ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY
Crustacean fossil from the 500 million-year-old Burgess Shale rocks. This is Waptia fieldensis, a marine crustacean that is now extinct. Only the rear end is seen. It steered through the water by using its telsons (tail-flaps, lower right). The front end resembled a lobster. It reached a length of around 7 centimetres. The Burgess Shale rocks, in the Canadian Rocky Mountains, were formed by avalanches of fine mud that preserved soft tissue details rarely found in the fossil record. This allowed them to record the Cambrian Explosion, the evolution of an extremely diverse array of invertebrate animals from which today's animals evolved.
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