Basal Cambrian microfossils from the Yangtze Gorges area (South China) and the Aksu area (Tarim block, northwestern China)

Abstract

The basal Cambrian marks the beginning of an important chapter in the history of life. However, most paleontological work on the basal Cambrian has been focused on skeletal animal fossils, and our knowledge about the primary producers—cyanobacteria and eukaryotic phytoplankton (e.g., acritarchs)—is limited. In this research, we have investigated basal Cambrian acritarchs, coccoidal microfossils, and cyanobacteria preserved in phosphorites and cherts of the Yanjiahe Formation in the Yangtze Gorges area (South China) and the Yurtus Formation in the Aksu area (Tarim Block, northwestern China). Our study confirms the occurrence in these two formations of small acanthomorphic acritarchs characteristic of the basal CambrianAsteridium–Comasphaeridium–Heliosphaeridium(ACH) assemblage. These acritarchs include abundantHeliosphaeridium ampliatimi(Wang, 1985) Yao et al., 2005, commonYurtusia uniformisn. gen. and n. sp., and rareComasphaeridium annulare(Wang, 1985) Yao et al., 2005. In addition, these basal Cambrian successions also contain the clustered coccoidal microfossilArchaeophycus yunnanensis(SonginLuo et al., 1982) n. comb., several filamentous cyanobacteria [Cyanonema majusn. sp.,Oscillatoriopsis longaTimofeev and Hermann, 1979, andSiphonophycus robustum(Schopf, 1968) Knoll et al., 1991], and the tabulate tubular microfossilMegathrix longusL. Yin, 1987a, n. emend. Some of these taxa (e.g.,H. ampliatum, C. annulare, andM. longus) have a wide geographic distribution but occur exclusively in basal Cambrian successions, supporting their biostratigraphic importance. Comparison between the stratigraphic occurrences of microfossils reported here and skeletal animal fossils published by others suggests that animals and phytoplankton radiated in tandem during the Cambrian explosion.