Abstract:

The Kuanti Formation, with the discovery of the Xiaoxiang Vertebate Fauna in 2007, has become the focus of global research on early vertebrates. However, its stratigraphic subdivision, correlation and age, especially the subdivision and age of its lower part, remain contentious, despite many biostratigraphic attempts made since 1914. In recent years, we conducted a series of extensive geological investigations in the Kuanti Formation near the Xiaoxiang Reservoir in Qujing, Yunnan Province. Based on the recently obtained data from the Liujiachong Section (dominated by the lower part of the Kuanti Formation) and the preliminary study of the newly discovered microvertebrate remains, we documented the sequence of the Kuanti Formation of the Liujiachong Section, and further explored the geological age of the related strata. The Kuanti Formation of the Liujiachong Section can be subdivided into three members, in ascending order according to the main lithological changes: Member Ⅰ (Yuejiashan Member), Member Ⅱ (Chongjiawan Member), and Member Ⅲ (Cailian Member). The lower part of Member Ⅰ is characterized by light yellow and yellow-green gravelly silty mudstones and calcareous siltstones lacking any fossils, while the upper part is composed of dark grey and grey-black carbonaceous shales with rare Lingula sp. and bivalves. Member Ⅱ is dominated by grey-green and purple-red shales, intercalated with light grey thin-bedded or lenticular limestones or bioclastic limestones containing many invertebrate fossils (brachiopods and crinoid stems) and abundant microvertebrate remains. Member Ⅲ, partially exposed in the section, is composed of purple-red and grey-green silty and calcareous mudstones or marls intercalating with minor purple-red or yellow-green shales or siltstones containing brachiopods, fish, and stout tubular trace fossils. A thin layer of medium-thick-bedded fine sandstone is developed on the bottom of the member, which marks the boundary between Members Ⅲ and Ⅱ. The microvertebrate remains from several beds in Member Ⅱ show high diversity, including four early vertebrate groups: agnathans, placoderms, acanthodians, and osteichthyans. They are characterized by the Yangtze Fish Assemblage of the Xiaoxiang Vertebrate Fauna. The discovery of the acanthodians Nostolepis spp. in this section reveals the stratigraphic correlation and the precise geological age of the lower part of the Kuanti Formation. Based on the acanthodians from Member Ⅱ, coupled with previous palaeontological data, we suggest the age of Member Ⅱ to be late Gorstian Stage of the Ludlow, in the Silurian. The upper part of Member I, containing the placoderm ‘Wangolepis’, might be referred to as early Gorstian. However, the age of the lower part of Member I without fossils is uncertain, and the possibility of its base extending down to the Wenlock is not excluded. This study offers new palaeoichthyological evidence for the Silurian biostratigraphy of China and provides a new chronostratigraphic scheme in the study of early vertebrate evolution.

Key words: microvertebrate remains, Kuanti Formation, Liujiachong Section, subdivision and correlation, Qujing, Yunnan