Researchers have discovered 650-to-640 million-year-old sponge fossils at Australia's Flinders Ranges. These fossils predate the oldest known animal fossils by 90 million years and antecede the blossoming of life during the Cambrian Explosion ~524 million years ago.
Princeton University's Adam Maloof and his team discovered these fossils by meticulously grinding up an ancient fist-sized rock and mapping the millimeter-long variations using 3D imaging. In an article published today in Nature Geoscience, the researchers describe both their findings and methodology:
The Neoproterozoic era was punctuated by the Sturtian (about 710 million years ago) and Marinoan (about 635 million years ago) intervals of glaciation. In South Australia, the rocks left behind by the glaciations are separated by a succession of limestones and shales, which were deposited at tropical latitudes. [These are] millimetre- to centimetre-scale fossils from the Trezona Formation, which pre-dates the Marinoan glaciation. These weakly calcified fossils occur as anvil, wishbone, ring and perforated slab shapes and are contained within stromatolitic limestones. The Trezona Formation fossils pre-date the oldest known calcified fossils of this size by 90 million years, and cannot be separated from the surrounding calcite matrix or imaged by traditional X-ray-based tomographic scanning methods. Instead, we have traced cross-sections of individual fossils by serially grinding and scanning each sample at a resolution of 50.8 μm. From these images we constructed three-dimensional digital models of the fossils. Our reconstructions show a population of ellipsoidal organisms without symmetry and with a network of interior canals that lead to circular apertures on the fossil surface.
And here's a video of the fascinating modeling process from New Scientist:
New Scientist also notes that Oxford palaeobiologist Martin Brasier believes that this discovery lends credence to the theory that life facilitated the creation of a "Snowball Earth," or that the earth cooled down during the Marinoan Glaciation ~635 million years ago. The presence of mulitcellular sponges would've abetted a carbon sink that would've caused the global cooling, but Brasier also warns that the Flinders fossils could be large single-celled organisms or slime molds.
Top image of the Cambrian Explosion via Astrobiology Magazine.