SGER: Search for Earliest Metazoans

寻找最早的后生动物亚利桑那州立大学新发现的最新的前寒武纪池塘沉积物沉积在西澳大利亚前寒武纪将探索身体化石和后生动物的痕迹化石，可能已经存在之前，寒武纪生命爆发的海洋。 有争议的分子钟结果表明，后生动物进化在晚前寒武纪，但详尽的搜索前寒武纪海洋沉积岩没有得到丰富和多样化的后生动物的证据。 海洋中的溶解氧可能太低了，直到前寒武纪晚期大量盐的沉积降低了盐度。 后生动物在低盐度的非海洋环境中广泛进化为软体生物，随后进入海洋，创造了寒武纪爆发的物种多样性特征。湖泊沉积物被快速侵蚀离开大陆，而之前没有已知的晚前寒武纪的例子来验证这些想法。 澳大利亚的池塘沉积物是在为其他目的进行的实地工作中发现的，尚未进行测绘或研究。 它们充填在元古宙艾略特山脉白云岩上形成的古溶面（古岩溶）上的孤立洼地。它们立即被非海相砂和最新的前寒武纪冰碛岩覆盖。 它们包括与页岩互层的层状碳酸盐旋回。 碳酸盐岩含有置换燧石和磷酸盐物质的不溶性残留物，已知这些物质甚至可以保存年轻地层中最小的后生动物化石。 SGER赠款将支持实地工作，以确定和绘制最好的池塘沉积物，寻找痕迹化石，并获得广泛的收集最新鲜的材料薄切片和随后的扫描电镜和显微镜检查不溶性残留物。 斯坦利Awramik（加州大学，圣巴巴拉）和亚利桑那州立大学的学生希望约翰斯顿将参加实地工作和实验室调查。 这些岩石中的后生动物的证据表明，后生动物生命的巨大扩张是先前进化的谱系进入海洋的结果，而不是一些环境或生物变化，使它们能够在很短的时间内起源于海洋。 目前的努力，以了解什么可能的环境或生物变化允许生命的爆炸将转变为一个新的努力，以了解古代和非海洋环境中的生命进化，这是如此罕见地保存在地质记录。

Search for Earliest MetazoansL Paul KnauthArizona State UniversityNewly 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.