TENDAGURU SERIES 267 



Hennig^^ is convinced that there is no break in deposition in the Ten- 

 daguru series, and that the three dinosaur horizons are intimately united 

 by transition zones with the three interbedded sandstones that are clearly 

 of marine origin. The transition zones are proved to be such not only by 

 their petrographic nature and transitions, but by their included faunas 

 as well. The whole consists of both marine and brackish-water deposits, 

 though the latter in the proximity of rivers may have been completely of 

 fresli- water origin. Ammonites persist longest in the transition zones, 

 but most often it is the guards of belemnites which occur, and they may 

 be as common as the associated dinosaur ])ones. Hennig writes :-^ 



"The vanishing of the generally varied and rich life of the marine horizons 

 is somewhat sudden toward the dinosaur zones. There appears to be a re- 

 markable loss of marine forms : aside from a very few molluscs, there are 

 only belemnites, and these persist into the base of the dinosaur zones. They, 

 too, then disappear, and of nmrine inhabitants that go to disprove the con- 

 tinental origin of the dinosaur beds there are, besides [ganoid] fishes, a few 

 [brackish- water] bivalves. Of these quite a number were found, but this is 

 mainly due to the extensive quarrying for dinosaurs. We have here a Mytilus 

 and one or more species of Cyrena. The former is a marine shell, while the 

 latter is a brackish-water genus, though both may occur in either habitat." 

 "In particular abundance and at times in colonies so as to make entire beds 

 within the dinosaur zones occur cyrenas and less commonly Mytilus. The 

 former are especially common at the base of the upper dinosaur zone, and the 

 latter at the base of the middle dinosaur zone. ... In the transition beds 

 between the Nerinea and middle dinosaur zones, and even at the base of the 

 latter, in connection with skeleton p, were associated nests of [marine] snails. 

 Locally throughout the upper dinosaur zone occur very rarely genera of [pul- 

 monate] snails. . . . Highly interesting was the occurrence of a small 

 marine fauna located within the ribs of a great sauropod at Mtapaia. This 

 skeleton was found at the top of the middle dinosaur zone, practically in the 

 transition bed, for immediately above it came the hard sandstone of the Tri- 

 gonia smeei horizon, rich in marine shells." In fairly typical dinosaur clay 

 there was found, in one case only, a single bone overgrown with oysters. 



In the marine zones the species of Trigonia play an important part and 

 are good guide fossils if one takes the time of their greatest individual 

 abundance as the zonal marker. The species themselves may have a long 

 time range, passing even into other zones (T. smeei is rare in the T. 

 srJiwarzi and Nerinea zones), but the time when they make up whole beds 

 of shells occurs but once. The gastropod Nerinea is often abundant, but 

 is not a trustworthy guide.^*^ Dinosaur bones are of very rare occurrence 



28 Op. cit, part iii, pp. 164-166. 



2» Op. cit, part ii, pp. 17-18. 



=*o Hennig : Op. cit., part ii, pp. 15, 16. 



