Burgess Shale

A Closer Look

Animals in the period known as the Cambrian Explosion sported bizarre combinations of legs, spines, segments, and heads found in no present-day animals. Many of these animals became extinct, leaving no descendants, whereas others may have evolved into groups that are familiar to us today. Most of our knowledge about these early life forms comes from the Burgess Shale, a 540-million-year-old formation of black shale discovered in 1909 by Charles Walcott in the Rocky Mountains of British Columbia. The unique process of fossilization that occurred in the Burgess Shale allowed exquisite preservation of these early animals. While in most cases a reaction to oxygen causes the soft parts of animals to rot away prior to fossilization, the Burgess Shale animals were killed instantly by a mudslide deep in the ocean, where there is a lack of oxygen. After burying the animals, the mud hardened into shale, preserving the soft animal parts. At the time of his discovery, Walcott was able to classify the fossils as ancestors of modern animals. The Burgess Shale was reexamined in the mid-1960s, and many new, unknown fossils were found. When Harry Whittington, Derek Briggs, and Simon Conway Morris studied these new fossils in the 1970s and 1980s, they realized that many of them did not fit into the modern classification system. The implication that there were more basic animal forms in the Cambrian Period than there are today shook up traditional ideas about evolution. In 1989 Stephen Jay Gould brought the Burgess Shale to wide public attention with the publication of his book Wonderful Life.