Smithian (Olenekian, Early Triassic) Conodonts from Ammonoid-Bearing Limestone Blocks at Crittenden Springs, Elko County, Nevada, USA

Abstract

The ammonoid-bearing limestone blocks at the classic Crittenden Springs ammonoid site belong to the lower part of the Lower Triassic Thaynes Group. These 0.5–1.2 m thick blocks, consisting mainly of bioclastic floatstone and rudstone, contain abundant macro- and micro-fossils such as ammonoids, gastropods, bivalves and scaphopod shells, as well as conodont elements, fish and echinoid remains. Ammonoid and conodont assemblages obtained from three blocks are utilized for biostratigraphical analysis. Twelve informal Smithian ammonoid biostratigraphic intervals from a previous study are condensed into four ammonoid beds, based on the range of age-diagnostic taxa, in ascending order: Meekoceras millardense-M. aff. olivieri beds, Owenites koeneni beds, Anasibirites multiformis bed, and Condensoceras youngi bed. Conodonts recovered from the three blocks consist of 30 species distributed among 15 genera, including one newly described taxon, i.e., Guangxidella minuta Maekawa and Jenks sp. nov. The blocks are divided into two conodont interval zones, i.e., the Novispathodus ex gr. waageni Interval Zone and Nv. pingdingshanensis Interval Zone, based on the first occurrences of their eponymous taxa. Additionally, four conodont range zones, i.e., the Paullella meeki Range Zone, Guangxidella bransoni Range Zone, Scythogondolella milleri Range Zone, and Borinella buurensis Range Zone are recognized, based on the ranges of these four index species. Conodonts within these interval and range zones vary in age from middle Smithian to latest Smithian. The presence of key ammonoid and conodont taxa regarding the Smithian-Spathian boundary (SSB) such as the AW (Anasibirites and Wasatchites) and GXP (Glyptophiceras, Xenoceltites and Pseudosageceras) ammonoid assemblages and the conodonts B. buurensis, Nv. pingdingshanensis and S. milleri demonstrate that the study area is an important reference site for the SSB in the eastern Panthalassa area.