SUCCESSION OF FAUNAS 



153 



Upper 

 Triassu 



Middle 

 Jurassic 



Upper 

 Jurassic 



Upper 

 Cretaceous 



X- 

 X- 



-X- 

 -X 



-X- 



-X- 



Teriiary 

 X 



-X 

 X 



X- 

 X- 



X- 

 X 



-X 



-X 



X 



X 



X 



-X 



X 

 X 



X 



Multitubcrculata 



Tritylodontidae 



Microcleptidae 



Plagiaulacidae 



Ptilodontidae 



Taeniolabididae 

 Triconodonta 



Triconodontidae 

 Symmetrodonta 



Spalacotheriidae 



Amphidontidae 

 Pantotheria 



Amphitheriidae 



Paurodontidae 



Dryolestidae 



Docodontidae 

 Marsupialia 



Didelphiidae 

 I nsectivora 



Zalambdalestidae 



Leptictidae 



Deltatheridiidae 



At first glance several broad facts regarding the known succession are notable. 

 The poorly known upper Triassic fauna does not contain the ancestors of any later 

 mammals with the possible exception of the single genus Stereogiiathus of the middle 

 Jurassic. The middle and upper Jurassic faunas of England are, to be sure, rather dif- 

 ferent but there is reason to believe that they were essentially continuous and that the 

 differences are largely due to the considerable lapse of time and to the imperfection of 

 our knowledge. The break between the upper Jurassic and the upper Cretaceous is vast. 

 Except for the transitional steps between reptiles and mammals, this is far the greatest 

 remaining gap in the knowledge of mammalian history. The break between the upper 

 Cretaceous and the Paleocene is slight from an evolutionary point of view, the known 

 Cretaceous groups mostly continuing with but slight advances in structure, but it is 

 strongly marked in the known areas by the introduction from some unknown source 

 of numerous new mammalian groups, not shown in the table. 



Further discussion of the Triassic fauna and of its relationships to the successive 

 waves of expansion within the order would be futile. It may represent a primary 

 spreading of non-ancestral groups from the center of mammalian origin, but it is so 

 poorly known that it offers no valid grounds for conclusions, either positive or nega- 

 tive, save that mammals of a sort were then in existence but that the known forms have 

 no significance with regard to the origin of later mammals. 



