic region — Holarctica, which 

 includes Europe, Asia, and North America. The 

 comparison is therefore significant. (See table on 

 p. 18.) 



The classification presented in this monograph is 

 more than phylogenetic: it is polyphyletic. Lin- 

 naeus (1758.1), when he wrote the several editions 



