Basal Cambrian Microfossils from the Yangtze and Tarim Blocks

Most paleontological work on the basal Cambrian has been focused on skeletal animal fossils, and little attention has been focused on the primary producers—cyanobacteria and eukaryotic phytoplankton (e.g., acritarchs). However, Cambrian acritarchs can be useful index fossils for biostratigraphic correlation, and they can also provide insights into the evolution of primary producers. To improve our knowledge about early Cambrian evolution of primary producers and to facilitate biostratigraphic correlations between early Cambrian faunal and floral assemblages, we have investigated basal Cambrian acritarchs, coccoidal microfossils, and cyanobacteria preserved in phosphorites and cherts of the Yanjiahe Formation in the Yangtze Gorges area (Yangtze Block, South China) and the Yurtus Formation in the Aksu area (Tarim Block, northwestern China).

Our study confirms the occurrences the characteristic basal Cambrian Asteridium–Comasphaeridium–Heliosphaeridium (ACH) small acanthomorphic acritarch assemblage in these two formations. These acritarchs include abundant Heliosphaeridium ampliatum (Wang, 1985) Yao et al., 2005, rare Comasphaeridium annulare (Wang, 1985) Yao et al., 2005, and a new acritarch form of moderate abundance. In addition, these basal Cambrian successions also contain clustered coccoidal microfossils, several filamentous cyanobacteria [Cyanonema, Oscillatoriopsis, and Siphonophycus], and the tabulate tubular microfossil Megathrix longus L. Yin, 1987. Some of these taxa (e.g., H. ampliatum, C. annulare, and M. 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.