17 minute read
Lead Story: A Trip to the Burgess Shale
LEAD STORY
A TRIP TO THE BURGESS SHALE
BY DAVID MOORE
The trail to the Burgess Shale is frosty this early on a clear mid-summer morning. Deceptively delicate flowers are covered with fine frost at this hour. Gathering near Takakkaw Falls in Yoho National Park, British Columbia, our journey begins at 1500m elevation. We will hike about 11km and gain 800m in elevation as we make our way to the Walcott Quarry. It is a well-used trail; this route has been worn by hikers, horses and paleontologists for well over 100 years. Most people walk right by the quarry and if it were not for the abundant signage warning them that it is a closed area, most would not even know it was there. The scenery is stunning. Even in July, snow-capped mountains surround us, and glaciers are visible on several of the nearby peaks. A series of switchbacks makes short work of the elevation gain while the air still has an early morning chill. Stopping only for a quick break at a pristine alpine lake, the anticipation builds. For some, this is a beautiful walk in a gorgeous setting, but for many it is a pilgrimage. This is the journey to the place most associated with the origins of complex animal life and ultimately the origins of life like us. For many people, making the pilgrimage to the Burgess Shale is a once in a lifetime, bucket list adventure.
Our destination is the site of one of the most important and famous fossil discoveries in the world. The Burgess Shale, currently understood to be about 510 million years old, has become synonymous with the spectacular, weird and wonderful Cambrian fossils that were first found here in 1909. For amateur fossil lovers and professional paleontologists alike, this is a very special place. The fossils found high on Mt Field have changed the way we view Cambrian life, the origins of complex animals, and our understanding of evolution itself. You would be hard pressed to name a fossil site anywhere in the world that has done more to contribute to our knowledge of early animal life. The Burgess Shale is a UNESCO World Heritage Site and lies within a closed area of Yoho National Park, with quarry access limited to guided excursions.
Walcott Quarry. The vertical drill holes visible are from the work of the Royal Ontario Museum. Walcott’s original drill marks are still visible on the back wall of the quarry.
Photo: David Moore
As we make our way into the alpine, above tree line, the blue-green stillness of Emerald Lake is far below. Finally the tiny ledge of the quarry, hewn out of the shale over the decades, is visible above us. The last bit of the trail, closed to the public, is steep and narrow but no one seems to mind. All day we have been walking in the footsteps of Dr. Charles Walcott. Walcott had a lifelong interest in fossils - particularly of the Cambrian period. He had been Chief Paleontologist and then Director of the US Geological Survey, then President of the Geological Society of America before accepting the role of Secretary of the Smithsonian Institution in 1907. Walcott was an avid explorer and fossil collector and travelled throughout the United States and Canada discovering important Cambrian fossil sites in New York, Vermont and elsewhere.
As the westward expansion of the transcontinental railway in Canada opened up new opportunities for fossil exploration, Walcott ventured west. In August 1909, Walcott was travelling by horseback with his wife, Helena, towards the Burgess Pass on the trail we have been using today. The path was covered with scree which had fallen from the yet undiscovered Fossil Ridge above them. Amongst the scree were slabs with multiple fossils on them, some which would have been instantly familiar to him, like the many trilobites, and some completely unknown. A further examination revealed that in addition to the shells and hard parts there appeared to be preservation of the soft tissue both of known organisms but also of strange creatures he had never seen before — most of which had no hard parts at all.
Walcott returned the following summer and discovered the source of these slabs a couple of hundred meters above the trail. His discovery profoundly altered our understanding of early life and ultimately evolution itself. Here on a treeless shaley slope high in the Canadian Rocky Mountains was a treasure trove of mid-Cambrian fossils, most of which were completely new to paleontology. New forms, weird and wonderful organisms that were unknown to paleontology, were found at the Burgess Shale in abundance. In addition to these new taxa, the preservation of many of the fossils was unlike anything he had seen before. The vast majority of fossils throughout history are the preserved hard parts – bones and shells – of long dead organisms. Here though, in addition to the hard parts of trilobites and brachiopods, soft parts of dozens of different, novel organisms had been beautifully preserved. This was at the time revolutionary, and is still today exceedingly rare. Soft part preservation offers a window very rarely seen into the whole of the animals and the ecosystem. Walcott must have known immediately that this was unlike anything else ever found to date. This was the most complete view of any early fossil assemblage ever discovered. Soft part preservation requires a unique set of circumstances, the fossils must be rapidly buried in very fine-grained sediment in an anoxic environment in the absence of anoxic decaying organisms. Alternatively, oxygen may have been present and mineralized seeps may have been responsible for the lack of scavenging and burrowing. For reasons not well understood this phenomenon was much more common in the Cambrian than in any subsequent geologic time period.
