Algae fossils. Light micrograph of an assortment of fossilised algae. These algae date from about 2.1 billion years ago, during the earliest geological era (the Pre-Cambrian). Primitive algae like these generated Earth's oxygen atmosphere. Algae fossils occur because many species grow a protective, mineral shell. These fossils were found in gunflint. Flint rocks are sedimentary rocks made from compressed, silicaceous material. Flint rocks are still forming from the deposition of the shells of radiolarian algae on the ocean floor. Photograph of gunflint from Ontario, Canada. Magnification: x325 at 35mm size with brightfield illumination.
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