120 Diener — Critical Phase in History of Ammonites. 



Art. X. — A Critical Phase in the History of Ammonites ; 

 by C. Diener. 



The extinction of ammonites, those masters of the 

 Mesozoic seas, near the close of the Cretaceous period 

 is a fact well known to all students of palaeontology. 

 The number of their families and genera is diminishing 

 gradually during the Senonian epoch. Five species only 

 reach into the stage of the Maestrichtian. Not one passes 

 the fatal border of the Danian. 



It is, however, less known, that the existence of ammon- 

 ites was threatened by a similar crisis at a considerably 

 earlier period of the Mesozoic era. They passed through 

 a very critical phase at the boundary of the Ehaetic and 

 Liassic stages. All but one phylum of Triassic ammon- 

 ites became extinct at the close of the Ehae tic epoch. By 

 the survival of this single phylum, which in the Lower 

 Lias gave rise to the development of a new and rich fauna, 

 the ammonites were saved from complete extermination. 



E. v. Mojsisovics was the first to notice this remarkable 

 crisis in the history of Triassic ammonites. For fuller 

 details the reader is referred to J. F. Pompeckj, "Am- 

 moniten des Ehaet" (Neues Jahrb. f. Mineral., etc., 

 1895/11, pp. 1-46) and to some of my own memoirs. 



The Upper Triassic deposits of Tethys are divided 

 generally into three subdivisions, the Carnic, Noric and 

 Ehaetic stages. E. v. Mojsisovics divided both the Carnic 

 and Noric stages into three substages, thus imparting to 

 the Ehaetic stage a taxonomic value inferior to that of 

 the two preceding ones. Many genera belonging to all the 

 known families of Upper Triassic ammonites reach 

 the acme of their development in the Carnic stage. 

 Although a considerable number of older genera are 

 found for the last time at this level, the ammonite fauna 

 of the Noric stage is a continuation and evolution of the 

 Carnic fauna in every branch of life. The last life phase 

 of the Noric stage seems to be the first which is distin- 

 guished from the preceding by the apparent extinction 

 of numerous wide-spread and important genera and by 

 the absence of any new elements either of foreign origin 

 or derived from endemic forms. Nevertheless it is doubt- 

 ful whether a single family of lower Noric ammonites 

 becomes really extinct. 



