Geoscience 240
LAB 06: THE BURGESS SHALE FA UNA, ITS FACIES AND
PRESERVATION – MIDDLE CAMBRIAN ~505 MA
INTRODUCTION
The Cambrian period saw the appearance of diverse hard-bodied marine invertebrate animal life. The
Cambrian was the onset of the first Calcite Ocean and original mineralogy was all Low Magnesian
Calcite. This Hothouse, Low Mg-Calcite time extended through the Early Paleozoic (CambrianDevonian) despite regional glaciations in Middle Ordovician. While there is a scant soft-bodied animal
record (sea pens, sponges) for the Ediacaran, stromatolites and biochemical traces going back to
Archean, the hard-bodied ecosystem seems to appear by fiat in its full complexity, perhaps with even
more phyla than are recognized as living today. This paleontological riddle is called the Cambrian
Explosion of Life circa 545 Ma. A mere 40 Ma later, the Burgess Shale, Middle Cambrian has some of
the most complex and detailed soft body part preservation for these nascent invertebrates. Examine
the phylogenetic tree for the relationships and sequence of evolution for biologic organisms. Note that
evolution proceeds to the right but time does not do so uniformly for all branches of the tree. For
example Fungi seem primitive yet did not fluoresce or evolve until after land plants and oxygenated
atmosphere were well established. There were certainly soft bodied sponges by 700 Ma from organic
marker compounds but spicules did not come until much later. There were probably soft bodied
precursors to many of the Middle Cambrian Hard bodied invertebrates including various kinds of
worms, slugs (Mollusca) and arthropods. Heads, eyes, bodies, tails all came before armour. Some of
the Cambrian fauna may have had zooplankton precursors.
The chart comes from The Animal Cognition Network at:
http://www.animalcognition.net/phylogeny.html
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The Animal Cognition logo above combines the tree of life with a neurological network. The tree of life
is drawn from current data obtained from the Tree of Life website, however, the relationships between
different organisms are subject to change. Note that all animals and plants are eukaryotes, which
means they have several distinct organelles contained within their cellular membranes: nucleus,
mitochondria, Golgi bodies, chloroplasts etc. More primitive chemoautotrophs like sulfur reducing
bacteria, iron reducing bacteria can subsist by chemosynthesis and are often involved within more
complex living organisms or are implicated in their fossilization (anoxic). Some researchers favour
that Archaea are the ancestors of all modern life, not Eucarya.
In the tree of life above, all organisms starting from Porifera (Sponges) belong to the Animal Kingdom.
Only the Porifera (and possibly the Placozoa, which are not very well understood) do not have nerve
cells.
The Burgess Shale outcrops at the Walcott Quarry on Mt. Stephen near Field, B.C. are a world
heritage site. They were discovered and first studied by Charles Dolittle Walcott in 1909. The most
productive fossil bearing shales occur in the middle of the, thick, deep water?, facies of the Stephen
Formation. Its relationship to the underlying and neighboring Cathedral Formation is shown here.
The Walcott Quarry is the small yellow rectangle. The Stephen formation is the overlying and
enveloping olive layer. The underlying and neighboring Cathedral formation is shown in grey. The
orange fin is the dolomitized Cathedral Escarpment which figures prominently in most theories for the
excellent Burgess Fauna’s preservation.
The shale contains the best collection of Middle Cambrian marine invertebrate life. All ancestral
marine animal phyla are present and since then the life web has become apparently less diverse and
whole body plans have been lost. The Burgess collections, from shales and carbonate facies, include
not only the typical hard bodied benthic invertebrates but many soft bodied forms as well. The details
of preservation are rare and unique. The cause of this preservation is still under debate. Papers that
formed the basis for Gould’s book “It’s a Wonderful Life” favour an anoxic basin that was capable of
receiving landslide debris and instantly burying and preserving delicate soft body parts. As a warm up
exercise, read the exchange of letters below, and examine S. J. Gould’s book and the collection of fossils
we have on loan from the Royal Ontario Museum, to make your own labeled drawings and
observations. Note the detailed preservation of delicate internal organs, soft body parts and
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integumentae! Any sedimentary deposit with excellent fossil preservation is termed a Lagerstatten,
irrespective of age. Other notable Cambrian Lagerstatten fossil beds are found in the Wheeler Shale in
Utah and the Maotainshan Shale in China. The Burgess remains the most famous and best studied.
Hypothesis testing: “Any hypothesis demands the test of preliminary doubt.” (Why is the soft bodied
preservation so excellent here?)
