New morphological observations for Paleoproterozoic acritarchs from the Chuanlinggou Formation, North China

Acritarchs from the Chuanlinggou Formation (ca. 1700 Ma) were first reported by Xing and Liu (1973) [Xing, Y.S., Liu, G.Z., 1973. On Sinian micro-flora in Yenliao Region of China and its geological significance. Acta Geological Sinica 1, 1-64] and some were later described as eukaryotes by Yan (1982) [Yan, Y.Z., 1982. Schizofusa from the Chuanlinggou Fromation of Changzhougou System in Jixian county. Bulletin of the Tianjin Institute of Geology and Mineral Resources 6, 1-7]. The eukaryotic affinity has been questionable, however, for lack of indisputable morphological evidence. Chuanlinggou acritarchs are some of the oldest acritarchs found on Earth and are well preserved for ultrastructure study. In this study, Chuanlinggou acritarchs were examined by jointly using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In most cases, an ovoidal group of acritarchs represents a half vesicle following a complete longitudinal rupture, which is a morphological model different from a whole envelope with medial splits as proposed by earlier studies. This ovoidal group displays a bipolar morphology, longitudinal rupture, and occasionally striated wall structures that are consistent with a eukaryotic affinity. Thus, the Chuanlinggou ovoidal acritarchs probably extends the eukaryotes body fossil record into the Paleoproterozoic, ∼200 million years earlier than the morphologically more complex acritarchs from Mesoproterozoic Roper Group (ca. 1500 Ma).