SUCCESSION AND DISTRIBUTION OF LATER MESOZOIC 
INVERTEBRATE FAUNAS IN NORTH AMERICA 
T. W.. STANTON 
IX 
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 itis 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 was 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 FAUNAS 
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 
the northwest corner of Colorado, part of Idaho, and a considerable 
« A full discussion of the marine Trias may be found in the published writings of 
Professor James Perrin Smith. See especially Proc. Cal. Acad. Sci., 3d Ser., ‘‘Geol- 
ogy,” Vol..I, No. 10, 1904; and Von Koenen Festschrift, pp. 377-434, 1907- 
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