Lasting Impressions – Fossils in the Canadian Rockies
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.
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
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
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
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 www.Burgess-Shale.bc.ca.
All Material © Ward Cameron 2005