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Prior to the discovery of Ediacaran
fossils, the Precambrian time was
referred to as the “Azoic” eon (i.e.
“without life”) .
Was the evolution of early cells from simple
organic compounds in Precambrian oceans a
slow, improbably process?
Or did life simply need...
- liquid water
- lack of free oxygen
- simple amino acids
- shelter from heavy meteorite showers
The 3.5 billion years old Archean cells
preserved in chert, and the low C13/C12
ratios of graphite inclusions in 3.8 billion
year old BIFs suggest that life arose
within 100 million years of the end of
heavy meteoritic bombardment.
Biomarkers suggest that eukaryotes may
have evolved from prokaryotes 2.7 billion
years ago.
Ediacaran fauna, the arliest probably
animals, evolved 570 million years ago.
Ediacaran remains are generally found in
nearshore sandstones.
In finer-grained sedimentary rocks of
Neoproterozoic age (570-545 million
years ago), the presence of crawling
traces and burrows suggest that other
kinds of animals were also evolving.
Are they the traces of predators and
grazing herbivores? Did they put an end to
the Ediacaran lifestyle, and restricted the
extent of stromatolites?
Is the Cambrian explosion “for real”?
Evidence of life forms becomes more
obvious in strata of Cambrian age.
Animals started to produce hard
(mineralized) parts (exoskeletons, shells,
teeth, sclerites…) of calcium carbonate
(CaCO3) and calcium phosphate (material
similar to the mineralized part of our
bones and teeth).
Fossils preserved as flat impressions are
easy to overlook. They are flat and
visible only on bedding surfaces.
The rock must be broken
up into plates to reveal
the fossils.
Hard parts are usually easier to find than
impressions of soft-bodied animals
because:
• they are often more resistant than the
rest of the rock in which they were
preserved. Weathering tends to make
such fossils “stand out” from the rock.
• Hard parts are sometimes replaced by a
mineral more resistant than the rest of
the rock.
The fossil record suggests that animal
life evolved faster during the 40 million
years of the Cambrian period than during
the rise of the Ediacaran fauna.
Three intervals have been recognized:
1) SSF
2) Tommotian fauna
3) larger, skeletalized fauna
The lowermost Cambrian contains a
“small shelly fauna” (SSF) of simple
fossils: vase- and tube-shaped, or
teeth-like.
(All are a few millimeters in size.)
The SSF is succeeded by the richer
Tommotian fauna, first discovered in
Siberia.
- all are small hard parts, most < 1 cm.
- many are unlike any hard parts found in
living animals, or fossils from strata
younger than Cambrian age.
- a few belong to groups that survive
today.
Many of the smaller
fragments were probably
sclerites, i.e. scales and
spines which covered
small, armored, wormlike animals.
Aldamella, a primitive mollusk
Anabarella,
ancestral to
present-day
mollusks
This Tommotian fauna was “short-lived”,
lasting perhaps 3-4 million years.
The Tommotian fauna is followed by
fossils from much larger animals, reaching
a few centimeters to nearly two meters.
Most of them belonged to phyla that have
survived to this day.
The phylum Arthropoda, today, includes
insects, spiders, crabs… All animals
without a spine or internal skeleton,
but with a light exoskeleton, a
segmented body and jointed legs.
horseshoe crab
scorpion
Trilobites were the earliest arthropods. They
appear in Cambrian strata and diversified
rapidly.
Benthic
(crawler)
Planktonic
form
Trilobite enrolled
(floated for protection.
or swam)
Trilobites may have been around earlier but
became “visible” in the Cambrian when they
started to reinforce their exoskeletons with
CaCO3...
Traces similar to those found with Cambrian
trilobites occur in Neoproterozoic strata and
lowermost Cambrian strata.
Proterozoic traces
are shallow, simple,
often “resting
traces” rather than
deeper meandering
feeding traces.
resting traces
of jellyfish?
Fortune Head, Newfoundland: the
reference section for base of Cambrian
Trichophycus pedum
is a trace fossil at
the base of Cambrian
strata.
The trace fossil Trichophycus pedum
- marks the first occurrence of fairly
complex metazoan animals.
- occurs nearly worldwide
- sometimes with the last Ediacaran
fossils, but usually in strata above them.
The first shelly fossils clearly appear
later.
Trichophycus pedum was officially chosen
in 1991 as the most useful fossil to mark
the boundary between the Proterozoic
and the Cambrian.
Other Cambrian phyla include:
benthic animals (bottom-dwellers)
Edrioasteroid
(primitive
echinoderm,
ancestral to
starfish) on a
brachiopod
shell.
Cambrian trilobites, lacking claws or
specialized mouth parts for chewing, were
probably not predators… There were larger
animals around.
Scattered remains
of a diverse
Cambrian softbodied fauna were
found first in the
Rocky Mountains
of B.C., in the
Burgess Shale…
Tuzoia
TuzoiaAnomalocaris hybrid
Some of the middle Cambrian fossils were so
unlike anything known today that the first
reconstructions mistakenly included parts
from different animals... If Tuzoia is a
bivalved shrimp: is this its shrimp-like body?
Paleontologists were groping in the dark until
complete specimens were found in Early
Cambrian strata from Chengjiang, China.
- soft bodied fauna, like the Ediacarans
- but 30 million years younger
- evidence of predation and burrowing
- most phyla surviving today are represented
- but also complex fossils of uncertain
affinities
Some re-assembly
was required...
jaws
interpreted as jellyfish
claws
= Anomalocaris
Up to 2 meters long