Between 1910 and 1924 Walcott returned to the site many times, excavating and removing tens of thousands of fossils from the Burgess Shale, and sending them to the Smithsonian Institution in Washington DC, where they were studied, catalogued and preserved. The Smithsonian retains one of the most important collections of Burgess Shale fossils in the world.
The Geological Survey of Canada (GSC) has a long history with the Burgess Shale. It was their early discoveries on Mount Stephen in the latter 19th century that brought Walcott to the area. The GSC conducted their own research and collections at the site in the mid 1960s. Dr. Harry Whittington and his group from Cambridge were part of this research. Whittington and his students would perform the first major re-evaluation of the Burgess Shale since Walcott’s time. Their research and conclusions would turn our understanding of earliest metazoan evolution on its head and was instrumental in the development of the theory of punctuated equilibrium by Niles Eldredge and Stephen Jay Gould. This re-evaluation would also form the basis of Gould’s 1989 best-seller Wonderful Life: the Burgess Shale and the Nature of History. More than any other publication, Gould’s book intensified interest in the subject and introduced the Burgess Shale to a wide audience.
Location of the Walcott Quarry of the Burgess Shale (red pin in NW quadrant), located in Yoho National Park within the Canadian Rockies. The city of Calgary is shown toward the eastern edge of the image
source: Google Earth
Beginning in 1975, the Royal Ontario Museum (ROM) began a decades-long research project, still underway today. The aim of the project is multi-faceted; to discover new organisms, to understand the origins of complex animal life and the interactions within the ecosystem and to shed light on the nature of evolution itself. Desmond Collins was the ROMs principal researcher and he devoted most of his career to exploring, collecting and understanding the Burgess Shale. Collins collected over 150,000 specimens, dwarfing the impressive collection at the Smithsonian. He was succeeded by Jean-Bernard Caron in the 2000s. Collins and Caron both discovered new localities of Burgess Shale fossils and added exponentially to our understanding of these animals and their role in the history of life.
A trilobite from the Burgess Shale.
Photo: Sean Smith
Sidneyia, an enigmatic arthropod from the Burgess Shale. Charles Walcott named it after his son Sidney.
Photo: David Moore
Emerald Lake as seen from the Walcott Quarry.
Photo: David Moore
Arriving at the quarry, our eyes begin to pick out the characteristic dark marks on the light shale that represents the fossils. As most of the animals had no hard parts, the fossils are most frequently preserved as darkened films on the lighter carbonate rich shale.
The Walcott Quarry is unremarkable, and most visitors are surprised that it is so small. Given the outsized impact this little ledge of rock on the side of the mountain has had in our understanding of the evolution of complex life, its physical size is underwhelming. You could park a full-sized intercity bus here, but there wouldn’t be room for much else. The cameras, antenna and other security equipment hint that this is a special place. I open the large metal lockbox and the hikers’ skepticism evaporates.
The fossils of the Burgess Shale are contained within shale beds about 25-30mm thick which extend more than 50km laterally nestled up against a limestone escarpment known as the Cathedral formation. The fossils themselves are contained within the Burgess Formation. Several theories have been proposed regarding where the animals lived with respect to the Cathedral escarpment and how the sediment accumulated, and whether they are preserved in the environment in which they lived or died. A number of other localities containing similar fossils to the original Walcott Quarry exist along the edge of the Cathedral Formation. All Cambrian life was marine, but the precise depositional environment of the Burgess Shale has been debated for years. The scarcity of trace fossils suggests that perhaps the assemblage did not live where the fossils are found. The conventional model suggests they were living at the top of a submarine cliff and were buried at the bottom of this feature, but there are many other hypotheses.
The organisms discovered by Walcott captured the imagination of geologists, paleontologists and the educated general public. Here were very complicated forms seen for the first time in the fossil record - with eyes and legs and defensive adaptations such as hardened shells and spines. Animals such as Hallucigenia with its double row of spines on its back which probably evolved as protection from predators from above. The giant arthropod Anomalocaris had grasping appendages to trap prey and a hardened shell for protection. A carnivorous priapulid, Ottoia, is abundant and some specimens are so exquisitely preserved their digestive tracts and contents are preserved. The arthropod Opabinia has five stalked eyes and a single nozzle-like appendage extending from the centre of its head. The hard shells of trilobites likewise provided protection from predation from above, while legs allowed them to seek out nourishment using eyes and other sensory apparatus to find food in an increasingly competitive world.
