Cell wall ultrastructures of the Proterozoic acritarch Leiosphaeridia asperata and their implications for biological affinity

Abstract

Abundant sphaeroidal acritarch Leiosphaeridia asperata, with vesicle size ranging from 13 to 360 μm, occurred in the Proterozoic Liulaobei Formation shales in Huainan, Anhui Province. TEM/SEM studies of these sphaeroidal acritarchs have revealed complex ultrastructures, including ridges, bands, and possible trilaminar structures (TLS). Ridges, spaced ∼1 μm apart, are distributed regularly on the vesicle internal surface of one specimen. Some specimens have alternating electron-dense and electron-tenuous bands that are perpendicular to vesicle walls. Some specimens have trilaminar structures (TLS), while a few others have both bands and TLS. These wall ultrastructures provide evidence that the Liulaobei leiosphaerids are of green microalgae. The variation in wall ultrastructures may represent various stages in a life cycle or developmental phases in the cyst formation analogous to some modern chlorophyceaen microalgae.