The Letters: Discussing The Facies of the Burgess Shale Fauna and the reasons for The enhanced
preservation of these Middle Cambrian Fauna near Mt. Stephen – Field B.C.
From: Tark Hamilton
To: tyler.beatty@NRCan.gc.ca
Cc: alan gell
Sent: 22/03/02 5:52 PM
Subject: Burgess shale facies question
Hey Tyler,
I got your id from Bob Thompson. He said you ran the Burgess field party for 2 summers and are
currently doing a masters on related geology. I am Ex-GSC and teaching at Camosun College right now.
In doing a sedimentology lab I presented about 30 specimens from the Burgess Shale and showed the
NFB/CBC videos. The models presented for the facies involved an anoxic basin at the foot of a cliff
(reef of some kind?) with either proximal submarine landslides carrying shallow fauna to preserve
and bury or distal turbidites originating possibly from Wapiti Pass region.
The films showed what looked like Bouma D-E finely laminated shales but no coarse components in
the Walcott Quarry location, favouring the second model.
The collection contained wispy filamentous algae branching from single points that appeared to be
points of attachment. There was also a specimen with a limpet like mollusc, of many different sizes all
in apparent living position but no signs of rotation or abrasion. How do delicate, seemingly in situ
shallow water fauna figure into the physical sedimentology picture?
I'd appreciate your thoughts or other references on the subject to clarify this for me. If you have any
thoughts please cc. Alan also in your reply.
Thanks,
Tark
tyler beatty <twbeatty@sfu.ca> wrote:
Hello Tark,
Your observations are both refreshing and astute and I will try to address them while giving you my
take on the sedimentology of the Burgess Shale (BS).
First things first, most of the tales spun on the BS have been an attempt to address what it is most
famous for, soft body preservation. This has led most authors to invoke some sort of rapid burial and
anoxia. Another issue is the fossils that receive all the attention seem to occur in specific layers which
may have DE characteristics, if only in grain size. The hard fauna however, is fairly evenly distributed
throughout the coarser grained (silt sized) interbeds that are distinctively more calcareous. Trilobites,
brachiopods, and monoplacophorans (Scenella- your limpets) are found in similar abundance in both
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soft body bearing beds as well as only hard part bearing beds leading to the hypothesis that sediment
chemistry may be the real limiting factor in terms of preservation type.
The community structure ranges between individual fossiliferous beds, from robust benthic
ecosystems to neritic dominated to planktic dominated (in the case of the algal beds). Studies done
which support episodic transport of these communities to the base of the escarpment have largely
used these neritic fauna dominated beds as evidence, random orientation of swimmers however is of
no surprise. Many of the benthic dominated communities show spatial distribution of faunal elements
that suggest in situ deposition. Our problem then becomes one of establishing a community at the
base of a 300m escarpment.
Without resorting to Ludvigson's (1989) ramp model it becomes difficult to hypothesize that such
communities could thrive below the photic zone, especially since many of the arthropods have well
developed eyes. I do believe in the escarpment however, as I have seen the contact at several sites
now.
My hypothesis lies in the escarpment as a dynamic feature that, once it was generated (evidenced by a
carbonate debris apron at the base of the formation), it became the site of algal reef growth and
therefore it grew coincident with the BS. The problem then becomes reconciling this with Fritz's
(1971) trilobite biostratigraphy. It is here that Ludvigson probably got things right, he suggested
applying a dual biostratigraphic zonation that accounted for spatially controlled trilobite biozones i.e.
restricted platform species and open marine species.
If we then apply a sequence stratigraphic paradigm, we see that the biozones match up with the
internal lithologic divisions of the BS formation (of Fletcher and Collins, 1998) from shale dominated
(transgressive) lithomembers which house open marine trilobites i.e. the Walcott Quarry member to
limestone dominated (regressive) lithomembers which harbour restricted species.
Additionally the limestone lithomembers tend to have an abundance of what I have interpreted as
tempestite beds that contain algal coated bioclasts and well preserved blue-green algae that were
likely derived from rim or platform lagoon environments.
So to return to the original question, we probably are dealing with in situ communities or relatively so
(10s m transport).
Whether these communities were stable or we have a record of pioneers into a variably anoxic
environment is yet to be seen. However, with the abundance of neritic organisms, the lack of partially
decomposed organisms and organisms such as Ottoia found in dwelling structures, the evidence
suggests that beds that contain soft body preservation most likely represent rapid toxic kills.
