The Burgess Shale contains the best record we have of Cambrian animal fossils. The locality reveals the presence of creatures originating from the Cambrian explosion, an evolutionary burst of animal origins dating 545 to 525 million years ago. During this period, life was restricted to the world's oceans. The land was barren, uninhabited, and subject to erosion; these geologic conditions led to mudslides, where sediment periodically rolled into the seas and buried marine organisms. At the Burgess locality, sediment was deposited in a deep-water basin adjacent to an enormous algal reef with a vertical escarpment several hundred meters high.
The Cambrian World:
The map at left shows the Earth as it was in the Cambrian Period. Green areas represent land above sea-level at that time, with red indicating mountain ranges. Dark blue indicates deep ocean basins, while light blue denotes shallow seas of the continental shelf. (For clarity, the outlines of present-day continents have been added to the map.)
The location of the Burgess fauna is indicated by a star on the continent of Laurentia (western North America). Notice that Canada is located just south of the equator!
Through additional work on the site, it was determined that the Burgess Shale included multiple fossil bearing layers of about 2 meters thick stacked 150 meters high and over 60,000 unique fossils have been found. The dominant fossils found are arthropods, but other fossils are found in great abundance, including worms, crinoids, sea cucumbers, chordates, and other organisms with no mineralized shell. Most fossils were found in underwater banks known as the phyllopod beds.
The Burgess fauna contains many fossils of soft bodied animals as well as those with hard parts. This is a tremendous opportunity for researchers because the soft bodied fossils are rarely found anywhere else -- such organisms are usually destroyed before they can be preserved as fossils. When an organism is completely soft, the body usually rots away before it can become fossilized. Hard parts are important because they are more easily preserved; they help the organism last long enough to become fossilized.
Walcott's Quarry : This is the site where Charles Walcott (pictured at left) first discovered fossils of the Burgess Shale. In the distance, you can see the peak of Mount Wapta. The slate and shale of the Stephen Formation are the gray layers, and this is where most of the fossils have been found.
The fossils of the Burgess Shale are spectacular, and many of them preserve exoskeletons, limbs, and infillings of the gut. In some rare fossils there is evidence of gut contents and muscle. For example, the internal organs are especially well preserved in the fossilized Ottoia worms. However, there are also many fossils that did not withstand geologic forces of the past 500 million years. Many empty shells have been found because the soft parts decayed and include Scenella and the brachiopods Lingulella and Diraphora. In the case of trilobites, entire exoskeletons have been found without appendages. These trilobites may have molted at the time of their death. The soft tissues of the appendages may have decayed along with the highly perishable body contents, and only the exoskeleton (shell) left behind became fossilized. Another arthropod, Canadaspis, is found in clusters. These clusters have a mixture of fossils exhibiting soft tissues and fossils exhibiting an absence of tissue. The mudslides may have brought moltings and whole animals together, another indication of the history of sediment flow in the Burgess Shale. The phyllopod bed is therefore exceptional for possessing both typical fossil bearing characteristics and unusual soft tissue preservation. This bed is highly important because it records the first appearance of organisms with hard parts in the fossil record.
Many fossils from the Shale exhibit a characteristic dark stain. This provides evidence of the existence of soft bodied organisms during the Cambrian period. After the death of these organisms, radioactive carbon in the soft tissues of the organisms was converted into a film of mica and silicates, creating a stain in the rock layer. Some fossils show this characteristic stain where the soft parts would have been.
Walcott and later workers have used several different methods to excavate these highly delicate fossils. First of all, erosion has accomplished part of the uncovering, brining the fossil bearing layers to the surface. As stated in Whittington (p.40), "erosion of the transported piles of rocks has produced the mountains of today and exposed the shales on the ridge". The first and most inefficient method used to remove specimens was to simply saw through the slabs to get to the fossil, but this proved a poor method of recovery. Preservation of the fossil in its entirety was difficult because the outline of the remains was not known, and risk of sawing through the remains was high. In addition, the crude saw caused fragmentation and disintegration of delicate shales and fossils. Newer techniques have been used over time. Dissolving the rocks in dilute acetic acid (vinegar) has retrieved fossils from limestones. Another helpful excavation method uses UV light at a low angle to show the overall shape, and at a high angle to highlight reflective parts. This makes it easier for researchers to recognize and dig out the fossils without breaking off parts.
Many unicellular organisms are also preserved here as microfossils. A great many of these fossils are of uncertain systematic classification, not recognizable as relatives of any particular living species. The microfossils are spheres and may occur alone or in chains. This arrangement is also observed in a great diversity of modern algae, and at least some of the fossils may represent those organisms. Other unicellular organisms found include bacteria, cyanobacteria, dinoflagellates, and other protists.
Macroscopic algae are common fossils in the Burgess Shale site. The most commonly found species among them is Morania confluens. This species is found in crowded fragments in the rock slabs. Oddly enough, there are no other species of alga found fossilized in the same vicinity. There are also very few animals found with this alga. Some of the rare animals found with Morania include the polychaete worm Burgessoechaeta and rare examples of Burgessa and Marella. Another alga found is Marpolia spissa. This alga is a rarer find because of its delicate branching, which is easily broken and disentegrated over time. Maripolia has also been found in masses of broken fragments. The animals found near this alga are Eldonia and Wiwaxia. Fragments of both types of algae are preserved in thin, shiny carbon films. By covering a specimen with transparent balsam, J. Walton was able to peel these films off. Upon investigation through a microscope, individual branches showed darker, longitudinal strips, while others had transverse dark bands. Walton concluded that these strips and bands were evidence of cell structures.
An oddity regarding the two algae found is they are in almost complete isolation of one another and from neighboring fauna. Perhaps the environment supporting the algae prior to the mudslides was different from that of most of the animals. In the vicinity of many fossilized animals, there is an absence of these algae, indicative of a heterogeneous environment. Another hypothesis involves the differential height in which the living algae were found. Perhaps over time, portions of these algae were periodically broken off and swept to a separate burial location in clumps.
Since its discovery in 1909, the Burgess Shale has become the authoritative picture of life in the Cambrian Period. No longer solely relying on the remnants of hard shells or exoskeletons, we now have a much better and richer picture of early animal communities. The sediment flow fossilization of the Burgess Shale has produced unique dark stained fossils that reveal the countless variety of soft bodied organisms. Soft-bodied organisms are now know to have existed in greater number and variety than those Cambrian organisms exhibiting hard parts. Additionally, quarries of the Burgess Shale contain evidence of the existence of our chordate ancestors, with fossils so finely preserved that they display traces of a notochord. Most importantly, the Burgess Shale tells of the Cambrian explosion, a huge radiation of marine animal life that included sponges, soft bodied arthropods and those with hard exoskeletons, the first chordates, worms, and trilobites, as well as the strange spiked creatures such as Wiwaxia, and the large predator Anomalocaris. The Burgess Shale represents a snapshot of the evolution of a marine biota that would come to dominate the world's oceans for the next 300 million years.
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