Early Dienerian ammonoids and nautiloids and a new late Griesbachian ammonoid site from the Dinwoody Formation (Thaynes Group) at Crittenden Springs, Elko County, Nevada.

Abstract

International audienceOngoing exploration in the Dinwoody Formation of the Thaynes Group at Crittenden Springs, Elko County, Nevada, motivated by the earlier reported discovery of late Griesbachian ammonoids and nautiloids, has led to the discovery of an early Dienerian ammonoid and nautiloid assemblage as well as a new late Griesbachian ammonoid site. The early Dienerian ammonoid fauna includes the genus Gyronites, heretofore known only from the Tethys and South Primorye, and the Crittenden Springs assemblage represents the only low-paleolatitude early Dienerian ammonoid and nautiloid occurrence in eastern Panthalassa. Early Dienerian ammonoid taxa include Gyronites rigidus (Diener), Gy. frequens Waagen, Gy. bullatus Ware and Bucher, Ussuridiscus varaha (Diener), Ghazalaites roohiae Ware and Bucher, Proptychites sp. and gen. et sp. indet. This assemblage correlates broadly with the early Dienerian of the Northern Indian Margin (NIM: Salt Range, Pakistan, Spiti, Himalaya and South Tibet, China). Nautiloids include Xiaohenautilus mulleni Jenks et al. and a new taxon, i.e., Xiaohenautilus chatelaini n. sp. The new late Griesbachian ammonoid site has yielded Wordieoceras wordiei (Spath) and W. mullenae Jenks et al. Several early Dienerian and late Griesbachian conodont taxa, including Sweetospathodus kummeli, the conventional basal Dienerian index marker, have been recovered from the study area. However, the validity of this taxon as a marker for the G/D boundary is questioned because of its first occurrence (FO) ~30 m above the FO of basal early Dienerian ammonoids