I hope that you can glean something from my ramblings, I have an abstract available from the
GeoCanada 2000 meeting in Calgary. If you can't find someone with a copy of the conference CD I can
probably e-mail it to you.
It's refreshing stirring up these old ideas, as I have now left the BS behind and have crawled up into the
carboniferous to study conodonts, its a shame we never unearthed a conodont animal in Walcott's
Quarry.
Cheers and thanks for the interest, Tyler
Tyler W. Beatty B.Sc.
M.Sc. Candidate, Dept. of Earth Sciences
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Simon Fraser University, Burnaby, BC, V5A 1S6
04/05/2002
Dear Tyler,
Thanks so much for your scholarly and open discussion.
It makes things a lot clearer than the "sales take" of the films. Makes more sense for the actual
specimen collection and lithologies I saw. Your communique will be a good addition to any
subsequent attempts to teach this suite and problem.
Cheers, Tark
More background available in the introductory films on the Burgess Shale & Books in Lab
THE COLLECTION
Carefully and non-destructively examine the collection material taking care to keep the identification
with each specimen. Draw, label and give scales for the fossils below. Next to your drawing describe
what role this life form has in the Middle Cambrian ecosystem (what did it do, where did it live, what
did it eat, what did it produce or leave behind): Most of the Burgess life fauna were either: 1) infauna
(burrowers, mud dwellers) within the shallow seabed, 2) sessile benthos (fixed benthic animals
(corals, sponges, bryozoans) that could filter feed from passing currents or 3) vagile benthos
(crawling, slithering bottom dwellers) who could prey on smaller animals or scavenge. There is very
little evidence of nektonic animals (swimmers). An ecosystem or trophic web is never based on
animals, but on primary producers. The Burgess assemblage has 2 candidates: algae (floating or fixed)
or bacterial mats (chemoautotrophs). The algae and bacteria mats at the Burgess site, Walcott
emphasized as being of equal importance to the animals.
- Filamentous Algae. Green algae had just evolved and today are abundant oxygen producers as a byproduct of their metabolism and production of complex organic molecules.
The green algae (singular: green alga) are the large group of algae within which the embryophytes
(higher plants) emerged in the charophytes. However, as the embryophytes are traditionally
excluded, they form a paraphyletic group, although the clade including both green algae and
embryophytes is monophyletic and may be referred to as the Viridiplantae and as the kingdom
Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as
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well as various colonial, coccoid and filamentous forms, and macroscopic seaweeds. In the Charales,
the closest relatives of higher plants, full cellular differentiation of tissues occurs. There are about
8,000 species of green algae. Many species live most of their lives as single cells, while other species
form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.
Though there are several Late Proterozoic fossils (both unicellular and multicellular) which
superficially resemble green algae, fossils which can be assigned to the group with some confidence
are not known earlier than the Cambrian. This is, perhaps in large part, because the group has
traditionally been defined by pigment composition (2 pigments Chlorophyll A and B which can be
extracted from bulk sediment like porphyrins), and not with more easily identified morphological
characters. Fossils were thus assigned to this group because of overall resemblance to modern taxa,
Recent morphological work has added a number of characters which should aid in recognition of
members of this group.
The Dasycladales has perhaps the best-known fossil record of any group of green algae. More than 120
fossil genera have been described, some dating back to Cambrian and Precambrian strata. This fossil
diversity contrasts remarkably with the fact that today there are only eight extant genera, including
the familiar "mermaid's cup" Acetabularia. The Dasycladales began to rapidly diversify in Middle
Ordovician, and are common in all strata until the lower Cretaceous. Both living and extinct species are
known primarily from warm marine waters.
Many members of the Dasycladales secrete lime (calcium carbonate) which increases their chances for
preservation and later discovery as fossils. The group is easily recognized by their radial symmetry,
with a central nonseptate axis to which are attached whorls of lateral appendages which may or may
not be branched. Fossil forms are cylindrical, club-shaped, or have a spherical appearance from the
density of their branches. Because of lime secretion (like coralline algae today) even these primitive
plants can contribut substantially to allochem load and sediment accumulation. When these occur as
sessile benthos (fixed to one place on the seabed) they additionally help stabilize sediment, aiding
accumulation and opposing erosion. The big advance of the Green Algae is having 2 different
chlorophyll types for efficient photosynthesis in higher versus lower light levels. Once this evolved
along with tissue differentiation, the pathway to modern plants was set.
