A palynological assemblage from the Cambrian (Series 2, Stage 4) of Shandong Province, China, and its implications to the transition from algae to land plants

Pre-Darriwilian (Middle Ordovician) spore-like palynomorphs are significant in understanding the plant origination and the evolutionary transition from algae to land plants (TAP). Cambrian cryptospore, though its concept and affinity remain controversial, shows indispensable evidence of algal ancestor preceding occurrence of land plants. In this study, we recognize a Cambrian (Epoch 2, Age 4) palynological assemblage from the Zhushadong Formation of Shandong Province, northern China, which shows more diversity than previously reported from the same section. The Zhushadong palynological assemblage (ZPA) is proposed and composed of cryptospores (15.4%), clustered forms (31.5%), acritarchs (20.1%), filaments (30.3%) and indeterminate specimens (2.7%). The ZPA shows the earliest record of Cambrian cryptospores or spore-like palynomorphs and sheds light on the TAP occurring in the Cambrian of northern China. Palynomorphs from the ZPA are comparable with those from roughly coeval sequences in North America and other areas. Our study also suggests that ancestral streptophyte algae started to adapt to subaerial life in several separated regions and probably earlier than the Epoch 2 of the Cambrian.