Chuaria and Tawuia fossils from 1.0 Ga rocks in North China: Implications for a polyphyletic origin of Chuaria and a potential biological link between these two widespread Proterozoic taxa

The Proterozoic witnessed the emergence and rise of early eukaryotes and Proterozoic carbonaceous compression macrofossils provide a valuable window for our understanding of this ecological expansion. Among them, the discoidal Chuaria and tomaculate Tawuia, which often co-occur in the same bedding layer, are perhaps the most common fossil forms in Proterozoic successions with extremely long temporal and broad geographical distributions. However, their phylogenetic interpretations and the relationship between them have been debated for decades, due to their simple morphologies lacking phylogenetically diagnostic features. In this study, we used an integrated application of reflected and transmitted light microscopy, scanning electron microscopy, biometric analysis, and Raman spectroscopy to characterize well-preserved carbonaceous compression macrofossils of Chuaria and Tawuia from the Tonian Liulaobei Formation in Huainan region, northern Anhui province, North China. The fossils are abundantly preserved in a ca. 1-2 cm thick homogeneous mudstone layer and can be considered to be closer to an ecological community than fossils from multiple different layers or localities. The comprehensive analysis shows that even Chuaria vesicles from the same layer may have a polyphyletic origin and three groups of Chuaria vesicles have been identified. Among them, small Chuaria vesicles with minimum diameter < 1.2 mm can be either related to the sphaeromorphic acritarch Leiosphaeridia or interpreted as multicellular aggregates. In contrast, large Chuaria vesicles with minimum diameter >= 1.2 mm are more comparable to the type material of Chuaria circularis from the Chuar Group and more closely related, either phylogenetically or ontogenetically, to Tawuia vesicles. This study takes a step on the path to solving the mystery of the most common and widely distributed Proterozoic macrofossil assemblage (i.e., Chuaria-Tawuia assemblage), and urges thorough and systematic evaluations to be taken on the possibility that Chuaria vesicles from the same Chuaria-Tawuia assemblages or sole Chuaria assemblages in other localities might also be polyphyletic.