RECOGNIZING AND INTERPRETING THE FOSSILS OF EARLY EUKARYOTES

Using molecular sequence data, biologists can generate hypotheses of protistan phylogeny and divergence times. Fossils, however, provide our only direct constraints on the timing and environmental context of early eukaryotic diversification. For this reason, recognition of eukaryotic fossils in Proterozoic rocks is key to the integration of geological and comparative biological perspectives on early protistan evolution. Recognizing Proterozoic microfossils as eukaryotic can be a challenge, despite the myriad molecular and ultrastructural differences that separate living prokaryotes and protists. Most of these diagnostic characters do not survive fossilization, leaving paleontologists with morphology and preserved lipid molecules to study in ancient sedimentary rocks. Preservable features known to be formed by eukaryotic microorganisms but unknown among prokaryotes include large, well-defined processes and/or other surface ornamentation; morphologically differentiated excystment structures; and wall structure, ultrastructure and chemistry. Microfossils preserved in shales of the ca. 1500 Ma Roper Group, northern Australia, display several of these features, permitting them to be recognized as eukaryotic. For example, Tappania plana displays cylindrical, occasionally branching processes as well as probable excystment structures. Viewed under SEM, the wall of Satka favosa consists of interlocking polygonal plates, whereas the wall of Valeria lophostriata is ornamented by concentric striations formed on its inner surface. And large cylindrical tubes that appear longitudinally striated under the light microscope display a distinct alternation of electron-dense and electron-tenuous bands when viewed by TEM. Despite confidence that these taxa were eukaryotic, they cannot at present be related to any particular protistan clade. This will require continuing research on wall ultrastructure and microchemistry. In combination, TEM/SEM and microchemical analysis promise new insights into the nature and systematic relationships of early eukaryotic fossils, and the timing and patterns of protistan evolution on the Proterozoic Earth.