What the Burgess Shale reveals is a complex and sophisticated fauna highly specialized for the many different niches available in the middle Cambrian environment. Precursor forms were nowhere to be found; it was as if the Burgess Shale fossils had suddenly appeared in the fossil record from nowhere. This led to the idea of the Cambrian explosion or radiation, the concept that complex life had “exploded” into the fossil record via a unique and unprecedented evolutionary radiation.
Takakkaw Falls as seen from near the trailhead.
Photo: David Moore
Trace fossils at the Burgess Shale are rare but do exist. All niches in the environment were exploited by the Burgess Shale animals. Representatives of pelagic, benthic and planktic organisms are all preserved. Most organisms are small, ranging from a few millimeters to centimeters in size, although Anomalocaris and several others reached sizes as great as a metre or more.
Darwin understood that the lack of intermediate or transitional forms in the fossil record made his idea of gradual evolutionary change difficult to prove. While he accepted that evolutionary rates for different taxa were necessarily different, he believed that evolutionary change took a long time and was a proponent of Charles Lyell’s idea of gradualism.
Ultimately the Burgess Shale and other fossil sites would lead researchers in the mid 20th century to develop the theory of punctuated equilibrium. Contrasting with the older idea of gradualism, punctuated equilibrium suggested that under specific favourable conditions rapid speciation can and would occur. Niles Eldredge and Stephen Jay Gould published a seminal paper on the subject in 1972 arguing that most of the fossil record represents periods of stasis where life is more or less static without much evolutionary change occurring.
Due to the poor resolution of the fossil record it can seem that new organisms appeared almost instantly, but these punctuations take many generations and can transpire over hundreds of thousands of years. Internal and external causes of rapid evolutionary change both occur. Internal drivers include chance mutations that can take hold and rapidly change the genome of long-static taxa. External forces include environmental changes, geologic or environmental catastrophes, extinction events and other forces which disrupt the stasis.
Dr. Charles Doolittle Walcott about 1920. Walcott discovered the Burgess Shale and would spend many summers collecting fossils from the quarry that bears his name.
The Cambrian explosion or evolutionary radiation has been the topic of much discussion and controversy for many years. The fossil record suggests that all major extant phyla first emerge near the beginning of the Cambrian, about 540 MYA. The beginning of the Cambrian marks the first occurrence of most modern metazoans. The Cambrian radiation lasts approximately 10-20 million years and by the end metazoan life has diverged and the ancestors of almost all later high-level taxa are firmly established. At the phylum level, the basic body plan of all complex life that comes later has already been set. All subsequent metazoan evolution builds on the basic body plans first seen in the Cambrian.
Life on Earth is estimated to have begun as early as the environment would have supported it. Earliest estimates suggest life began at least approximately 3.8 billion years ago. That early life was for billions of years the exclusive domain of single-celled organisms. It was long believed that multicellularity was a phenomenon of the Phanerozoic, the last 540 million years of evolution. While it is now understood that multicellular organisms may be up to 1 billion years old, life forms like us are a Phanerozoic development.
If the Burgess Shale captures a moment in time shortly after a great evolutionary radiation, the Cambrian Explosion, then what is the evidence and what are the proposed causes or mechanisms of this radiation?
Researchers have pointed to a number of potential causes, although a complete answer probably contains elements of many of these. Changes in the environment, a warming climate at the end of a great glaciation, increasing oxygen levels in the atmosphere and degassing from increased volcanic activity all probably contributed to creating the essential prerequisites. Others have pointed to evolutionary developments themselves, notably the appearance of eyes. But these evolutionary advancements were likely a response to changing circumstances rather than the cause of them. Perhaps the most likely candidate for a cause of the Cambrian radiation is the development of a new behaviour, predation. While tantalizing evidence of bilateralism and the precursors to the type of life so familiar today exists prior to the Burgess Shale, it is perhaps this development that drove the rapid evolutionary advancement of the early Cambrian. Predation changes everything, both in terms of the predator and the prey.
Eyes, sometimes considered a catalyst for the Cambrian radiation, likely evolved in response to the pressure from predation. Both predators and prey would have benefited immensely from being able to see their surroundings, both to find food and to avoid becoming food. Eyes are expensive from an ontological perspective, they therefore must provide a benefit that exceeds the cost of creating them. By the Cambrian, the world had changed drastically. It was no longer sufficient for heterotrophic organisms to wait for food to come to them - they had to go out and find it. The evolution of eyes provided a very significant advantage to those taxa unable to make their own food.
