124 Diener — Critical Phase in History of Ammonites. 



many beds of the Ladinic or Noric stages. Our knowl- 

 edge of Rhaetic ammonites is certainly not more limited 

 than that of Permian ammonites after the discovery of 

 the Artinsk and Sosio fauna. There is, consequently, as 

 much evidence of a decline of the group during the Rhsetic, 

 as there is of a decline of the trilobites during the Car- 

 boniferous and Permian. 



Such are the facts. They show us a great dying-out 

 of ammonites towards the close of the Triassic and a 

 rebirth, as it were, of a new fauna in the early Liassic, 

 giving rise to the great wealth of Jurassic ammonite 

 evolution. In entering into a discussion of the probable 

 causes of this remarkable event in the life history of 

 ammonites, we have to face the grave problem of the 

 repeated extinction of large and flourishing groups of 

 organisms. That this extinction has been partial only, 

 affecting all but one stock of Triassic ammonites, marks 

 the special case of our problem. 



If we reflect on the multitude, the variety, and the 

 complexity of the facts to be explained, and the scantiness 

 of our information regarding them, we shall be ready to 

 acknowledge that a full and satisfactory solution of so 

 profound a problem is hardly to be hoped for, and that 

 the most we can do in the present state of our knowledge 

 is to hazard a more or less plausible conjecture. 



In discussing the possible causes of the decay and final 

 extermination of Upper Triassic ammonites, it will be 

 best to follow the lines which have been traced by H. P. 

 Osborn in his memoir on the causes of extinction of 

 Mammalia (Amer. Naturalist, XL, 1906, p. 769). 



Changes of Geographical Conditions. — The Triassic 

 was on the whole a geocratic (land) period. A transgres- 

 sion of marine Rhaetic beds is confined to the coasts of 

 western Europe. It is counterbalanced by a regression 

 of the sea in southern China, Japan, North and South 

 America, where the Rhaetic stage is represented by 

 deposits of terrestrial and lacustrine origin only. There 

 is but a small change in the distribution of land and sea 

 during the Lower Lias, which in some regions of the 

 ancient geosynclines is marked by a transgression of a 

 rather limited range. The extinction of Triassic ammon- 

 ites consequently does not admit of an explanation by 

 changes of geographical conditions. 



Changes of Climate.— The importance of this factor has 



