CHAPTER IX 



SUCCESSION AND DISTRIBUTION OF LATER MESOZOIC 

 INVERTEBRATE FAUNAS IN NORTH AMERICA' 



T. W. STANTON 



EARLIER MESOZOIC FAUNAS 



In early Mesozoic time the marine invertebrate faunas of North 

 America were closely confined to the borders of the present continent, 

 and particularly to the western border. The land-area, or at least 

 the area above sea-level, was nearly as large as it is now. The early 

 Triassic sea with a rich ammonite fauna extended as far as eastern 

 Idaho but its area was apparently restricted and its most probable 

 connection with the ocean w^as through Utah, Nevada, and southern 

 California. Later Triassic marine faunas are not known east of 

 western Nevada and eastern Oregon in the United States. They 

 occur also at many localities in British Columbia and Alaska, and in 

 a very limited area near Zacatecas, Mexico. The occurrence of 

 fresh-water shells (Unio) in the Upper Triassic of New Mexico, and 

 the character of the vertebrate remains found there and at other points 

 farther north, attest the non-marine character of the Triassic deposits 

 in the Rocky Mountain region. The scanty invertebrates found in 

 the Newark group of the east also indicate non-marine deposits. 

 Early Jurassic (Liassic) faunas are apparently restricted to an area 

 still smaller than that of the marine Trias. ^ 



LATE JURASSIC FALTNAS 



Marine fauna. — At or near the close of the Middle Jurassic the 

 sea again invaded the continent and covered a large part of the Rocky 

 Mountain region. It extended east to the Black Hills, south to south- 

 ern Utah, and covered much of Montana, Wyoming, and Utah, with 



' Published by permission of the Director of the U. S. Geological Survey. 



2 A full discussion of the marine Trias may be found in the published writings of 

 Professor James Perrin Smith. See especially Proc. Cat. Acad. Sci., 3d Sen, "Geol- 

 ogy," Vol. I, No. 10, 1904; and Von Koenen Festschrift, pp. 377-434, 1907. 



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