PHYLOGENY AND RELATIONSHIPS 



143 



same process of diversification and selection on a smaller scale. The unsuccessful or 

 only temporarily successful lines become extinct and only their fossil remains are 

 found to puzzle the taxonomist. They will not enter into the divisions of classifications 

 based, as classifications almost always have been, only on the much better known sur- 

 viving lines. Within the subclass Eutheria this is clearly seen in the Paleocene and 

 Eocene mammals. In addition to the various groups which are ancestral to later mam- 

 mals and which are relatively easy to classify in our present system, there are others 

 which can only be characterized as aberrant and extinct side lines — aberrant, usually, 

 only in the sense that they differ from the more successful and more familiar phyla. 



This apparently was equally true of the whole class in its early history. Of the 

 four major groups in the Jurassic, — Multituberculata, Triconodonta, Symmetrodonta, 

 and Pantotheria, — only one, the Pantotheria, shows evidence of having any signifi- 

 cance in regard to Tertiary mammals. This order (in a broad sense, not necessarily 

 any actually known family or genus) was the only potentially successful and fruitful 

 Jurassic group and was destined to give rise to most or all recent mammals (save 

 monotremes). 



CENOZOIC 



MONOTREMAIA 



MARSUP 

 ^ 



ALIA PLACENTALIA 



ME50Z01C 



1 



SYMMETRODi 



TRJCONODONTA 



1ULTITUBERCULATA 



MAMMALS 



'reptile's' 



PANTOTHERIA 



CYNODONTIA 



Fig. 58. Suggested relationships of the major groups of mammals. 



The multituberculates were independently derived from the ultimate mammalian 

 common ancestry, probably within a group which must be called reptilian by defini- 

 tion. They had their own great and eventful history throughout the Mesozoic and 

 Paleocene and even into the true Eocene, but they then became extinct. They are rela- 

 tively well known and it is justifiable to place them in a distinctive subclass, the 

 Allotheria. 



The triconodonts represent another early side line, probably of equally remote 

 derivation and equally independent of other mammals. Their history, while not short 

 even in comparison with a number of Tertiary orders, was more limited and less event- 



