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Lasting Impressions – Fossils in the Canadian Rockies

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Introduction to Mountain Fossils

Fossils are one of the most exciting finds in the mountains. Suddenly, while wandering through the high country, you stumble across an impression of a species of animal that lived hundreds of millions of years ago. Usually, it is a cast of the animal’s shell, or in the case of plants, an impression of a leaf or stem. Since the hard parts of an animal, its shell in particular, remain in ocean sediments long after their softer flesh has disappeared, they are usually our only hint of what type of creatures lived in a particular time period. Occasionally, as in the case of the Burgess Shales, the soft bodied parts were preserved along with their shells. This is exceedingly rare, and sites like the Burgess are jealously protected.

The entire process of fossilization is a geological accident. It may occur when the remains of an animal or plant are buried under deep sediments, where oxygen is in limited supply. Oxygen is one of the major factors in the breaking down of dead plant and animal material. If an animal dies near the edge of a shallow continental shelf, his carcass may wash down into the deep waters adjacent to the shelf. These deeper waters are oxygen poor, and this allows his shell to remain in the muds for a significantly longer period before slowly breaking down. As more sediments pile up on top of the shell, it leaves an impression in the ever compressed sediments surrounding it. Later, when the shell itself has broken down and vanished, the impression remains as a permanent record. Some times, the mold left behind will be filled with minerals like quartz or calcium carbonate, creating a perfect cast of the shell. The chances of fossilization occurring are exceedingly small, and this helps account for the rarity of high quality fossils.

To geologists, fossils represent specific periods of time. Through careful examination of rock structures and ages, along with a detailed study of the remaining fossils, scientists can date the rock formation. The study has become so detailed, that with little more than fossils, scientists can estimate the age of the rock from which the specimen was collected.

Fossils are not always found as individual shells. In many cases fossil deposits may be huge, as in many of the limestone cliffs surrounding us. Much of this limestone represents the remains of microscopic shells of tiny organisms. In other cases, the limestone resulted from the fossilization of ancient coral reefs. Whether large or small, searching for fossils provides one more way of appreciating the mountains around us. Remember though, within the parks, the fossils are protected so please leave them where you find them.

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The Burgess Shales

Yoho National Park contains several rock outcrops collectively known as the Burgess Shales. These fossil bearing shales have forced scientists to completely revise many of their theories on the evolution of life.

Discovered by Charles Walcott in 1909, it is one of few sites worldwide where soft bodied animals left almost perfect fossils.

This may not seem important until you realize that most of the study of early life is the study of hard parts – trying to learn something about an animal from a mold or cast of its shell. This tells us little about what lived inside the shell. This quarry uniquely preserved a huge variety of soft bodied creatures along with their shells.

How did this high level of preservation occur? Scientists believe that a mud slide came down into a shallow bay, and then washed the animals caught in its flow down into the oxygen poor depths far below. Since oxygen is a major factor in decay, this strange incident allowed their soft carcasses to persist in the sediments long enough for their remains to be preserved as fossils.

The timing of this tragic slide was also critical. It occurred in middle Cambrian times, around 505 million years ago. This puts it right in the middle of the greatest explosion of life in the history of the planet. The Cambrian was the first period in which multi-cellular life appeared, and within a geological blink of the eye, the world was populated with a diversity never before (or since) experienced. In no other site on the planet can scientists match the high level of preservation of this site along with such excellent timing.

As Walcott began to grapple the significance of this site, he began to collect ravenously, carrying tens of thousands of specimens back to Washington, D.C. Unfortunately, he never found the time to unravel their true significance. In the limited research he completed, he forced the fossils to fit within groupings of animals with which the scientific world was already familiar. While this may seem a natural conclusion, it became his greatest error. Many of the animals present in the Burgess Shales cannot be classified based on an analysis to life on earth today.

Harry Whittington, one of Walcott's successors began to discover the secret of the Burgess by careful examination of the fossils left behind. Whittington began in 1971 with a creature called Marella. This animal was clearly an Arthropod (the group of animals containing all the insects, spiders and crustaceans, as well as the extinct trilobites) but he found that it didn't fit into any Arthropod group known to exist today.

Moving on to Opabina, Whittington got an even bigger surprise. This creature didn't fit into any known Phylum (the Arthropods are one Phylum, and all living creatures fit into at least one known Phyla). Never before, at any other fossil site in the world, had fossils been found that simply could not be classified. This had far reaching consequences.

Most standard discussions of evolution show a single primitive ancestor giving rise to a wide variety of future species. We expect there to be less diversity early in the history of life – not more. In actual fact what we have found is that there were many more basic structural plans at the beginning of life than today.

Natural selection states that in any competition between two species, one will have some genetic advantage that will allow it to survive at the expense of its inferior competitor. This means that if we were to roll the clock of time back, each time these two competitors squared off, the same result would occur. 

Survival of the fittest may be invoked to say that these unclassifiable animals were merely inferior and failed to survive. However, scientists could not find anything that would indicate inferiority in these unique animals.

This led scientists towards a new and exciting possibility 'survival of the luckiest'. Some of the extinctions may have been based on some environmental, or otherwise unpredictable, fluke. The end result may be that animals perish, although not inferior in any way.

The long term results become very interesting. If chance plays a major role in the extinction of species, than perhaps if we were to turn back the clock of time, an entirely different outcome could arise. This could in turn change the entire sequence of events following this particular event – and could have even changed the eventual evolution of man.

In this same quarry, fossils of the first known Chordate (back-boned animal) were also discovered. Had it been one of the unlucky ones, we may have never evolved. This does bring a whole new view to evolution. This chance factor is known as contingency and is being widely accepted as a valid aspect of evolution today – and all because of a tiny quarry in Yoho National Park.

The Yoho-Burgess Shale Foundation operates guided hikes during the summer months. For more information visit

All Material © Ward Cameron 2005