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Fossils reveal that early in the Cambrian, burrowing was sparse and occurred only horizontally along the ground's surface (top). Late in the Cambrian, though, horizontal burrowing activity became more intense (middle) and burrows often extended vertically from the surface deep into the seafloor (arrow, bottom). Credit: K.N. Marenco and D. Bottjer/Palaeogeography, Palaeoclimatology, Paleoecology

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