LATER MESOZOIC INVERTEBRATE FAUNAS 187 



EARLY CRETACEOUS FAUNAS 



At the beginning of the Cretaceous the two faunal provinces which 

 have just been indicated were even more sharply defined than they 

 had been in the Jurassic, and in each area the characteristic elements 

 of the fauna are developed from the fauna that preceded it. The 

 Shasta fauna on the one hand and the Comanche fauna on the other 

 are always sharply contrasted, though each exhibited several facies. 

 When compared with European faunas, one is in the beginning chiefly 

 boreal and the other Mediterranean; one is associated with shales, 

 sandstones, and conglomerates, the other, mainly with limestones. 



Shasta faunas.— The boreal Aucella fauna of the Knoxville for- 

 mation is the earliest one in the Shasta series. It is distributed from 

 the Arctic coast of Alaska to southern California but south of the 

 Yukon never extending as far east as the late Jurassic fauna did 

 Cretaceous Aucella beds have been reported from Catorce, San Lui^^, 

 Potosi, Mexico, but it is probable that these are Jurassic on about 

 the same horizon at which Aucella occurs at Mazapil. 



The succeeding Horsetown fauna though at first showing a transi- 

 tion from the Knoxville fauna is, as a whole, remarkably distinct 

 from it. It is characterized by the great abundance and variety of 

 ammonites of types which in Europe are considered distinctive of 

 the deeper water facies of the Mediterranean province. The boreal 

 element is wanting, or at least inconspicuous. This early Horsetown 

 fauna is much less widely distributed than the Knoxville. In its 

 typical development it is known in a relatively small area in northern 

 California and in Oregon. Toward the close of the Horsetown the 

 fauna was greatly modified by the introduction of many types that 

 show relationship with the Cretaceous faunas of southern India and 

 also with those of Japan. This relationship was continued in the 

 succeeding Upper Cretaceous faunas to such an extent that it is 

 appropriate to speak of an Indo-Pacific province or region. This 

 later Horsetown fauna was more widely distributed along the Pacific 

 border and is well developed as far north as the Queen Charlotte 

 Islands. 



The marked change at the close of the Knoxville when the fauna 

 ceased to have a distinctively boreal character was probably due to a 

 northern uplift which closed Bering Strait and other direct connections 



