Ke Pang

The nature and origin of nucleus‐like intracellular inclusions in Paleoproterozoic eukaryote microfossils

The well-known debate on the nature and origin of intracellular inclusions (ICIs) in silicified microfossils from the early Neoproterozoic Bitter Springs Formation has recently been revived by reports of possible fossilized nuclei in phosphatized animal embryo-like fossils from the Ediacaran Doushantuo Formation of South China. The revisitation of this discussion prompted a critical and comprehensive investigation of ICIs in some of the oldest indisputable eukaryote microfossils-the ornamented acritarchs Dictyosphaera delicata and Shuiyousphaeridium macroreticulatum from the Paleoproterozoic Ruyang Group of North China-using a suite of characterization approaches: scanning electron microscopy (SEM), transmission electron microscopy (TEM), and focused ion beam scanning electron microscopy (FIB-SEM). Although the Ruyang acritarchs must have had nuclei when alive, our data suggest that their ICIs represent neither fossilized nuclei nor taphonomically condensed cytoplasm. We instead propose that these ICIs likely represent biologically contracted and consolidated eukaryotic protoplasts (the combination of the nucleus, surrounding cytoplasm, and plasma membrane). As opposed to degradational contraction of prokaryotic cells within a mucoidal sheath-a model proposed to explain the Bitter Springs ICIs-our model implies that protoplast condensation in the Ruyang acritarchs was an in vivo biologically programmed response to adverse conditions in preparation for encystment. While the discovery of bona fide nuclei in Paleoproterozoic acritarchs would be a substantial landmark in our understanding of eukaryote evolution, the various processes (such as degradational and biological condensation of protoplasts) capable of producing nuclei-mimicking structures require that interpretation of ICIs as fossilized nuclei be based on comprehensive investigations.

Electron microscopy reveals evidence for simple multicellularity in the Proterozoic fossil Chuaria

Multicellularity arose multiple times in the evolutionary history of eukaryotes, and simple multicellularity may have a deep history tracing back to the Paleoproterozoic. However, complex multicellular organisms with cellular and tissue differentiation did not appear in the fossil record until the Mesoproterozoic, and it is not until the Ediacaran Period (635–541 Ma) when diverse assemblages of complex multicellular eukaryotes evolved. In the intervening Tonian Period (ca. 1000–720 Ma), the fossil record of multicellular organisms is poorly documented. To address this knowledge gap, we investigated Chuaria and associated carbonaceous compression fossils from the Tonian Liulaobei Formation in North China. These fossils have been variously interpreted as unicellular or multicellular organisms. Our analysis using backscattered-electron scanning electron microscopy (BSE-SEM) revealed direct evidence for simple multicellularity in some of these fossils and suggests that Chuaria may have had a multicellular vegetative stage in its life cycle. This study demonstrates that BSE-SEM has the potential to unveil the hidden diversity of multicellular organisms in the Tonian Period, thus enriching our knowledge about the multiple origins of multicellularity in this critical geological period before Cryogenian glaciations.