280 C. SCHUCHERT MORRISON AND TENDAGURU FORMATIONS 



We have long been holding that land vertebrates, living under more 

 variable conditions, change far more quickly than do the marine inverte- 

 brates of long-enduring equable environments, and yet in the present case 

 hardly any of the species of the Trigonia smeei beds continue into the 

 time of T. schwarzi. There are only five marine bivalves known to bridge 

 this time, a class of long-lived animals and as such having the least value 

 as zonal markers; yet BracMosaurus brancai, B. fraasi, and the genera 

 Dicrceosaurus and Kenfrosaurus lived longer, across the time from Ju- 

 rassic into Lower Cretaceous. If the dinosaurs are correctly determined, 

 then all of the invertebrates of the T. smeei zone should be reinvestigated 

 to see if some of them can not be referred to the Lower Cretaceous and 

 so brought into harmony with the evidence of the dinosaurs, or the latter 

 seemingly must be shown to be dissimilar in the two upper horizons. 

 Under these circumstances, it appears best, at least for the present, to 

 folloAv the suggestion of Buckman, who regards the middle and upper 

 dinosaur zones as of Upper Jurassic age. This does not necessarily mean 

 that there is a time break between the upper dinosaur and the T. schwarzi 

 zones, though the present writer is inclined to look on the available evi- 

 dence as indicating a hiatus here between the Jurassic and the Lower 

 Cretaceous. 



On the other hand, it is plain that the faunas of the upper marine 

 horizons and the two upper dinosaur zones are more closely related to one 

 another than they are to those of the Nerinea and lower dinosaur hori- 

 zons. The writer therefore raises the further question whether the Ten- 

 daguru is actually a continuous series of deposits or is again broken at 

 the top of the Nerinea zone ? The fauna of the Nerinea beds is a small 

 one, with forms that have little stratigraphic value, and this nearly all of 

 the paleontologists concerned with the work have noticed. As for the 

 dinosaur evidence in the lowermost zone, it is as yet unknown other than 

 that they are all small, and this is a very significant fact ; all large sauro- 

 pods appear to be absent. The writer therefore thinks that the evidence 

 as it now stands indicates rather that the lower dinosaur zone and the 

 J^erinea beds are of earlier Jurassic time, followed by a break in the 

 Tendaguru series. Then come the two higher marine and the two main 

 dinosaur horizons that may be of continuous deposition and, if so, bridge 

 over the time from the Jurassic into the Lower Cretaceous. 



