ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY
Mollusc fossil shells from the 500 million-year- old Burgess Shale rock. These shells are Scenella sp. marine molluscs, which are now extinct. The Burgess Shale rocks, found 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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