ANCESTRY OF THE MAMMALIA DO 



supials. The most distinctive trait of the Multituberculates is 

 the many-cusped appearance of the molar teeth. Since these 

 animals existed at a period so early in mammalian history and 

 because of the fact that the probable ancestors of modern Mam- 

 mals when first encountered* present molars with several cusps 

 it has been suggested that the existing mammalian dentition 

 may have been derived from that of a primitive ancestor with 

 many-cusped molars. This hypothesis is known as the Primi- 

 tive Polybuny Theory. Later investigation has shown that the 

 Multituberculate dentition probably is either of independent 

 origin or derived by reduction from the primitive Prototherian 

 dentition, and although some modern Mammals recall this type 

 in certain features, the features in question are in reality sec- 

 ondary results of convergent evolution. The Multituberculates 

 therefore are no longer considered as the parent stem of modern 

 mammalian forms. 



THE JURASSIC MAMMALS 



It is in the next geological period, the Lower Jurassic, that we 

 find the first examples of mandibles which we can be sure are 

 mammalian and they indicate immediately that we are once 

 again observing Nature's experiments. The Jurassic Mammals 

 clearly fall into two categories each quite distinguishable from 

 the other, the Triconodonta and the Trituberculata. Of these 

 as of so many other fossil animals the mandible is the bone by 

 far most commonly found. 



The Triconodonta 



In the mandibles of certain Triconodonts illustrated in Fig. 

 14 can be observed a low lying sessile condyle recalling in its 

 position the articulation of the Cynodonts, a broad coronoid 

 process and an angle more or less inflected as in recent Mar- 

 supials. The teeth of Trieonodon show three cones arranged 

 in an antero-posterior line. Of these the middle one tends to be 



*See section on Trituberculates. 



