On the significance of tetrahedral tetrads of Precambrian algal cells

Abstract

Tetrahedral tetrads of coccoid cells are known from two Precambrian localities; one example has been reported from the late Proterozoic Bitter Springs Formation and a second example, morphologically different from the first, is here described from the middle Proterozoic Amelia Dolomite. Among modern algae, sheath- or wall-enclosed tetrahedral tetrads are regularly produced only by eukaryotes, either during meiotic or mitotic reproduction. In evaluating the significance of Precambrian tetrads, consideration should be given to the following: whether the tetrad geometry is truly tetrahedral; if so, whether the tetrahedral configuration is original or artifactual; and if original, whether the tetrad was produced during meiotic or mitotic reproduction. We suggest several kinds of evidence which may assist in making these determinations. Considering the Amelia Dolomite specimen, we conclude that it most closely resembles a mitotically produced green algal autospore tetrad, although this interpretation cannot yet be confirmed because of the difficulty in ascertaining whether the tetrahedral configuration is original. Considering the Bitter Springs specimen, we conclude that the organism probably was eukaryotic but that its mode of origin (whether through meiosis or mitosis) cannot be unequivocally ascertained from data presently available.