696 GEOLOGY 



Europe during the Permian period, it lingered about the Mediter- 

 ranean, in Russia, Turkestan, and northwestern India, and probably 

 on the continental platform in or near Siberia. The Mediterranean, 

 the Himalayan, and the Siberian regions are the best known tracts 

 into which the shallow-water marine life of the late Paleozoic 

 retreated and underwent transformation into the early provincial 

 faunas of the Mesozoic. It is quite certain that there was at least 

 one other area where important faunal reorganization took place 

 at about this time, for a notable fauna suddenly appeared in the 

 Middle Triassic, which does not seem to have originated in any of 

 these three districts. 



In each of these three areas an important remnant of Paleozoic 

 sea life seems to have undergone a radical and perhaps rather rapid 

 evolution, such as might be anticipated from the crowding of the 

 great faunas of earlier times into such limited areas. From these 

 areas the new faunas spread when the sea again extended itself upon 

 the land. 



The transition faunas. The most complete record of the tran- 

 sition from Paleozoic to Mesozoic marine life is found in India. Beds 

 containing fossils characteristic of the Permian are overlain con- 

 formably by beds containing forms characteristic of the Mesozoic. 

 In the Permian beds there are forms foreshadowing the Mesozoic 

 types, and in the beds above there are Permian types that lived on 

 and mingled with Mesozoic forms. The transition fauna of the 

 Mediterranean region appears to have been less rich than that of 

 India. Concerning the early stages of the Siberian fauna, little 

 is known; but its peculiarities, as revealed in a later stage of the 

 early Trias, leave little doubt of its independence of origin. 



General nature of the faunal changes. In nearly all Paleozoic 

 faunas, brachiopods were a leading element of the marine faunas, 

 while trilobites, crinoids, and corals, each in turn, gave distinctive 

 character to the successive faunas. In the Mesozoic era, the 

 ammonites took the first place, followed by the pelecypods and 

 the gastropods. The ammonites (Fig. 473) are peculiarly fitted 

 for distinguishing successive horizons, not only because they 

 were free forms, measurably independent of bottom conditions, 

 but because they were steadily and rapidly advancing in organ- 



