The distribution and evolution of microbial life in the Late Proterozoic Era

Until recently, it was possible to discuss pattern in the Precambrian fossil record in terms of three benchmark fossil assemblages conveniently dated at approximately 3000, 2000, and 1000 million years before the present (Ma BP). The oldest of the three biotas, that of the Fig Tree Group of southern Africa (now considered to be ~ 3400 Ma old), was known to contain extremely simple organic microspheres, some preserved in various stages of apparent binary fission. In contrast, the ~ 850-Ma-old Bitter Springs Formation, Australia,contains cyanobacterial and possible algal remains of strikingly modem morphology. The microbiota of the Gunflint Iron Formation, Ontario, Canada, appeared to be both temporally and morphologically intermediate between the other two, so that the overall picture of early biological history to emerge was one of gradual evolution over vast stretches of geological time. Whether or not this interpretation can be sustained depends in part on the extent to which the benchmark micro biotas can be considered representative of the range of organisms that existed at the time they were deposited. In this paper I will focus on a single brief interval of Late Proterozoic time and attempt to document the degree of paleoecological heterogeneity observable in the preserved paleontological record. If substantial environmental variation exists at any one point in time, then it must be taken into account in any attempt to unravel evolutionary patterns in early Earth history. The period chosen for examination is 800 Ma ago (± 100 Ma), a time that is unusually well represented by sedimentary rocks and microbial fossils.