Localities of the Cambrian:

The Burgess Shale

Introduction and Discovery of the Burgess Shale

The Burgess Shale is found in an area of the Canadian Rocky Mountains known as the Burgess Pass, and is located in British Columbia's Yoho National Park. Part of the ancient landmass called Laurentia, centered in Hudson Bay, the Burgess Shale represents one of the most diverse and well-preserved fossil localities in the world. These fossils have been gathered from shales of the Stephen Formation in two quarries opened between Mount Wapta and Mount Field. The upper quarry is known as Walcott's quarry and contains the most famous fossil-collecting site, the Phyllopod Bed. The lower quarry is known as Raymond's quarry, named after Professor Piercy Raymond of Harvard, a visitor of the site who opened the quarry in 1924. It is now appreciated that the Burgess Shale is a site of exceptional fossil preservation, and records a diversity of animals found nowhere else. In 1981, to protect the site from overgathering, UNESCO designated the Burgess Shale as a world heritage site.

The Burgess Shale contains the best record we have of Cambrian animal fossils. The locality reveals the presence of creatures originating from the Cambrian explosion, an evolutionary burst of animal origins dating 545 to 525 million years ago. During this period, life was restricted to the world's oceans. The land was barren, uninhabited, and subject to erosion; these geologic conditions led to mudslides, where sediment periodically rolled into the seas and buried marine organisms. At the Burgess locality, sediment was deposited in a deep-water basin adjacent to an enormous algal reef with a vertical escarpment several hundred meters high.

Cambrian World Map
The Cambrian World:

The map at left shows the Earth as it was in the Cambrian Period. Green areas represent land above sea-level at that time, with red indicating mountain ranges. Dark blue indicates deep ocean basins, while light blue denotes shallow seas of the continental shelf. (For clarity, the outlines of present-day continents have been added to the map.)

The location of the Burgess fauna is indicated by a star on the continent of Laurentia (western North America). Notice that Canada is located just south of the equator!

In 1909, the Burgess Shale was discovered by Charles D. Walcott of the Smithsonian Institution. The legend has it that his horse stopped in front of a rock which he then cracked open, discovering fossils. Walcott called the Burgess Shale the "fossil-bearing unit", but it received little attention until fifty years later. Finally in 1960, Alberot Simonella brought a renewed interest to Walcott's work when he began to redefine members of the Burgess Shale fauna. In 1967, the Cambridge project was intiated by Harry Whittington. Two graduate students, Simon Conway Morris (a worm specialist) and Derrick Briggs (an expert in arthropods) joined him in this endeavor. This trio of scientists became the leading authorities on the site. Recent work has been continued by Desmond Collins, the curator of the Canadian museum.

Through additional work on the site, it was determined that the Burgess Shale included multiple fossil bearing layers of about 2 meters thick stacked 150 meters high and over 60,000 unique fossils have been found. The dominant fossils found are arthropods, but other fossils are found in great abundance, including worms, crinoids, sea cucumbers, chordates, and other organisms with no mineralized shell. Most fossils were found in underwater banks known as the phyllopod beds.

The Burgess fauna contains many fossils of soft bodied animals as well as those with hard parts. This is a tremendous opportunity for researchers because the soft bodied fossils are rarely found anywhere else -- such organisms are usually destroyed before they can be preserved as fossils. When an organism is completely soft, the body usually rots away before it can become fossilized. Hard parts are important because they are more easily preserved; they help the organism last long enough to become fossilized.

C. Walcott Walcott Quarry
Walcott's Quarry : This is the site where Charles Walcott (pictured at left) first discovered fossils of the Burgess Shale. In the distance, you can see the peak of Mount Wapta. The slate and shale of the Stephen Formation are the gray layers, and this is where most of the fossils have been found.

