ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY
Trilobite fossil from the 500 million-year-old Burgess Shale rock. This is Kootenia burgessensis, an early trilobite. The segmented form of the shell is seen, and the head is at top. Trilobites were arthropods that fed as they crawled on the seabed. They lived between 500 and 300 million years ago and 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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