Arizona State University
Newly discovered latest Precambrian pond sediments deposited in Western Australia Precambrian will be explored for body fossils and trace fossils of metazoans that may have existed prior to the Cambrian explosion of life in the sea. Controversial molecular clock results suggest that metazoans evolved in the Late Precambrian, but exhaustive searches of Precambrian marine sedimentary rocks have not yielded evidence of abundant and diversified metazoans. Dissolved oxygen may have been too low in the seas until salinity was lowered by deposition of enormous salt deposits in the late Precambrian. The possibility arises that metazoans were extensively evolved as soft-bodied organisms in lower salinity non-marine environments and moved subsequently into the sea to create the species diversity characteristic of the Cambrian Explosion. Lake sediments are rapidly eroded off continents, and no previous late Precambrian examples are known to test these ideas. The Australian pond sediments were discovered during field work for other purposes and have not been mapped or studied. They fill isolated depressions on an ancient solution surface (paleokarst) developed on the Proterozoic Eliot Range Dolomite. They are immediately overlain by non-marine sands and latest Precambrian glacial tillite. They include cycles of laminated carbonate interbedded with shales. The carbonates contain replacement chert and insoluble residues of phosphatic material known to preserve even the smallest metazoan fossils in younger strata. The SGER grant will support field work to identify and map the best pond deposits, search for trace fossils, and obtain an extensive collection of the freshest material for thin sections and subsequent SEM and microscopic examination of insoluble residues. Stanley Awramik (University of California, Santa Barbara) and ASU student Hope Johnston will participate in the field work and lab investigations. Evidence of metazoans in these rocks would indicate that the great metazoan expansion of life was a result of previously evolved lineages moving into the sea rather than some environmental or biological change that allowed them to originate in the sea in a very short interval of time. Current efforts to understand what possible environmental or biological changes allowed an explosion of life would transform to a new effort to understand the antiquity and evolution of life in the non-marine environments which are so infrequently preserved in the geologic record.
