First Report of Small Skeletal Fossils from the Upper Guojiaba Formation (Series 2, Cambrian), Southern Shaanxi, South China

Abstract

A small skeletal fossil assemblage is described for the first time from the bioclastic limestone interbeds of the siltstone-dominated Guojiaba Formation, southern Shaanxi, China. The carbonate-hosted fossils include brachiopods (Eohadrotreta zhujiahensis, Eohadrotreta zhenbaensis, Spinobolus sp., Kuangshanotreta malungensis, Kyrshabaktella sp., Lingulellotreta yuanshanensis, Eoobolus incipiens, and Eoobolus sp.), sphenothallids (Sphenothallus sp.), archaeocyaths (Robustocyathus sp. and Yukonocyathus sp.), bradoriids (Kunmingella douvillei), chancelloriids sclerites (Onychia sp., Allonnia sp., Diminia sp., Archiasterella pentactina, and Chancelloria cf. eros), echinoderm plates, fragments of trilobites (Eoredlichia sp.), and hyolithelminths. The discovery of archaeocyaths in the Guojiaba Formation significantly extends their stratigraphic range in South China from the early Tsanglangpuian at least to the late Chiungchussuan. Thus, the Guojiaba Formation now represents the lowest known stratigraphic horizon where archaeocyath fossils have been found in the southern Shaanxi area. The overall assemblage is most comparable, in terms of composition, to Small skeletal fossil (SSF) assemblages from the early Cambrian Chengjiang fauna recovered from the Yu’anshan Formation in eastern Yunnan Province. The existing position that the Guojiaba Formation is correlated with Stage 3 in Cambrian Series 2 is strongly upheld based on the fossil assemblage recovered in this study.

The Internal Anatomy and Water Current System of Cambrian Archaeocyaths of South China

“…They appeared as the first animals to secrete thick carbonate skeletons and occurred in all continents in the early Cambrian period, with the most well-preserved and best-known examples from Siberia, Southern Australia, North America, and South China [6][7][8][9][10][11][12][13][14][15][16]. In South China, archaeocyathan fossils mostly occur in the Xiannüdong (Stage 3), Mingxinsi (Stage 3), Jindingshan (Stage 4), Tianheban (Stage 4), and, most recently, Guojiaba formations throughout the Yangtze Platform [15,[17][18][19][20][21][22][23][24]. Given their global distribution, limited stratigraphic extension, rapid evolution, and exquisite calcareous skeletons, archaeocyaths are crucial to our knowledge on the evolution of metazoans and early Cambrian stratigraphic correlation [15,25,26].…”

“…As a consequence, the study of the morphological structure and systematic classification of the taxon has been significantly limited. Recently, a few three-dimensional phosphatized archaeocyaths have been found [24,[44][45][46][47][48][49], which worked as an important supplement to thin-section analysis for the identification of important taxonomical features and development of the cups. Nevertheless, the reports of these fossils mostly focused on their occurrences and taxonomy, lacking comprehensive morphological and anatomical studies.…”