Habitat: fossil
tropical reef with exquisite invertebrate
fossils entombed in mudslides
Burgess shale,
the final resting place of thousands
of unusual fossils, is on the
mountain in the background. Years
ago, many years ago, this 7000'
foot high area was near the equator
and at sea level.
The Burgess Shale
Formation is one of the world's
most celebrated fossil localities,
and is famous for the exceptional
preservation of the fossils found
within it, in which the soft parts
are preserved.[2]
It
is 505 million years (Middle
Cambrian) in age, making it
one of the earliest fossil beds
to preserve the soft parts of
animals. The pre-Cambrian fossil
record of animals is sparse
and ambiguous.
The rock unit
is a black shale, and crops
out at a number of localities
near the town of Field in the
Canadian Rockies in the Yoho
National Park of British Columbia,
Canada[2]
Yoho Nat'l Park,
Canadian Rockies. The Burgess
Shale is visible as a ridge just
above the tree line.[1]
History
and significance
The Burgess Shale
was discovered by Charles Walcott
in 1909, towards the end of the
season's fieldwork. He returned
in 1910 with his sons, establishing
a quarry on the flanks of Fossil
ridge. The significance of soft
bodied preservation, and the range
of organisms he recognised as
new to science, led him to return
to the quarry almost every year
until 1924. At this point, aged
74, he had amassed over 65,000
specimens. Describing the fossils
was a vast task, pursued by Walcott
until his death in 1927. Walcott,
led by scientific opinion at the
time, attempted to categorise
all fossils into living taxa;
as a result, the fossils were
regarded as little more than curiosities
at the time. It was not until
1962 that a first-hand reinvestigation
of the fossils was attempted,
by Alberto Simonetta. This led
scientists to recognise that Walcott
had barely scratched the surface
of information available in the
Burgess Shale, and also made it
clear that the organisms did not
fit comfortably into modern groups.
[2]
Excavations were resumed
at the Walcott quarry by the Geological
Survey of Canada under the persuasion
of trilobite expert Harry Blackmore
Whittington, and a new quarry, the
Raymond, was established about 20
metres higher up Fossil ridge. Whittington,
with the help of research students
Derek Briggs and Simon Conway Morris
of the University of Cambridge, began
a thorough reassessment of the Burgess
Shale, and revealed that the fauna
represented were much more diverse
and unusual than Walcott had recognized.
Indeed, many of the animals present
had bizarre anatomical features and
only the sketchiest resemblance to
other known animals. Examples include
Opabinia with five eyes and a snout
like a vacuum cleaner hose; Nectocaris,
which resembles either a crustacean
with fins or a vertebrate with a shell;
and Hallucigenia, which was originally
reconstructed upside down, walking
on bilaterally symmetrical spines.[2]
With Parks Canada and
UNESCO recognising the significance
of the Burgess Shale, collecting fossils
became politically more difficult
from the mid 1970s. Collections continued
to be made by the Royal Ontario Museum.
The curator of invertebrate palaeontology,
Desmond Collins, identified a number
of additional outcrops, stratigraphically
both higher and lower than the original
Walcott quarry. These localities continue
to yield new organisms faster than
they can be studied.[2]
Stephen Jay Gould's
book Wonderful Life, published in
1989, brought the Burgess Shale fossils
to the public's attention. Gould suggests
that the extraordinary diversity of
the fossils indicate that life forms
at the time were much more diverse
than those that survive today, and
that many of the unique lineages were
evolutionary experiments that became
extinct. Gould's interpretation of
the diversity of Cambrian fauna relied
heavily on Simon Conway Morris' reinterpretation
of Charles Walcott's original publications.
However, Conway Morris strongly disagreed
with Gould's conclusions, arguing
that almost all the Cambrian fauna
could be classified into modern day
phyla.[2]
Burgess
Shale Formation
The fossiliferous deposits of
the Burgess Shale correlate to
the Stephen formation, a collection
of slightly calcareous dark mudstones,
about 505 million years old. The
beds were deposited at the base
of a cliff about 160 m tall, below
the depth agitated by waves during
storms.
Marrella,
the most abundant Burgess Shale
organism.
This vertical cliff was composed of
the calcareous reefs of the Cathedral
formation, which probably formed shortly
before the deposition of the Burgess
shale. The precise formation mechanism
is not known for certain, but the
most widely accepted hypothesis suggests
that the edge of the Cathedral formation
reef became detached from the rest
of the reef, slumping and being transported
some distance — perhaps kilometers
— away from the reef edge. Later
reactivation of faults at the base
of the formation led to its disintegration
from about 509 million years ago.
This would have left a steep cliff,
the bottom of which would be protected,
because the limestone of the Cathedral
formation is difficult to compress,
from tectonic decompression. This
protection explains why fossils preserved
further from the Cathedral formation
are impossible to work with —
tectonic squeezing of the beds has
produced a vertical cleavage that
fractures the rocks, so they split
perpendicular to the fossils. The
Walcott quarry produced such spectacular
fossils because it was so close to
the Stephen formation — indeed
the quarry has now been excavated
to the very edge of the Cambrian cliff.
It was originally thought
that the Burgess Shale was deposited
in anoxic conditions, but mounting
research shows that oxygen was continually
present in the sediment. The anoxic
setting had been thought to not only
protect the newly dead organisms from
decay, but it also created chemical
conditions allowing the preservation
of the soft parts of the organisms.
Further, it reduced the abundance
of burrowing organisms — burrows
and trackways are found in beds containing
soft-bodied organisms, but they are
rare and generally of limited vertical
extent. Brine seeps are an alternative
hypothesis - see Burgess Shale type
preservation for a more thorough discussion.
Burgess
Shale type preservation
There are many other comparable Cambrian
lagerstätten; indeed such assemblages
are far more common in the Cambrian
than in any other period. This is
mainly due to the limited extent of
burrowing activity; as such bioturbation
became more prevalent throughout the
Cambrian, environments capable of
preserving organisms' soft parts became
much rarer.
Biota
The biota of the Burgess Shale appears
to be typical of Middle Cambrian deposits.
Although the hard-part bearing organisms
make up as little as 14% of the community,
these same organisms are found in
similar proportions in other Cambrian
localities. This means that there
is no reason to assume that the organisms
without hard parts are exceptional
in any way; indeed, many appear in
other lagerstätten of different
age and locations.
The biota consists of
a range of organisms. Free-swimming
(nectonic) organisms are relatively
rare, with the majority of organisms
being bottom dwelling (benthic) —
either moving about (vagrant) or permanently
attached to the sea floor (sessile).
About two-thirds of the Burgess Shale
organisms lived by feeding on the
organic content in the muddy sea floor,
while almost a third filtered out
fine particles from the water column.
Under 10% of organisms were predators
or scavengers, although since these
organisms were larger, the biomass
was split equally between each of
the filter feeding, deposit feeding,
predatory and scavenging organisms.
Burgess Shales Video
This is part 2 of
a 6 part episode. To watch the full
show go to my channel (Zuke696) then
go to my Playlist, find the show then
click Play All - Paleontologist Des
Collins leads this exciting exploration
into the beginnings of advanced life
on the planet, which can fortunately
be studied by visiting primitive shores
where creatures that lived hundreds
of millions of years ago still flourish
today. Collins's destination: Burgess
Shale in Canada's Rocky Mountains.
Advanced computer technology enhances
the discovery as viewers get close
to some of the most fascinating small
beasts ever.
