Geos 240: Winter 2013_Lab 7
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Name
The Burgess Shale Fauna, its Facies and Preservation
Introduction:
The Cambrian period saw the appearance of diverse hard-bodied marine invertebrate animal life. While there is a scant softbodied animal record 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. 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.
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The chart comes from The Animal Cognition Network at:
( http://www.animalcognition.net/phylogeny.html
).
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, mitochontria, 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 and are implicated in their fossilization in anoxic environments.
In the tree of life below, 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 near Field,
B.C. are a world heritage site. They contain the best collection of Middle Cambrian marine invertebrate life. At the Cambrian explosion of life, circa 540 Ma the ancestors of hard bodied animals appeared, with no known precursors. All 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 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 integumentae!
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.
-----Original Message-----
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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 whispy 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
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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 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 spacial 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 ie. 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 ie. 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
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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
Simon Fraser University
Burnaby, BC, V5A 1S6
604-268-6626
604-803-0563
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
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More background available in the introductory films on the
Burgess Shale
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 1) Filamentous Algae, 2) 2 soft bodied animals including Ottoia, 3) Scenella (granddaddy of the mollusca) and 4)a Trilobite plus 2 more unusual invertebrates.
No scratching, prying or use of acid, just lenses and binocular scopes allowed. Note the kind of substrate each is found in and record this along with your other observations. Note whether each fossil is in place or transported. Compare to Gould’s book as appropriate. Identify each phylum and family if you can.
1) Filamentous Algae and any microbolite (20)
2) Ottoia and other soft bodied animals (pick ones with detailed delicate evidence of body parts and internal structures. (20)
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3) Scenella (mollusca, monoplacophoran) (20)
4) Trilobite (sp._________) and 2 other rarer invertebrates (20)
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Synthesis:
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 SO
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-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 of 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 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:
C
6
H
12
O
6
+ 2SO
4
-2 => 6HCO
3
+ 2HS + 2H
2
O empirical sugar aqueous ions water(+/- e )
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 end products of reduced native carbon or sulfur can occur. These kind of geochemical changes require anaerobic conditions and complex assemblages archaebacteria 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 crystals of carbonate minerals.
Now think back to whether the substrate had carbonate in it
(either to begin with or as cement). Contrast this with the shales and what was preserved there.
Consider the ways that: temperature, salinity and dissolved gases vary in natural sea water and how that water can become either layered or convected. Also consider physical variables such as: climate change, extreme weather events, hypersalinity, regression, subsidence or uplift. What kinds of changing environmental factors could trigger anoxic events in these water depths (lagoonal to bathyal)? Write up your own conjecture and conclusions below for whether changes in the community dictate what is preserved or changes in sediment, bacteria and pore water chemistry. Critique the prevailing interpretation and suggest ways to test alternate hypotheses.
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5) My synthesis and interpretation of the Burgess Shale’s depositional environment and fossilization. (20 points)
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