Ultrastructure and morphology of Chuaria circularis (Walcott, 1899) Vidal and Ford (1985) from the Neoproterozoic Visingsö Group, Sweden

The organic-walled microfossil Chuaria circularis [Walcott, C.D., 1899. Precambrian fossiliferous formations. Geol. Soc. Am. Bull. 10, 199–244] Vidal and Ford [Vidal, G., Ford, T.D., 1985. Microbiotas from the Late Proterozoic Chuar Group (Northern Arizona) and Uinta Mountain Group (Utah) and their chronostratigraphic implications. Precambrian Res. 28, 349–389] from the Visingsö Group in Sweden have been re-examined using reflected and transmitted light, scanning electron and (SEM) transmission electron (TEM) microscopy. Specimens extracted from the rock matrix and those in situ, compressed on the bedding planes, were both studied. Besides previously described external wall surface, a psilate internal surface of Chuaria vesicle was observed. Transmission electron microscopy revealed a single-layered, electron-dense and homogeneous wall ultrastructure of the microfossil. Similar wall ultrastructure has been observed in other acid resistant microfossils from Lower Cambrian rocks. Various stages of the microfossils’ wall degradation were observed and are discussed in the paper. An overview of morphological interpretations and the history of research of C. circularis is presented. © 2000 Elsevier Science B.V. All rights reserved.