Taphonomy and Study of Burgess Fossils

The Burgess Shale has contributed significantly to the fossil record because of the large number of well preserved fossils found within its layers. These fossils exemplify a unique method of preservation. In most forms of fossilization, the body of a dead organism settles to the bottom of a river or lake whereupon sedimentation slowly covers the body. In the Burgess Shale however, organisms lived in underwater mud banks known as phyllopod beds. Water currents would periodically cause sediments to flow in the form of mudslides, quickly burying the living organism in moving sediment. Fossils are therefore found in random orientation, indicative of a violent mudslide engulfing many of the Cambrian organisms. There is also evidence that these organisms died instantly. First, in the presence of an anaerobic environment, such as mud, marine invertebrates normally curl up upon dying. Fossils of the Burgess Shale locality do not exhibit this coiling. Secondly, there is no evidence of any attempt by these organisms to burrow out of their mud "prison." Killed instantly by the mudslides, preservation began immediately.

The fossils of the Burgess Shale are spectacular, and many of them preserve exoskeletons, limbs, and infillings of the gut. In some rare fossils there is evidence of gut contents and muscle. For example, the internal organs are especially well preserved in the fossilized Ottoia worms. However, there are also many fossils that did not withstand geologic forces of the past 500 million years. Many empty shells have been found because the soft parts decayed and include Scenella and the brachiopods Lingulella and Diraphora. In the case of trilobites, entire exoskeletons have been found without appendages. These trilobites may have molted at the time of their death. The soft tissues of the appendages may have decayed along with the highly perishable body contents, and only the exoskeleton (shell) left behind became fossilized. Another arthropod, Canadaspis, is found in clusters. These clusters have a mixture of fossils exhibiting soft tissues and fossils exhibiting an absence of tissue. The mudslides may have brought moltings and whole animals together, another indication of the history of sediment flow in the Burgess Shale. The phyllopod bed is therefore exceptional for possessing both typical fossil bearing characteristics and unusual soft tissue preservation. This bed is highly important because it records the first appearance of organisms with hard parts in the fossil record.

Many fossils from the Shale exhibit a characteristic dark stain. This provides evidence of the existence of soft bodied organisms during the Cambrian period. After the death of these organisms, radioactive carbon in the soft tissues of the organisms was converted into a film of mica and silicates, creating a stain in the rock layer. Some fossils show this characteristic stain where the soft parts would have been.

Walcott and later workers have used several different methods to excavate these highly delicate fossils. First of all, erosion has accomplished part of the uncovering, brining the fossil bearing layers to the surface. As stated in Whittington (p.40), "erosion of the transported piles of rocks has produced the mountains of today and exposed the shales on the ridge". The first and most inefficient method used to remove specimens was to simply saw through the slabs to get to the fossil, but this proved a poor method of recovery. Preservation of the fossil in its entirety was difficult because the outline of the remains was not known, and risk of sawing through the remains was high. In addition, the crude saw caused fragmentation and disintegration of delicate shales and fossils. Newer techniques have been used over time. Dissolving the rocks in dilute acetic acid (vinegar) has retrieved fossils from limestones. Another helpful excavation method uses UV light at a low angle to show the overall shape, and at a high angle to highlight reflective parts. This makes it easier for researchers to recognize and dig out the fossils without breaking off parts.

Animal fossils of the Burgess Shale

Many fossilized sponges have been found in the Burgess Shale. An extraordinary variety of these asymmetric, sessile invertebrates were found to exist during the Cambrian period. The most abundant of these is Vauxia gracilenta. It had a skeleton of spongy fibers woven into a net. The fossils reveal no spicules or spines, making them different from all other fossilized sponges. The sponge branches complexly, and has a bushlike shape. It bears resemblance to the living sponge Verongia, and is classified in the same family. Only 15 specimens of the sponge Eiffelia globosa remain. It was a sessile, complex globular sponge. The body was supported by calcareous spicules, which are characteristic of the modern calcerae sponges. Lepitomus was an elongated sponge with a thin wall of silaceous spicules. It lived attached to a substrate. It is the earliest known demosponge and an ancestor of diverse later sponges. It is found also in earlier Cambrian rocks.

