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Northern Sierra del Alamo (Triassic of Mexico)

Also known as Mina el Antimonium

Where: Sonora, Mexico (30.8° N, 112.6° W: paleocoordinates 6.6° N, 47.4° W)

• coordinate stated in text

• small collection-level geographic resolution

When: Tropites dilleri ammonoid zone, Lower Member (Antimonio Formation), Carnian (235.0 - 221.5 Ma)

• "Co-occurrence of the ammonoid Tropites subbullatus and the Sonoran ichthyosaur specimens was recognized by Smith in 1925 (King, 1939) and by Keller (1928). Smith additionally reported T. dilleri within the same layer. Working with faunas from the Hosselkus Limestone of Shasta County, California, Smith (1927) subdivided the T. subbullatus Zone into a lower "Trachyceras" Subzone and an upper" Juvavites" Subzone. These were changed, respectively, to the Dilleri Zone for the lower Upper Carian, and the Welleri Zone for the middle Upper Carnian (Silberling, 1956, 1959; Silberling and Tozer, 1968). The holotype of Shastasaurus altispinus was reported by Merriam (1908) as occurring in the "Trachyceras" beds (=present Dilleri Zone) of the Hosselkus Limestone in Shasta County. The presence of T. dilleri with UCMP 27141, 27142, and 27146 places them within the Dilleri Zone also; therefore, the holotype and the Sonoran specimens are both early Late Carnian in age."

• bed-level stratigraphic resolution

Environment/lithology: marine; lithified, red, sandy, calcareous mudstone

• "The unit is composed of red calcareous and sandy mudstones, thin beds of very fine-grained red calcareous and arkosic sandstones, and thin interbeds of red limestone. The ichthyosaur bones have a dark reddish hue derived from the matrix."

Size class: macrofossils

Collected by C. L. Baker in 1925; reposited in the UCMP

Primary reference: J. M. Callaway and J. A. Massare. 1989. Shastasaurus altispinus (Ichthyosauria, Shastasauridae) from the Upper Triassic of the El Antimonio District, northwestern Sonora, Mexico. Journal of Paleontology 63(6):930-939 [M. Carrano/H. Street] more details

Purpose of describing collection: general faunal/floral analysis

PaleoDB collection 102472: authorized by Matthew Carrano, entered by Hallie Street on 15.01.2011, edited by Jonathan Tennant

Creative Commons license: CC BY (attribution)