Upper Norian conodonts from the Baoshan block, western Yunnan, southwestern China, and implications for conodont turnover

Abstract

The Sevatian of the late Norian is one of the key intervals in biotic turnover and in changes of paleoclimate and paleoenvironments. Conodont faunas recovered from two sections of upper Norian strata of the Dashuitang and Nanshuba formations near Baoshan City in western Yunnan province provide new insights into the diversity and biostratigraphy of the Sevatian conodonts within China as well as globally. A lowerMockina(M.)bidentataZone and an upperParvigondolella(P.)andrusoviZone are identified in this area according to the first occurrences ofM. bidentataand ofP. andrusovi. Rich conodont fauna ofM. zapfeiis detailed and presents various intraspecific forms. A total of 19 forms of P1elements are presented, which, when combined with the reported conodonts in theM. bidentataZone, suggest that there was a peak in conodont diversity within theM. bidentataZone. A biotic crisis in the uppermostM. bidentataZone is recognized from the contrast between the diverse conodont fauna in theM. bidentataZone and the rare conodonts in theP. andrusoviZone. The conodont turnover during the middle Sevatian highlights the fact that the prolonged phases of the end-Triassic mass extinction probably began in the transition interval fromM. bidentataZone toP. andrusoviZone.