TRIASSIC 603 



ties, its nearest relatives being found in India and Siberia. It is there- 

 fore a reasonable inference, both from the faunal relationship and the 

 - areal distribution of the deposits, that there was no direct connection 

 between Atlantic and Pacific waters through the Mexican-Central Amer- 

 ican region. A later fauna in the Lower Triassic, the Tirolitic fauna, is 

 known only in the Mediterranean region and in Idaho, and hence an open 

 route of migration through the Central American region has been in- 

 ferred, but no deposits or other direct evidence in support of this inference 

 are known. 



The facts are similar for the Middle Triassic when the sea retreated 

 still farther, so that the known marine deposits are restricted to eastern 

 California, central Nevada, and British Columbia. In his recent mono- 

 graph of the Middle Triassic marine invertebrate faunas of North Amer- 

 ica, J. P. Smith states that "the faunas of the American and of the 

 Mediterranean regions during the Middle Triassic are more closely re- 

 lated to each other than either is to the Indian or to the Boreal fauna.'' 

 He further says that "in the zone of Oeratites trinodosus, in the West 

 Humboldt Eange of Nevada, out of more than 100 species more than 

 one-fourth are either identical with or very closely related to forms from 

 this zone in the Mediterranean region. It is possible that during the 

 Middle Triassic a connection was established between these regions 

 through some other way than the Indian branch of the old central Med- 

 iterranean, or 'Tethys.' " In another paper he definitely states that the 

 Central American portal was reopened at this time. Again, the oceanic 

 connection is based on faunal relationships of distant regions only. 



During Upper Triassic time the Mediterranean element in the Califor- 

 nia faunas continues large and is especially predominant in the Tropites 

 subhullatus fauna of the Hosselkus limestone, which is referred to the 

 Karnic epoch. The only marine Triassic deposits known in North Amer- 

 ica south of the United States are also of Karnic age and have yielded a 

 meager fauna which is closely related to that of the Mediterranean on 

 the one hand and to that of California on the other. These beds are in 

 central Mexico, near the city of Zacatecas, with possibly another area 

 southeast in Guanajuato. On account of these faunal and geographic 

 relations, Burckhardt has suggested a marine connection between the 

 Atlantic and Pacific across central Mexico, and Smith has inferred an 

 open "Central American portal." That there was a passage somewhere 

 between Panama 4nd central ^lexico seems reasonable. 



The Upper Triassic submergence was of brief duration, and not long 

 thereafter, either in latest Triassic or earliest Jurassic time, or possibly 

 in both, there was an epoch of continental sedimentation during which, 



