Smithian (Early Triassic) Cephalopods

During the Early Triassic rocks of the Thaynes Formation were deposited in the Sonoma Foreland Basin. Fossil Cephalopods are common in these rocks in western Utah, eastern Nevada and southeastern Idaho. Late in the Smithian the sea spread eastward onto the Moenkopi mudflats and the Sinbad Limestone Member of the Moenkopi Formation was deposited.

References: Mathews, Asa A. L., 1929, The Lower Triassic Cephalopod Fauna of the Fort Douglas Area, Utah, Walker Museum Memoirs Vol.1 No.1 University of Chicago Press; Hose, R. K., and Repenning, C. A., 1959, Stratigraphy of Pennsylvanian, Permian, and Lower Triassic Rocks of the Confusion Range, West-Central Utah, A.A.P.G. Bulletin vol. 43, no. 9; Tozer, E. T., 1971, Triassic Time and Ammonoids: Problems and Proposals, Canadian Journal of Earth Science, 8; Tozer, E. T., 1994, Canadian Triassic Ammonoid Faunas, GSC Bulletin 467; D. A. Stephen, K. G. Bylund, P. J. Bybee and W. J. Ream, 2010, Ammonoid Beds in the Lower Triassic Thaynes Formation of western Utah, USA, in: Cephalopods – Present and Past, edited by K. Tanabe, Y. Shigeta  and T. Sasaki & H. Hirano. Tokai University Press, Tokyo, p. 243-252

In the Thaynes Formation of the Confusion Range in western Utah are three assemblages of Smithian "Ayaxian" ammonoids. The lower two probably represent the Hedenstroemia bosphorensis / "romunderi" Zone with Meekoceras, Arctoceras and Aspenites in the lower beds, and Inyoites, Lanceolites and Pseudosageceras above. The upper assemblage contains Anasibirites and Wasatchites, representative of the Anasibirites nevolini / "tardus" Zone.  These are just assemblages as I see them in the beds in west-central Utah and are by no means considered biozones.

Meekoceras beds      Inyoites beds       Anasibirites beds

Anasibirites Beds Study

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