The Ediacaran Period, 635-540 MYA, begins at the end of the Marinoan Glaciation and ends with the beginning of the Cambrian Period. If precursor forms to the animals of the Burgess Shale exist, they will be found within the Ediacaran biota. The Ediacaran was unknown in Walcott’s time, although Ediacaran discoveries were made beginning in the mid-nineteenth century. It wasn’t until the 1950s that their place in the geologic time scale was fully understood. Originally discovered in the 1860s they were not considered to be fossils because they were stratigraphically below the Cambrian, which was considered to be the origin of life on Earth. Early 20th century Ediacaran fossils were assumed to be Cambrian in age. It wasn’t until the mid 20th century that our current understanding of the significance and age of these organisms became clear.
Every modern phylum can trace its origins to this small outcropping of exquisitely preserved organisms high in the Canadian Rocky Mountains. The Burgess Shale is the beginning in many ways of the incredible evolutionary journey that represents complex animal life on Earth. The genetic and morphological change from the Ediacaran to the Cambrian brought so much change in a relatively short period of time that it often has us questioning our understanding of the origins of modern life itself.
The nature of the Cambrian radiation is poorly understood. It is possibly the most significant evolutionary event in the history of life like us. Our oldest ancestors that feature legs, eyes and a mode of life including predation are very far removed from us, but they hint at the beginning of an evolutionary pathway that will ultimately lead to the types of life far more familiar to us in the early 21st century. New research suggests that the Ediacaran may hold more clues to the origins of the Burgess Shale organisms.
A major question remains in the study of earliest complex life. Do the Ediacaran fossils represent the precursors to later complex animal life or do they represent an evolutionary dead-end? Recent research suggests that the Cambrian radiation was more gradual than it may appear in the fossil record. As such, the seeds of the complexity in morphology and behaviour that the Burgess Shale and other early to mid-Cambrian fossil sites reveal may be more an artifact of the incompleteness of the fossil record than a single extraordinary evolutionary event. New discoveries and interpretations suggest that the Ediacaran biota was more complex than previously thought. Bilateralism, hollow body plans with guts, segmentation and other advanced features suggest that these organisms may indeed be the precursors of organisms of the early to middle Cambrian. If these so-called advanced features exist in the Ediacaran it suggests that the evolutionary radiation may have been less dramatic than previously thought. Research has also suggested that the Ediacaran underwent its own evolutionary radiation, the Avalon radiation, and that evolutionary events like this may be more common than previously thought.
In any case, the animals of the Burgess Shale have the power to inspire and get us to ponder some of the deepest questions in the history of life. Future work will no doubt further illuminate one of the most fascinating and mysterious time periods in evolutionary history.
As we make our way down the trail past the glaciers and alpine lakes the group is quieter, perhaps reflecting on the significance of what we have experienced, pondering the great questions that have been raised by the presence of these amazing fossils high on the mountain. Packs are a little lighter and the trail gradually loses elevation. The scenery is no less spectacular, but our thoughts are lost in the wonder of the possibilities of the earliest animal forms that would one day evolve into the rich diverse abundance of the Devonian oceans, the well-known large vertebrates including terrestrial reptiles and dinosaurs of the Mesozoic and ultimately to the fauna of our modern world.
Returning to our vehicles there is a shared sense of accomplishment, of a day very well spent. While some are happy to remove muddy boots from tired feet, we are all left with a feeling of having done something unique, revealing a part of our shared history and the incredible story of the evolution of life on Earth. For those who have not made the journey it is a pilgrimage that we should all undertake even if just once in a lifetime.
REFERENCES
Bobrovskiy, I., Hope, J.M., Golubkova, E. et al. (2020) Food sources for the Ediacara biota communities. Nature Commun 11, 1261.
Caron, JB., Gaines, R., Aria, C. et al. (2014) A new phyllopod bed-like assemblage from the Burgess Shale of the Canadian Rockies. Nature Commun 5, 3210
Eldredge, Niles and S.J. Gould (1972) Punctuated equilibria: an alternative to phyletic gradualism. In TJM Schopf (ed) Models in Paleobiology. San Franscisco: Freeman Cooper. pp. 82-115
Watson, Traci. (2020) The bizarre species that are rewriting animal evolution. Nature Vol. 586, No. 7831. pp. 662-665
Zhang, X., Shu, D., & Erwin, D. (2007) Cambrian Naraoiids (Arthropoda): Morphology, Ontogeny, Systematics and Evolutionary Relationships. Journal of Paleontology, Systematics and Evolutionary Relationships. Journal of Paleontology, 81 (S68) pp 1-52.
