Microfossils across the Llandovery–Wenlock boundary in Ziyang–Langao region, Shaanxi, NW China

A total of 80 samples were collected primarily for chitinozoans from the Wuxiahe Formation (late Llandovery–early Sheinwoodian) in the Ziyang–Langao region, Shaanxi Province, Northwest China. Besides chitinozoans, several other microfossil groups have been recovered unexpectedly, including conodonts, melanosclerites, acritarchs, and scolecodonts. Colonized acritarchs and club-shaped melanosclerites appear sporadically. Among the rare recorded chitinozoans, Angochitina longicollis is a Telychian–early Sheinwoodian species occurring throughout the studied interval of the Wuxiahe Formation at the Bajiaokou A section of Ziyang County. Conodonts of the late Telychian Pterospathodus eopennatus, Pterospathodus celloni, Pterospathodus a. amorphognathoides biozones and the early Sheinwoodian Pterospathodus p. procerus Biozone are identified at the Qiaoxi section of Langao County, where they co-occur with graptolites. The occurrences of melanosclerites and turbidite deposits support an open marine environment during late Llandovery to early Wenlock in the study region. Based on the recovered microfossils, the Llandovery–Wenlock boundary seems to be confined within a 0.73 m-thick interval at the Qiaoxi section.It is the first time that microfossils, especially conodonts, have been recorded in the Llandovery–Wenlock transitional interval in the Ziyang–Langao region, Northwest China. The discovery of these microfossil groups supplements biostratigraphic information in the Ziyang region, where a possible GSSP candidate for the base of the Wenlock Series is located. The potential input of these microfossil groups in defining the Llandovery–Wenlock boundary is discussed.