A NEW MIDDLE TRIASSIC FLAT CLAM (PTERIOIDA: HALOBIIDAE) FROM THE MIDDLE ANISIAN OF NORTH-CENTRAL NEVADA, USA

Abstract

In addition to ammonoids and conodonts, the enigmatic and poorly understood “flat clams” belonging to the genera Enteropleura Kittl, 1912, Daonella Mojsisovics, 1874, Halobia Bronn, 1830, and Monotis Bronn, 1830 have proven to be of biochronologic value through the Middle and Upper Triassic (e.g., Silberling and Nichols, 1982; Brack and Rieber, 1993; Campbell, 1994; McRoberts, 1997). These “flat clams,” many belonging to the family Halobiidae, can be extremely abundant in the marine Triassic rocks of the former Panthalassic Ocean and especially the Tethyan Seaway, where many of these species were first described. Their widespread nature and high speciation rates make these bivalves exceptional biochronologic indicators, especially when integrated with the temporal distribution of other, more conventional indices such as ammonoids and/or conodonts. We describe and name a poorly documented halobiid species, Enteropleura jenksi n. sp., from the latest middle Anisian and therefore provide a robust correlation datum for the Shoshonensis Zone. #### Geologic and stratigraphic setting The fossils described herein occur near McCoy Mine located in the Wild Horse Mining District, on the northeast side of the New Pass Range of north-central Nevada (Fig. 1). These fossils occur in two closely spaced stratigraphic horizons within the Middle Triassic Fossil Hill Member of the Favret Formation. Both at McCoy Mine and at its type locality in the Prida Formation at Fossil Hill on the east flank of the Humboldt Range (e.g., Muller et al., 1951; Nichols and Silberling, 1977; Silberling and Nichols, 1982), the Fossil Hill Member is composed of dark-grey to black, thin-bedded limestone, cherty limestone, and calcareous shale. At the McCoy mine locality, these strata were mapped as an informal unit of calcareous shale and thin-bedded limestone by Dane and Ross (1942). Neither the upper nor lower contact of the Fossil Hill Member is exposed …