A TALE OF TWO SEEMINGLY SIMPLE FOSSILS: ELECTRON MICROSCOPY REVEALS EVIDENCE FOR MULTICELLULARITY IN THE PROTEROZOIC FOSSILS CHUARIA AND TAWUIA

Multicellularity, including simple and complex multicellularity, independently arose multiple times in the evolutionary history of cellular life. Simple multicellularity—characterized by clusters, filaments, or sheets of cells that have limited cell-to-cell communication and cell differentiation—may have a deep history tracing back to 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 when diverse assemblages of complex multicellular eukaryotes evolved. In the intervening Tonian Period, the fossil record of multicellular organisms is poorly documented, partly because of the preservational limitations of soft-bodied organisms and the interpretation of morphologically simple forms. To address this knowledge gap, we investigated Chuaria, Tawuia (two of the most ubiquitous Proterozoic macrofossils) and other associated soft-bodied fossils from the Tonian Liulaobei Formation in North China, using a combination of optical and electron microscopy. Our examination using backscattered-electron scanning electron microscopy (BSE-SEM) revealed direct evidence of multicellularity in some of these fossils, which are interpreted as representing a multicellular stage in the life cycles of Chuaria and Tawuia. This study favors the long-suspected but controversial hypothesis that Chuaria and Tawuia might be multicellular organisms and possibly multicellular eukaryotes. Although Chuaria and Tawuia may represent a simple level of multicellularity (i.e., coloniality), they represent an important step toward the great diversity and cellular complexity as manifested by the diverse multicellular organisms that came afterwards in the Cryogenian and Ediacaran periods. It also suggests that BSE-SEM has the potential to open a floodgate of new microstructural information about the seemingly simple Precambrian carbonaceous compressions and to unveil the hidden diversity of multicellular organisms in the Tonian Period.