- Bacterial mats: Bacteria are often chemoautotrophs using some redox reaction of simple organic
molecules (like methanophores) or inorganic compounds like sulphate to sulphide or Fe+3 (or ferrates)
to Fe+ . Some form near hot vents/black smokers deriving their energy from sulphur oxidation. They
form gelatinous biofilms on rivers, ponds, in soil porosity, sediment water interface, or in the
subsurface in sedimentary porosity with the right chemical environment. Methanophores oxidize
methane at or below the sediment water interface and can leave carbonate deposits out of their
inorganic stability field (e.g. cold deep seafloor vents). Rather than colonies of a single type of
organism, these are often complex communities with many different commensal organisms with
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different chemical roles that collectively help the mats to grow and flourish. Today, many of these
colonies are extremophiles. The NOAA image above (left) is of a white carbonate forming bacterial
mat from a cold methane seep over a gas hydrate diaper on the Blake Ridge. They produce calcium
carbonate from the suboxic oxidation of methane coupled with sulphate reduction to sulphide. The
Blake Ridge is 3500 m depth SE of South Carolina on the Atlantic passive margin. The orange and grey
image on the right above is of bacterial mats on Mn crusts (grey) and Fe crusts, (Orange) at West Mata
in Lau Basin (Tebo & Davis, 2012). They also found Fe forming mats in cool hydrothermal setting and
white CaSO4 mats in warmer hydrothermal settings caused by sulphur oxidizing bacteria (right above).
Some of the bacteria rely on methane as a food source; others rely on dissolved inorganic solutes.
Some of these colonial mats bind sediment or even produce it as a by-product of their bio-activity.
Modern Mn crusts resemble old microbolites in their layering. Below are some microscopic images of
microbial mats. Left is from a geyser/hotsprings pool at Yellowstone (thermophyllic bacteria). Center
left is a layered mat with different communities and redox states. Centre right is from a Cambrian age
Phosphorite from Mongolia. Right is the typical elephant skin texture of a Burgess microbolite mat.
Microbial mats may be: layered, filamentous, branching, nodular or concentrically zoned and made of
clay size mineral crystals like those pictured in the delicate, branching, nodular phosphorite.
Fauna (Animals):
Priapulid (penis) worms, (left below) live in burrows, have an anchor end, a single central gut, and
an ornate proboscis. This modern one below resembles Ottoia from the Burgess whose last meals
were diverse bits of anything drifting along the bottom including broken bits of larger arthropods and
mollusks! For the Middle Cambrian equivalent of this dude see the Ottoia fossils.
Monoplacophorans (single shelled mollusks) Right below. Thought to be ancestral to cephalopods.
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Trilobite (see general trilobite body parts below. Trilobites like most other arthropods had to molt
and shed their old shell to grow (like spiders or crabs). Like those arthropods today (or you if you
outgrow your jeans), they shed the outgrown shells of the middle (thorax) and hind (pygidium) parts
but kept their heads on! Most trilobite fossils are incomplete shells lacking the cephalon. Part of the
quick kill idea for the Burgess preservation is that the trilobites “died with their heads on”.
Other Proto-Arthropods: Anomalocaris, Canadapsis, Marella, Sidneya
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Lab 6 Burgess Shale Drawings and Interpretations Name _____________________________________________
Draw and label the Fossils: No scratching, prying shales apart or use of acid, only hand lenses and
binocular scopes are allowed. Note the kind of substrate (grain size, sediment type, mineralogy,
allochems, matrix) each is found in and record this along with your other observations. Note whether
each fossil is in place and whole or transported and abraded. Compare to S.J. Gould’s book as
appropriate. Identify each phylum and family if you can. Infer what they produced, ate, did for a
living. You may use the books provided or the web.
1) Filamentous Algae and any microbolite (raspberry shapes, films, stromatolites) (20)
2) Ottoia and other soft bodied animals such as Wiwaxia (pick ones with detailed delicate
evidence of soft body parts and internal structures. For the Ottoia, how can you tell what
he/she did for a living? (20)
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3) Scenella (mollusca, monoplacophoran) plus a Brachiopod. For the Scenella, notice whether
this appears to be a living (in situ) or a death assemblage (transported) and explain how you
can tell. (20)
4) Trilobite (sp._________) (20)
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5) 2 other rarer invertebrates proto-arthropods? (Anomalocaris, Canadapsis, Marella, Opabina
Yohoia). (20)
SYNTHESIS
A Possible Cladogram for the Burgess Fauna:
6) Place the names for the 5 animal species you drew on this cladogram and note where they ate:
a-in the mud (organic detritus, bacteria, smaller in-fauna), b-filter feeding (zoo- or
phytoplankton), c-predation or scavenging. (10 points)
Onychophorans (Aysheaia)
Armored lobopods (Hallucigenia)
Kerygmachela
Opabinia
Anomalocaris, Laggania
Arthropods
From the lithofacies and sediment structures you observed, comment on what got preserved where.
