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cement textures found in adjacent carbonate rocks. In many cases the nodules formed soon after deposition, before burial initiated the compaction that bent encompassing sediments around them. Silica has no color of its own; the blackness of the nodules comes from included organic matter.
    Spitsbergen cherts contain abundant and remarkably well-preserved microfossils—exquisite tiny gems locked in a tomb of silica. Cherts in carbonates that formed above the high-tide mark usually contain only a single type of microfossil, thick-walled tubes about 10 microns across that form a tightly woven fabric in the rock ( plate 2a ). (A micron is extremely short—one-thousandth of a millimeter, or forty-millionths of aninch. An eyelash is more than ten times as wide as these fossils.) The tubes are interpreted as the extracellular sheaths of filamentous cyanobacteria ( plate 2b ), those hardy bacterial practitioners of “green plant” photosynthesis. The microbial weave indicates that these minute organisms formed the microbial mats whose signature is written in the wavy lamination of encompassing carbonates. Low-diversity cyanobacterial mats occur today along the shoreward edge of restricted embayments from the Florida Keys and the Bahamas to the Persian Gulf and the arid coast of Western Australia.
    On present-day tidal flats, microbial diversity increases toward the ocean, and Spitsbergen rocks show the same pattern. A series of mat-building cyanobacteria-like populations subdivided the ancient tidal gradient, forming discrete communities of mat builders and dwellers (organisms that lived in but did not contribute to the formation of the mat—like clams that nestle among frame-building corals in modern reefs).
    Proterozoic microfossils have long been compared with living cyanobacteria, but how close is the comparison? Most “blue-greens” have simple shapes, and the morphological similarity between ancient and modern forms might mask deep physiological differences. Do we really mean to imply that cyanobacteria found today evolved before trilobites graced the oceans? One beautiful Spitsbergen population provides unusual insight into this issue. Polybessurus bipartitus consists of spheroidal cells 10 to 30 microns in diameter atop stalks made of extracellular secretions ( plate 2c ). Along the seaward edge of the ancient tidal flat, Polybessurus fossils are found as isolated individuals, but in more frequently exposed areas, they occur in dense populations that formed patchy crusts on the sediment surface. As my then graduate student Julian Green (now at the University of South Carolina) first recognized, morphological variations in the preserved fossils allow us to reconstruct their life history. Cells settled on the tidal-flat surface and, as they grew, began to secrete a series of extracellular envelopes. The stalks formed by successive envelopes enabled cells to maintain their position at the sediment-water interface despite an influx of lime mud. Once individuals reached a certain size, they divided repeatedly without intervening growth to form small cells that dispersed and settled again on the sediment surface, beginning the cycle again.
    That’s a lot to know about a Precambrian microfossil, enough for us to seek meaningful comparisons with living organisms. Frustratingly, published compendiums of cyanobacterial biology do not describe living populations with the suite of characters observed in the Spitsbergen fossils. But we knew something else about our microfossils; they lived along a tidal flat that bordered a subtropical to tropical seaway where carbonate sediments accumulated.
    Armed with this knowledge, I traveled with my friend, academic neighbor (at Boston University), and cyanobacteria guru Steve Golubic to the closest modern environmental analogue we could identify—the Bahama Banks. (Science occasionally compensates those who summer in Spitsbergen.) There, on the lonely western edge of Andros Island, we found