Name: Burgess Shale
Location: British Columbia
Age: Middle Cambrian (505 Ma)

Geological Setting and Age

During the Middle Cambrian 505 million years ago, the world was very different. For example, one study suggested that days lasted only 21 hours and an average year had 417 days. The theory of plate tectonics, whereby continents are attached to plates that float on a partly molten layer of the earth known as the asthenosphere, allows scientists to determine the position of continents in ancient times. North America did not yet exist, and the Burgess Shale animals lived on the western edge of a much more ancient continent known as Laurentia.
Laurentia, which was composed of what is now North America and parts of Europe, lay roughly along the equator and was surrounded by calm seas. There was no significant ice accumulation in the poles during this time, and that the climate was probably warmer and more uniform than that of today. Laurentia itself was likely to be warm because of its equatorial location and the presence of geological markers, such as evaporites - deposits that suggest evaporation of water due to high temperature.
Figure 1. Continental map of Middle Cambrian earth. Courtesy of the Smithsonian Institution, Washington, D.C.

The only life on the earth at this time existed in the oceans. Life on land would not evolve for yet at least another 50 million years. The barren surface was a vast desert, characterized low topographic relief (i.e., no mountains) and by wind-blown sand, silt, and clay. The levels of oxygen in the atmosphere, however, was probably only slightly less than present levels. Warm tropical reefs predominated on the edges of Laurentia, much of which was flooded by the large and shallow (epeiric) Sauk Sea. The Burgess Shale fauna lived a reef with high concentrations of algae. This implies its local environment was warm shallow water in the photic zone (light-penetrating zone) because all algae are photosynthesizers, i.e. require light to make their own energy food. Since photosynthesizers produce oxygen as a byproduct of their metabolism, the water was likely highly oxygenated.

The movement of tectonic plates would eventually transport the Burgess Shale far from its original location. About 175 million years ago during the Jurassic Period, orogenesis, or mountain building processes, uplifted the Burgess Shale from the bottom of the ocean and carried it eastward and northward. These processes eventually deposited the Burgess Shale in its current location in Canada.
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Geological Setting and Age
Flora and Fauna
References and Links

Section author: Alexei A. Rivera

This section is part of a Fossil Lagerstätten web site which has been built up as a result of the efforts of the 2002-3 MSc Palaeobiology class in the Department of Earth Sciences at University of Bristol, as part of a course in Scientific Communication.

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