Brachiopoda, an abundant group among Cambrian fossils, possesses a shell that is lined with a sheet of tissue called a mantle. Micromitra burgessensis represents an example of an inarticulated brachiopoda found in the Burgess Shale. These species show several shells that were originally attached to spicules of the sponge Pirania. These fossils show a possible evolutionary symbiotic relationship between the brachiopoda and Pirania. Evidence of articulated brachiopoda such as Diraphora bellicostata was also found at the Burgess Shale locality. This species exhibits no traces of soft parts, possesses a presumably empty shell, and has two valves linked together. Due to past geological events, only impressions of this species remains (shell is dissolved).

Odontogriphus omalus belongs to the super phylum Lophophorata. This group includes animals having a lophophore such as the brachiopods and bryozoans. The Odontogriphus omalus is found only as an irregularly bounded, darker patch on the shale. It is roughly oval in outline, about 6 cm long, and crossed by many different fractures. It was most likely a gelatinous body that was ring shaped, according to the closely spaced, parallel, dark lines that traverse across the fossil. Also on one end of the fossil is a horseshoe-shaped structure that has evidence of tooth-like objects each with a broad base and a long thin cusp. This was interpreted as a feeding apparatus or lopophore for the flat-bodied animal. The horseshoe shaped structure was also seen as two tentacle bearing arms. It is rare and it bore tooth like objects, making it a very important fossil. These fossils with toothlike features are called, have been objects of great speculation as to their function and zoological affinities.

The Cnidaria include Mackenzia, which was a large organism in the shape of a sac. It lived in a seabed. The body was expandable, with the stomach in the center. A member of the order Actinaria, and a relative of the sea anemone, Wolcott originally described it as a sea cucumber. The interpretation has been problematic because it was all soft bodied, and only an impression has been preserved. Not maany specimens have been discovered, and the creature was probably not prominent in the shale fauna.

The Burgess Shale also provides evidence for the existence of mollusks in the Cambrian period. The mollusk Scenella has a small shell shaped like a cap, but none of its soft parts have been preserved. One of the most common fossils in the Burgess Shale, Scenella belongs to the primitive class Helcionelloida. Members of this class are oval shaped with a sub-central apex creating a cap-like shape. It is related to modern Scenella, but affinities are unclear, and the shell lacks many features of the modern Scenella. Hyolithes carinatus was an elongate, tapering mollusk, cone shaped in cross section. The shell was a tall cone with one flat side. It was sessile, and often became prey. Fossilized specimens have been found in the gut of Ottoia and Sidneya, the worms.

Both priapulid and annelid worms lived in the Burgess Shale. Priapulid worms are short worms that grasp prey with a proboscis. The most common priapulid was Ottoia prolifica, which is very common in the Raymond Quarry. It burrowed, and lived in sediments. A carnivore, many fossils were found to reveal shells in the stomach. It is thought that Ottoia profilica was a cannibal as well. This priapulid does not have any known relations to any extant family. Another worm was Selkirka columbia. Unlike Ottoia, Selkirka was a priapulid with its body encased in a tube. It was also a carnivore, but prey has not been recognized in its gut. It is assigned to its own family. The Annelid worms include the class Polychaeta. The most abundant polychaete worm was Burgessochaeta. Its trunk had 24 segments, and it had long tentacles at the head. It also is not related to any modern family.

The predominant fossils in the Burgess Shale are the arthropods. These are segmented animals with jointed limbs and exoskeleton, and this group today includes insects and crustaceans. The arthropods are the most numerous and varied animals alive today. The most common arthropod found and the most common Burgess fossil found overall is Marrella splendens. This species was used to demonstrate that the specimens had been transported in a turbulant cloud of sediment and buried at various angles in the mud. The morphology of the Marrella is primitive, with a large number of body segments bearing identical limbs. Marrella was previously thought to be similar in morphology to the ancestor that gave rise to any of the three great aquatic arthropod groups (crustaceans, chelicerates, or tribolites). Presently however, the ancestor is thought to be a primitive crustacean. This specimen was interesting because many of the ones found had a dark stain surrounding their rear end. This stain was found to be the results of fluids seeping from the body during early decay.

