The palynoflora of the Lower Cretaceous strata of the Yingen-Ejinaqi Basin in North China and their implications for the evolution of early angiosperms

A total of 41 spore species and 63 pollen taxa, including seven angiosperms, were identified from 24 samples of the Early Cretaceous succession (latest Barremian to Aptian) at the Wulan Section in the east part of the Yingen-Ejinaqi Basin, Inner Mongolia, China. In a stratigraphically upward order, three types of palynomorph assemblages were identified. Palynological assemblage I (PA I) was dominated by bisaccate pollen, whereas palynological assemblages II (PA II) and III (PA III) were characterised by abundant Classpollis associated with more spores. PA III contained the highest spore content. Angiosperm pollen display an increasing trend in diversity and abundance from PA I to PA III. PA I is most likely correlated to the Chinese Cretaceous palynofloral Transitional Zone and northern Disacciatrileti - Cicatricosisporites Province, whereas the assemblages II and III represent the transitional zone. The composition of these assemblages suggests that the climate was cool and humid within the basin during the latest Barremian, subsequently warmer and drier during the Early Aptian and, finally, became slightly wetter and warmer during the Late Aptian. Early angiosperm pollen occur frequently in this section, including that of Clavatipollenites, Asteropollis and Songipollis. On the stratigraphical order, these angiosperm pollen show a clear and continuous increase regarding the relative abundance and diversity of angiosperm, which most likely reflects the incipient dispersion of angiosperm plants during the latest Barremian to Late Aptian in inner East Asia. Lastly, a comprehensive compilation of published literature indicates that the emergence and evolution of early Asteropollis was not simultaneously worldwide, most likely occurring earlier in Eurasia than in other regions with a dramatic increase in both diversity and abundance prior to the emergence of tricolpate pollen.