Anisian ammonoids from Qingyan, southwestern China: biostratigraphical implications for the age of the Qingyan Formation

Abstract

The Upper Qingyan Formation in the northeastern vicinity of Qingyan, Guizhou Province, southwestern China, yields an ammonoid sequence comprising a latest middle Anisian assemblage in its lower part followed by an earliest late Anisian assemblage in its middle and upper parts. Age–diagnostic genera of the lower assemblage include Bulogites and Acrochordiceras; additional genera comprise Proarcestes, Sageceras, and Beyrichitinae gen. indet. This assemblage well correlates with the Bulogites mojsvari Subzone in northwestern Nevada and the Bulogites zoldianus Zone in Lombardy and Hungary. The upper assemblage comprises the age-diagnostic genera Rieppelites and Judicarites; additional characteristic genera include Ptychites and Gosauites. This assemblage correlates with the Billingsites cordeyi Subzone in northwestern Nevada and the Rieppelites cimeganus Zone in Lombardy. The middle/late Anisian boundary is situated in the upper middle part of the highly fossiliferous section at Leidapo. Judging from re-study of available and figured specimens, reports of stratigraphically younger taxa such as Paraceratites trinodosus from the Upper Qingyan Formation of this area are based on misidentifications. The time interval represented by the Qingyan Formation at its type locality is not equal to the Anisian as has long been supposed in Chinese stratigraphy. An earliest late Anisian age for the uppermost strata of the Qingyan Formation in this area is the only available, wel-cumented age-constraint.