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6.5. The Burgess Shale - A Window into the Middle Cambrian Biosphere The Burgess Shale records a period just following the Cambrian explosion. This time would see the emergence of some new forms but not to the same dramatic degree or rapidity of the Early Cambrian. In effect, life was starting to evolve within the body plan "rules." Even so, this time sees the evolution of articulate brachiopods (brachiopods with calcareous shells with valves fastened together by teeth and sockets) and an important early chordate, the conodonts ( not to be confused with the earlier conodontomorphs). Figure - The Burgess Shale Quarry, Yoho National Park, B.C. Photo from the International Subcommission On Cambrian Stratigraphy (http://www.palaeontologie.uni-wuerzburg.de/) . The Burgess Shale outcrops in a quarry in Yoho National Park in B.C. (http://www.pc.gc.ca/pn- np/bc/yoho/natcul/natcul15_E.asp) , just next to the border with Alberta. Land Acknowledgement The Burgess Shale site is located within the traditional territory of the Ktunaxa people which extends from south-eastern BC to western Alberta and into Idaho, USA. The Ktunaxa people are also known at the Kootenay, Kootenai, and Kutenai. Interesting fact: Yawunik kootenayi , a species of early arthropod found within the Burgess Shale, is named after the Ktunaxa Nation, hence the inclusion of “kootenayi”. “Yawunik” is a legendary sea monster central to Ktunaxa’s creation story, which describes the origin of humanity. You can read more about Yawunik kootenayi here: https://onlinelibrary.wiley.com/doi/full/10.1111/pala.12161 (https://onlinelibrary.wiley.com/doi/full/10.1111/pala.12161) and you can read about the Ktunaxa story on which the name was based here: https://www.ktunaxa.org/who-we-are/creation-

story/ (https://www.ktunaxa.org/who-we-are/creation-story/) . These extra resources are just for interest and will not be examined. Discovery of the Fossils The fossils here were discovered by Charles Walcott in 1909 who collected more than 65,000 specimens from the quarry between 1911 and 1914. Walcott's specimens were deposited at the Smithsonian Institute in Washington, D.C. but further collections have been housed at the Royal Ontario Museum (http://www.rom.on.ca/) . It wasn't until the late 1960s that the Burgess Shale fauna was recognized for its significant diversity and uniqueness. This special status was due to the work by Harry Whittington, Derek Briggs, and Simon Conway Morris of the Cambridge project. The Burgess Shale is an important fossil find due to the preservation of creatures that lacked significant hard parts. Such softer-bodied creatures very rarely survive the processes of fossilization. Until recently, it was thought that most of the Burgess Shale organisms lived in shallow warm waters on top of a large submarine cliff called the Cathedral Escarpment. It was believed that they were periodically transported off the escarpment by underwater avalanches, called turbidity flows, that were triggered by earthquakes. However, more recent work has suggested most of the Burgess Shale organisms actually lived at the base of the Escarpment rather than on the top. Evidence indicates that this part of B.C. (and all the other areas where Burgess Shale fossils have been found) was close to the equator at this time.

Figure - "Cambrian Sea Mural" at the Field Museum in Chicago. Reconstruction of marine life in the Burgess Shale Reef, 510 million years ago. Anomalocaris sails over a Pikaia and a trilobite. Illustration by Karen Carr (http://www.karencarr.com/) copyright by the Field Museum (http://www.fieldmuseum.org/) . Current Scenario for How This Deposit Happened The Cathedral Escarpment was about 200 meters high. Usually the environment at its base was calm and safe with many animals living either on or in the sea floor or swimming just above it. Fossilized algae suggest sunlight probably penetrated right to the sea floor. Occasionally, there were mudflows which buried the organisms living there in a disorganized jumble. Gradually, over several hundered thousand years, these successive mud flows and other sediments filled in the basin. Figure - A reconstruction of the Burgess Shale site and its surroundings as it were 510 Ma. Image from the Smithsonian National Museum of Natural History (http://paleobiology.si.edu/burgess/) . Several conditions may have contributed to slowing decay and protecting the organisms trapped in the sediment from predators, thereby enhancing the preservation of their soft body parts: 1. The water was relatively under-saturated in oxygen. 2. A layer of sediment rapidly covered the animals. 3. Clay minerals in the surrounding sediment inhibited bacterial activity. Finally, the carbonate rock that made up the escarpment may have protected the animals from the full force of later metamorphic processes. Course Recording of Your Instructor (Louise Longridge) talking about Hiking to the Burgess Shale In this Course Recording I give insights into what it was like to hike up and visit the famous Walcott Quarry of the Burgess Shale, where I went as part of a fieldtrip organized for the Canadian Paleontology Conference in 2003.

Now that you have watched this recording, you should proceed to learn more about the amazing animals found at this quarry and the other quarries that form part of the Burgess Shale! These hypothesized circumstances resulted in the preservation of a complete record of the creatures that lived on top of that algal reef in the warm shallow Cambrian ocean. This included the soft-bodied forms that are not usually preserved. Of the 140 species recovered, around 60-70% are forms that are not usually preserved in the fossil record. This allows us to appreciate that the fossil record is generally biased to those creatures that possess significant external or internal hard parts. Significant Specimens in the Burgess Shale Some of the significant specimens from the Burgess Shale are described below. See more images and several wonderful animations at the Burgess Shale website hosted by the Royal Ontario Museum. Start here (http://burgess-shale.rom.on.ca/en/sea-odyssey/) , or explore further by going to the home page (http://burgess-shale.rom.on.ca/) of that web resource. (Videos may take a minute or so to load; be patient, it's worth it! ) When seeing these images and animations you should ask yourself "How much of this image is based on evidence, how much is well-founded conjecture (i.e. based on observations but not certain) and how much is almost certainly imagination?". Ponder these three questions by yourself, or discuss on- line & with friends if you like. Pikaia : Early Chordate It is from the chordates that vertebrates and ultimately humans will evolve. Pikaia is one of our earliest ancestors but was only a very minor component of the Burgess shale fauna. If you were an alien observing the Burgess shale creatures back in the Cambrian, it's unlikely that you would bet on the vertebrates as being the group that would eventually be manipulating their environment and creating technologies that would permit them to venture outside of their planet of origin. Figure - Reconstruction of Pikaia , a detail from Cambrian Sea Mural. Illustration by Karen Carr (http://www.karencarr.com/) copyright by the Field Museum (http://www.fieldmuseum.org/) .    0:00 / 7:27 1x 1x 

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