Another arthropod, Canadaspis, is the most completely known of the crustaceans in the Burgess Shale. It has a carapace that is made up of two valves connected by a straight hinge. The limbs of this organism were biramous and these limbs were used to churn up the sediment in search of the small animals and organic particles upon which it fed. The characteristic of this species that marks it clearly as a crustacean lies in the limbs of the head. A small number of Canadaspis fossils found show two pairs of antennae, the mandibles, and the maxillae characteristics of crustaceans.

Some of the most celebrated Burgess Shale fossils are those of the arthropods in the class Trilobita, or the trilobites. These fossils are known only from the dorsal exoskeleton because it alone was mineralized in life. These fossils preserve limbs and additional "soft" tissues well. There are thirteen Burgess Shale trilobite genera with calcified exoskeletons. The interesting fact about these groups is that they do not preserve many soft parts even though many trilobites usually do. In some cases the explanation for this is that the certain types are rare. In other cases the trilobites may have been buried alive and the soft tissues decayed away in place.

Sidneya inexpectans was an arthropod fossil named after Walcott's son Sidney. Its spiny legs functioned to dig into the mud and grab small prey such as hyolithids and trilobites. It has a head shield, segmented body and tapered tail region. This active deep-water predator had an unusual head with only antennae and stalked eyes. All other appendages were located on its trunk, similar in morphology to the living horseshoe crab but with a tail similar to modern lobsters and shrimps.

There were also many shrimplike arthropods found as well. Waptia fieldensis and Yohoia tenuis are two such examples. Waptia fieldensis had a large head region consisting of an outer hinge-like head plate, stalked eyes, extended antennae, and four pairs of walking branches. The remaining six limbs were blade-shaped filaments forming a thin cylindrical extended tail region. Waptia swam with its blade-like filaments and walked on the ocean floor with its four walking branches. Its motility was maximized on the ocean floor, so it was probably a bottom dweller. Yohoia tenuis had a much smaller head region in comparison to its trunk and tail. The trunk is divided in to 13 segments with blade-like plates. Yohoia had a pair of large jointed appendages ending in four spikes attached to the head. Like Waptia, Yohoia was a bottom dweller and used these unique appendages to scavenge and capture prey.

Burgessia bella was a large, predatory arthropod that had a circular trunk with a wide angular gap in the rear, where a long spine extends from the under body. This bottom dweller had a segmented walking branch under the globular shell indicating motility along the ocean floor. Long flexible antennae served as a sensory organ for blind Burgessia in finding small amounts and organic particles in the sediment.

Only 12 species of Helmetia, large arthropod swimmers, have been found. These predators had flat, wide trunks, a dual spiked head shell, and a large tail region. No evidence of a walking branch and appearance of numerous filamentous appendages indicate that Helmetia was a swimmer. Its wide body is not streamlined and not adapted to catch swimming prey so Helmetia was likely a floating filter feeder.

Chordata is a phylum that includes species that have a notochord. Pikaia gracillens represents a species in this phyla that gives evidence of the existence of vertebrates in the late Cambrian, and is an important fossil in the Burgess Shale because it shows the early origin of the chordates. The preservation of a trace of the notochord in the fossils of Pikaia is unique and defines it as a chordate.

Hallucigenia, similar to a spiky caterpillar, is one of the most famous fossils in the Burgess Shale. This species was first thought to walk on its spines with seven tentacles waving in the water. Later, it was learned that the opposite was true. Claws were found on the ends of its tentacles, indicating the use of tentacles for mobility and the spines for protection.

