Original paper
Early Carnian ammonoids from China Mountain (Tobin Range, Nevada, USA)
Marchesi, Ruben; Balini, Marco; Jenks, James F.
Palaeontographica Abteilung A Band 325 Lieferung 1-6 (2023), p. 69 - 109
45 references
published: Jan 30, 2023
published online: Aug 8, 2022
manuscript accepted: Apr 4, 2022
manuscript received: Dec 2, 2021
Abstract
We herein document a newly reported Early Carnian ammonoid fauna from China Mountain (Tobin Range, north-central Nevada, USA). Although the general locality was reported in the 1970s as representing the second site in North America with an ammonoid fauna belonging to the lowermost Carnian Trachyceras desatoyense Zone, neither the section nor the ammonoid fauna has ever been described. The area was surveyed in 2018, and the stratigraphic succession, i.e., the Panther Canyon and Smelser Pass members of the Augusta Mountain Formation and the overlying Cane Spring Formation, is herein described. The fossil-bearing interval occurs in the uppermost portion of the Smelser Pass Member, just a few meters below the unconformity at the base of the Cane Spring Formation. Four fossiliferous localities (A–D) were found, of which two (B and C) yielded many well preserved ammonoids. Taxonomic studies of these specimens provided unexpected results, namely that the fauna is dominated by Cryptomorphiceras, a new genus of the subfamily Trachyceratinae, which occurs with very rare specimens of Perrinoceras Badiotites Silenticeras Trachyceras and Clionitites Cryptomorphiceras n. gen. includes C. compressum (Johnston, 1941; type) from South Canyon (New Pass Range) and C. crassum n. sp., C. levicostatum n. sp. and C. sp. ind. from China Mountain. Cryptomorphiceras n. gen. is characterized by a weakly furrowed venter, which is crossed by ribs and a suture line with a shallow external lobe and wavy to weakly indented saddles. The internal mold of the new genus exhibits weakened ornamentation with respect to the outer surface of the test, due to the peculiar, triple-layered structure of the test. An earliest Carnian age assignment of the China Mountain ammonoid fauna is based on the occurrence of Cryptomorphiceras n. gen. at South Canyon, where the new genus is confined to the lowermost Trachyceras silberlingi chronozone. Based on its features, Cryptomorphiceras n. gen. does not occupy a clear position within the evolutionary trend of Muenster- ites → Daxatina → Trachyceras, the only trend thus far recognized in the Trachyceratinae across the Ladinian/Carnian boundary. The evolution of this group of ammonoids is more complex than previously thought, but ongoing investigations at South Canyon are expected to provide more information for a better understanding of this issue.
Keywords
: Upper Triassic • Carnian • Taxonomy • Ammonoids • Chronostratigraphy