Keep in mind that to preserve soft organic tissues requires an absence of oxygen. Even below the
seabed, there is enough SO4-2 anion for sulphate reducing bacteria to feast on most organic matter,
breaking down Carbon-Hydrogen bonds to produce: water, bicarbonate ions and hydrogen sulphide or
bisulfide ions. This in turn causes most fossilization building mineral replicas of animal or plant
tissues of mineral matter: calcite, dolomite, pyrite etc. These minerals appear as films, botryoidal
aggregates or replacive permineralization of the original organic tissues. A generic reaction for the
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bacterial metabolism of an organic molecule is given below, where the “sugar” is oxidized and the S+6
inside the sulphate molecular anion is reduced:
C6 H12 O6
+ 2SO4-2 ==> 6HCO3- + 2HS- + 2H2O
empirical sugar
aqueous ions
water(+/- e-, H+ )
The bicarbonate is used to generate carbonate mineral cements or fossil structures, while the bisulfide
is used to make pyrite or other iron sulfides (greigite, smythite. Depending on the chemical
environment (SO4-2 limiting versus excess) end products of either reduced native carbon or sulfur can
occur. These kind of geochemical changes require anaerobic conditions and complex assemblages of
archaea and bacteria so it is in essence the bacteria that do the fossilizing as a byproduct of their
metabolizing organic matter. In some systems instead of sulphate reduction is it the reduction of ferric
or manganic ions or other transition metals. Watch for pyrite and other metal sulfides on these
Burgess Shale specimens in addition to films and platy rhombohedral or scalenohedral crystals of
carbonate minerals.
Now think back to whether the substrate (sediment) had carbonate in it (either to begin with or as
cement). Contrast this with the shales and what was preserved there. Where carbonate minerals are
abundant as in micrites, it is easy to saturate calcite from Ca+2 ions in sea water or pore water. Fossil
preservation is preferentially as calcite. By contrast, in anoxic black shale basins which are more
acidic, fossils are often only preserved as carbon films.
7) The Burgess shale is dominantly: (circle correct answer)
a) micrite
b) siliciclastic
Interpreting the Causes for the Excellent Preservation and Fossilization.
From the geochemical history of Earth presented in the beginning of Carbonate Lab 5, we can infer
some general global background conditions for the Middle Cambrian Earth. After Proterozoic, oxygen
fills all potential reservoirs: weathering on land, solubility in the warming oceans to the point that it
has built up to between 11 and 20% of the atmosphere as a free gas. 20% is like today and permits
abundant and vigorous animal life and a fast carbon cycle for biodegradation. Estimates of RCO2
(~17) are still near their all time high from the Late Proterozoic Cap Carbonates and about 35 x higher
than the 400 ppm in the present atmosphere. Basically Early Paleozoic is still near the high point of
limestone deposition. Recall that at about ~542 Ma the Cambrian Explosion happened giving us all
modern animal phyla and many which are since extinct (Hard Bodied Shells: Rudists, Corals, Mollusca
etc.), 1st Green algae.
Consider the ways that: temperature, salinity and dissolved gases vary in natural sea water and how
that water can become either layered or convected in a restricted marginal basin. Also consider
physical variables such as: climate change, extreme weather events, hypersalinity, regression,
subsidence or uplift. Tyler notes that the coarser sediments (not represented here) have throughgoing records of trilobites and brachiopods, so are these more transported bio frags and allochems?
The soft bodied preservation and more diverse assemblages are only seen in the shale interbeds.
What kinds of changing environmental factors could trigger anoxic events in these water depths
(lagoonal to bathyal)? Life itself is inherently prone to a boom or bust type of population dynamics
and can itself give rise to blooms followed by eutrophication and die offs. (Think of that forgotten
stuff on the back shelf of your fridge!) Write up your own conjecture and conclusions below for
whether changes in the community dictate what is preserved or changes in sediment reworking versus
quiet deposition, special chemistry, bacteria and pore water chemistry. Critique the prevailing
interpretation for episodic large intact slump blocks and suggest ways to test alternate hypotheses.
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8) Put your own observations, synthesis and interpretation of the Burgess Shale’s depositional
environment and explanation for the excellent preservation and fossilization. (20 points)
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