Wiwaxia was characterized by a dorsal shell consisting of flattened scales and extended spines. Its underside had no protection (naked), and the site of this species mouth filled with two rows of teeth. Due to the location of the mouth on its ventral surface, Wiwaxia was most likely a bottom feeder and used its scales and spines for protection from predators. Its taxonomic relationships are still being debated.

Anomalocaris approached half a meter in length making it the largest and longest Burgess Shale animal. Built with a streamlined flat outer shell, this predator had two claw-like spiked appendages, growing up to 20 cm in length, attached to the head region that were used for capturing prey. Two large eyes on both sides of the head region allow sight precision required for predation. These appendages form constricting rings that crushed hard-shelled organisms like the trilobite common in the diet of this monstrous arthropod. The size and efficient body plan of Anomalocaris made it the dominating predator in Cambrian marine life.

Algae and unicellular fossils

Many unicellular organisms are also preserved here as microfossils. A great many of these fossils are of uncertain systematic classification, not recognizable as relatives of any particular living species. The microfossils are spheres and may occur alone or in chains. This arrangement is also observed in a great diversity of modern algae, and at least some of the fossils may represent those organisms. Other unicellular organisms found include bacteria, cyanobacteria, dinoflagellates, and other protists.

Macroscopic algae are common fossils in the Burgess Shale site. The most commonly found species among them is Morania confluens. This species is found in crowded fragments in the rock slabs. Oddly enough, there are no other species of alga found fossilized in the same vicinity. There are also very few animals found with this alga. Some of the rare animals found with Morania include the polychaete worm Burgessoechaeta and rare examples of Burgessa and Marella. Another alga found is Marpolia spissa. This alga is a rarer find because of its delicate branching, which is easily broken and disentegrated over time. Maripolia has also been found in masses of broken fragments. The animals found near this alga are Eldonia and Wiwaxia. Fragments of both types of algae are preserved in thin, shiny carbon films. By covering a specimen with transparent balsam, J. Walton was able to peel these films off. Upon investigation through a microscope, individual branches showed darker, longitudinal strips, while others had transverse dark bands. Walton concluded that these strips and bands were evidence of cell structures.

An oddity regarding the two algae found is they are in almost complete isolation of one another and from neighboring fauna. Perhaps the environment supporting the algae prior to the mudslides was different from that of most of the animals. In the vicinity of many fossilized animals, there is an absence of these algae, indicative of a heterogeneous environment. Another hypothesis involves the differential height in which the living algae were found. Perhaps over time, portions of these algae were periodically broken off and swept to a separate burial location in clumps.

Since its discovery in 1909, the Burgess Shale has become the authoritative picture of life in the Cambrian Period. No longer solely relying on the remnants of hard shells or exoskeletons, we now have a much better and richer picture of early animal communities. The sediment flow fossilization of the Burgess Shale has produced unique dark stained fossils that reveal the countless variety of soft bodied organisms. Soft-bodied organisms are now know to have existed in greater number and variety than those Cambrian organisms exhibiting hard parts. Additionally, quarries of the Burgess Shale contain evidence of the existence of our chordate ancestors, with fossils so finely preserved that they display traces of a notochord. Most importantly, the Burgess Shale tells of the Cambrian explosion, a huge radiation of marine animal life that included sponges, soft bodied arthropods and those with hard exoskeletons, the first chordates, worms, and trilobites, as well as the strange spiked creatures such as Wiwaxia, and the large predator Anomalocaris. The Burgess Shale represents a snapshot of the evolution of a marine biota that would come to dominate the world's oceans for the next 300 million years.

For more information about the Burgess Shale:

Photo of Charles Walcott courtesy the Hooper V.P.M. Images of the Burgess locality and most fossils courtesy Andrew MacRae. Map of the Cambrian world showing the location of the Burgess Shale modified from an existing map of unknown origin.

Sources: