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It was in 1909 that Charles Walcott of the Smithsonian Institution, noticed an unusual fossil in Canada’s Rocky Mountains in Yoho National Park. It was the discoloured, extremely thin remains of a soft-bodied marine creature. Now wait a minute, he must have thought! What we normally find as fossils are the hard parts such as shell or bone from once living creatures. Naturally intrigued, Walcott returned to British Columbia for several summers. He found a whole collection of soft bodied creatures previously unknown to science.

The fossils were astonishing in their character when first discovered, and now a century later, they are even more amazing. In connection with the Burgess Shale (Walcott’s quarry), scientists coined the term “Cambrian Explosion.” The nature of an “explosion” is “sudden and powerful.” So what was there about the Burgess Shale that seems similar to an explosion? It was not the nature of the soft-bodied fossils, squashed so flat, that was explosive but details surrounding their occurrence.

Cambrian rocks are the first layer in which you encounter traces of many-celled animals  as you move up from the lowest deposited rocks toward more recent sediments. Below Cambrian rocks are sediments which contain only microbes. It is not surprising that Cambrian rocks should contain many-celled animals rather than just microbes, but it is the sudden appearance in these rock layers of a vast array of different kinds of animal which attracts attention. According to evolutionary expectations, these complicated creatures should have slowly descended from simpler organisms. These simpler ancestors should have been preserved in the rocks lower down (older) than the Cambrian rocks. Yet these ancestors are not there. Thus we wonder why we find this amazing collection of different body plans all at once. (A body plan is a pattern of organization of the creature’s body. Fruit flies, flat worms and fish, for example, all have very different body plans.)

Various scientists have described how surprising the Cambrian Explosion is (as discovered initially from the Burgess Shale). Naturally scientists like to speculate about what could have caused an evolutionary process to go into astonishing overdrive and then stall. Stephen Jay Gould articulated this in his book Wonderful Life (which was about the Burgess Shale). He declared: “Since then, more than 500 million years of wonderful stories, triumphs and tragedies, but not a single new phylum, or basic anatomical design, added to the Burgess complement.” (p. 60) What we actually see in the Burgess Shale, Gould says is the opposite of evolutionary expectations. (p. 62) He didn’t abandon his support for evolution however.

The Burgess Shale was the first discovery of soft-bodied fossils of fantastic descriptions. Now we know about several more sites. It was in Australia in the 1950s that the next Burgess Shale community of marine organisms was found fossilized at Emu Bay on Kangaroo Island. However the community was judged to have been deposited earlier. This site included a typical collection of Burgess Shale type organisms including the large predator Anomalocaris (1 metre or more long).

Then in 1984 two amazing sites were discovered with similar Burgess Shale communities of marine fossils. These were Chengjiang in Yunnan Province, China and Sirius Passet in Greenland. Both these communities lie at a very low level in the rocks compared to the Burgess community. Both contain similar communities but with new types as well. At Chengjiang there was Myllokunmingia (possibly a jawless fish), and another similar organism with a backbone. Sirius Passet, for its part, exhibited a moderate number of arthropods (including trilobites and crustaceans), and sponges and worms with spiny exteriors. These worms are found at Burgess too, but not at other similar sites.

In the late 1990s, several sites were found in southeastern Morocco, called the Fezouata community. The interesting thing about the Moroccan fossils is that they lie at a much higher level. The rock in Morocco is not even considered Cambrian, but a higher lying category called Ordovician. The interesting thing is that the Burgess creatures were believed to have become extinct, missing from any rocks above mid-Cambrian levels. But here many were found along with other newly occurring Ordovician creatures like horseshoe crabs. These latter animals are famous as “living fossils”, occurring today along sea shores such as the Atlantic coast in the United States. Interestingly, no animal with a backbone was found in the Moroccan deposits although some are found at lower levels at other sites.

But the discoveries just keep coming!! In 2012 a Burgess type community was found at Marble Canyon, in Kootenay National Park, about 40 km from the original Walcott quarry. Scientists consider the Marble Canyon fossils only slightly younger than Burgess, but “shockingly” different in what is present. One of the key differences between the Walcott Burgess community and Marble Canyon is that the latter contains creatures found in the much lower lying fossil community in China. For example the arthropods Misszhouia  and Primicaris were previously known only from China.

As far as animals with backbones are concerned, the Burgess Shale exhibits much lower diversity than the lower lying Chinese beds. The Marble Canyon site however has yielded many specimens of Metaspriggina (with backbone), previously known only from two poorly preserved specimens from the Walcott quarry. Overall, the preservation and appearance of the fossils at Marble Canyon are remarkably similar to the Chinese fossils which lie at a much lower level.

It is evident that the various characteristics of these Burgess type communities do not fit with evolutionary expectations. As expert Desmond Collins declared in 2009, the centenary of the discovery of Walcott’s quarry: “Additional Cambrian material is now coming from the Chenjiang fauna in China (particularly new chordates, the group that includes humans), and the Sirius Passet fauna in Greenland. Along with the Burgess Shale animals, they demonstrate that virtually all animal groups alive today were present in Cambrian seas.” (emphasis mine)(Nature 460 Aug. 20 p. 953)

Up until the late 1990s, no Burgess type creatures had been found at levels higher than the Walcott quarry. Scientists believed that these creatures were extinct above this point. But now similar creatures have been found along with organisms which were supposed to have evolved long after the middle Cambrian layers. This meant that absence from the fossil record did not necessarily mean that organisms were extinct. So Burgess and the much higher Fezouata community included some types of organism in common. Marble Canyon and the much lower Chenjiang communities were similar. In addition, chordates (animals with backbones), common in the lowest Chinese deposits, but missing from the highest Moroccan deposits, and pitifully few in the Walcott quarry, were well represented at Marble
Canyon. The take home lesson is that it is extremely difficult to draw conclusions about the fossil record based on presence or absence of particular specimens or collections of specimens. There is no sign that marine communities ever evolved. They were created with sophisticated organisms and ecology, and some of their members were suddenly engulfed in sediments deposited in the flood!

 


Margaret Helder
December 2014

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