Middle Telychian (Llandovery, Silurian) graptolite and chitinozoan faunas and biostratigraphy of the upper part of the Xiushan Formation in the Xiushan area, southwestern China

The middle Telychian (Llandovery, Silurian) Xiushan Fauna (characterized by abundant benthic brachiopods, cephalopods and trilobites) from South China is important in understanding the re-radiation of the Early Palaeozoic Evolutionary Fauna after the Late Ordovician Mass Extinction. Although previous integrated studies based on graptolites, conodonts and chitinozoans have resulted in the erection of a biostratigraphical framework for the Xiushan Fauna, the precise biostratigraphical correlation has long been debated, due to the sporadic occurrence of biozonal index fossils, e.g. only a few graptolites in carbonate clasts and siliciclastic rocks. Here, we report a moderately well-preserved graptolite fauna from the interbedded shaly layers of the Xiushan Formation in Chongqing Municipality, southwestern China. The graptolites and chitinozoans from the Xiushan Formation at the Hujiadongkan section suggest a middle Telychian (Llandovery, Silurian) age for the upper part of the Xiushan Formation, possibly corresponding to the upper part of the Monoclimacis crenulata to the lower Oktavites spiralis biozones, for the graptolite assemblages, and the Angochitina longicollis chitinozoan Biozone. This age identification, based on the co-occurrences of graptolites and chitinozoans at the Hujiadongkan section, agrees with earlier biostratigraphical studies based on other fossil groups, e.g. conodonts for the Xiushan Fauna recovered from the upper part of the Xiushan Formation. The graptolitic shales in the upper part of the Xiushan Formation may provide new evidence for high sea-levels in the middle Telychian, as have been reported worldwide.