Dienerian (Early Triassic) ammonoids from the Candelaria Hills (Nevada, USA) and their significance for palaeobiogeography and palaeoceanography

摘要

A well-preserved ammonoid fauna of Early Dienerian age has long been known from the lower portion of the Candelaria Formation in the old Candelaria silver mining district in Mineral and Esmeralda Counties, Nevada, but for a number of reasons, this fauna has never been studied in detail nor illustrated. Previous authors assigned this ammonoid fauna to the Early Dienerian Proptychites candidus Zone of Canada. In reality, it more closely resembles the Tethyan faunas than the higher palaeolatitude Canadian faunas, thus indicating the presence of some degree of equatorial faunal exchange between opposite sides of the Panthalassic Ocean during Early Dienerian time. It also indicates the onset of a provincialism, which contrasts with the cosmopolitan Griesbachian faunas. A rigorous taxonomic analysis of the Candelaria fauna allows us to differentiate the following ten species, which include two new species and one new genus (Mullericeras nov. gen.) belonging to the new family Mullericeratidae: Ambites lilangensis (Krafft, 1909), Ambites aff. radiatus (Brühwiler, Brayard, Bucher and Guodun, 2008), Ussuridiscus sp. indet., “Koninckites” aff. kraffti Spath, 1934, Mullericeras spitiense (Krafft, 1909), Mullericeras fergusoni nov. sp., Mullericeras sp. indet., Proptychites haydeni (Krafft, 1909), Proptychites pagei nov. sp., Vavilovites sp. indet. and Parahedenstroemia kiparisovae Shigeta and Zakharov, 2009. This Early Dienerian fauna correlates with the Ambites fauna known from the base of the Ceratite Marls in the Salt Range and from the base of the “Meekoceras” beds in Spiti (northern Gondwanian margin). The fauna also permits the precise dating of a shelfal anoxic episode on the equatorial North American margin. This anoxic event correlates in time with similar palaeoceanographic changes in the southern Tethys, which indicates that the Early Triassic biotic recovery was at least partly shaped by such discrete, short events rather than by pervasive and lingering adverse environmental conditions.