Formation

Formation of the Burgess Shale

The Cambrian explosion marked the first appearance of multi-cellular organisms in the fossil record. The ‘explosion’ took place 570 million years ago within the span of a few million years, which is quite short in geologic terms. The Burgess Shale was formed 530 million years ago (Morris 1989), slightly after the Cambrian explosion, (Middle Cambrian Period) near the town of Field in Yoho National Park, British Colombia (Gould, 1989).

The Burgess Shale is an incredibly important site because it contains the only soft-bodied fauna from this time (there are some other Cambrian soft-bodied organisms preserved but they are not precisely contemporaneous to the Burgess Shale). All of the fossils in the Burgess Shale lie in the Stephen Formation, next to the Cathedral Escarpment. The Cathedral Escarpment was a massive, nearly vertical reef that was created by algae (Gould, 1989). Habitats such as these are known to be shallow and well lit; consisting of highly diverse Precambrian marine fossils.

The basin in front of the Cathedral Escarpment took over 2 million years to fill. Mud began to settle through the water, building up layers of sediment on the sea floor (Coppold and Powell, 2000). Currents moved the mud near the front of the Cathedral Escarpment. Occasionally, the mud slumped because of earthquakes; and any creature that lived on the sea floor would be deposited within the mud flows. The benefit of the mudslide that buried the fossils of the Burgess Shale is that it protected them from any Precambrian scavengers, as well creating anoxic conditions.

The mudflows buried the fossils quickly, creating an anoxic environment (oxygen deprived) so that the organisms covered in the mudslide were protected from bacteria that could decompose their bodies. The preservation of the Burgess Shale fossils was possible because they were buried by clay. The organic molecules that are necessary for decomposition stuck to the surfaces of very fine grained clay that coated and preserved the fossils (Coppold & Powell, 2000). The results are the thin films and soft-bodied fauna that define the Burgess Shale today.

Coppold and Powell.(2000).  A GeoScience Guide to The Burgess Shale.  Field B.C, The Yoho-burgess Shale Foundation.

Gould, Steven J.(1989).  Wonderful Life:The Burgess Shale and the Nature of History. New York, W.W  Norton &Company.

Morris, S.C. (1989). The community structure of the Middle Cambrian phyllopod bed (Burgess Shale). Palaeontology 29(3) 423-467.

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