Gloppy mats of microscopic life left the same signature on coastal and river bank sediments 3.48 billion years ago that they do today. Earth scientists recently discovered that signature, known as a “microbially induced sedimentary structure” (MISS), on rocks from 300 million years earlier than any previously known MISS fossils.

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A rock surface displaying “polygonal oscillation cracks” in the 3.48 billion years old Dresser Formation, Pilbara region, Western Australia. (Nora Noffke)

To this day, MISS still forms in pools of stagnant water along rivers and lakes or in the muck of coastal mud flats. Complex communities of microorganisms form layers of slimy life in the moist environment. Over time the microbial mat etches its biological graffiti into rock after being buried by sediments and eventually turned to stone. MISS consists of visible polygonal cracks and gas domes in the rock, along with tell-tale microscopic features.

“The structures give a very clear signal on what the ancient conditions were, and what the bacteria composing the biofilms were able to do,” said Nora Noffke of Old Dominion University, lead author of the paper published in Astrobiology, in a press release.

Noffke and her colleagues mapped the fossilized microbial community down to the millimeter scale in rocks from the Pilbara district of western Australia. That district already claimed paleontological fame for the window on ancient life provided by fossilized stromatolites from there. Stromatolites look like large rocky mushrooms or cow poop plops. For the past 3.5 billion years, microscopic life has constructed the stony lumps by trapping sediments.

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Stromatolites once dominated ancient seashores. Now, they form only in a few locations, including Shark Bay, on the coast of western Australia relatively close to where both the fossilized MISS and ancient stromatolites exist.

Top Image: A modern day bacterial mat in run-off from Black Pool at West Thumb, Yellowstone National Park, Wyoming, USA. (Acroterion, Wikimedia Commons)