Wall ultrastructure of an Ediacaran acritarch from the Officer Basin, Australia

Well-preserved organic-walled microfossils referred to as acritarchs occur abundantly in Ediacaran deposits in the Officer Basin in Australia. The assemblages are taxonomically diverse, change over short stratigraphical intervals and are largely facies independent across marine basins. Affinities of this informal group of fossils to modern biota are poorly recognized or unknown, with the exception of only a few taxa. Morphological studies by use of transmitted light microscopy, geochemical analyses and other lines of evidence, suggest that some Precambrian acritarchs are related to algae (including prasinophytes, chlorophytes, and perhaps also dinoflagellates). Limitations in magnification and resolution using transmitted light microscopy may be relevant when assessing relationships to modern taxa. Scanning electron microscopy reveals details of morphology, microstructure and wall surface microelements, whereas transmission electron microscopy provides high-resolution images of the cell wall ultrastructure. In the light of previous ultrastructural studies it can be concluded that the division of acritarchs into leiospheres (unornamented) and acanthomorphs (ornamented) is entirely artificial and has no phylogenetic meaning. Examination of Gyalosphaeridium pulchrum using transmission electron microscopy reveals a vesicle wall with four distinct layers. This multilayered wall ultrastructure is broadly shared by a range of morphologically diverse acritarchs as well as some extant microalgae. The chemically resistant biopolymers forming the comparatively thick cell, together with the overall morphology support the interpretation of the microfossil as being in the resting stage in the life cycle. The set of features, morphological and ultrastructural, suggests closer relationship to green algae than dinoflagellates.