SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY
Trace fossils in sandstone. These fossils, known as Cruziana, are the fossilised remains of excavation marks left by Trilobite organisms. These indicate that the sandstone dates from the ordovician period (485 to 443 million years ago). Photographed in Inakacheker, Tassili du Hoggar, southern Algeria.
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