The Early Paleozoic World:

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The Early Paleozoic World the Cambrian explosion
of life
EPSC233 Earth Life History (Fall 2002)
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Recommended reading STANLEY Earth System
History Chapter 13, pp. 344-350.
Keywords Phanerozoic eon, Paleozoic era,
Cambrian period, Tommotian fauna, trace fossils,
evolutionary radiation ( diversification),
invertebrate metazoans.
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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. The Ediacaran
fauna (earliest probable animals) evolved 570
million years ago.
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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) .
This was most irritating to the early defenders
of the hypothesis that life had evolved from
simpler to more complex forms... including
Darwin. Like today, these early evolutionists
eager to find primordial fossils did eventually
find them!
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A metamorphic rock from the Grenville province
(1.1 billion years-old) found in 1858 by William
Logan, the founder of the Geological Survey of
Canada.
In 1864, Sir William Dawson (father of North
American paleobotany) named the fossil Eozoon
canadense and affirmed its parentage with
protozans (eukaryotic unicellular organisms). To
the end of his life, he defended the thesis of a
biological origin for this primordial fossil.
Darwin eagerly accepted Eozoon as support for his
ideas.
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In 1894, the detailed description of a green and
white layered metamorphic rock collected near
Vesuvius (Italy) convinced geologists that the
layering must have formed without the influence
of life.
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The 1946 discovery of cm-sized fossils in the
Ediacara hills of the Flinders Range, Australia,
presented a new enigma to paleontologists.
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Was the Garden of Ediacara a community of
organisms without predators?
A modern sea pen (essentially a soft coral).
Sea pens leave only scattered spicules (mineral
needles) when they die.
Ediacaran fossils are generally found in
nearshore sandstones. It would have taken a tough
skin to resist abrasion by quartz grains.
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• What were the Ediacarans/Vendozoans?
• Were they animals with a quilted structure
  (thick walls separating compartments like air
  mattress)?

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• What were the Ediacarans/Vendozoans?
• Were they endosymbionts, i.e. a primitive
  eukaryotic animal that lived in symbiosis with
  photosynthetic bacteria (a bit like modern
  corals?)...
• Were they lichens?
• Were they preserved in sandstone because they
  were covered (and protected) by cyanobacterial
  colonies?

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Was the diverse Ediacaran fauna a short-lived
evolutionary phase, spanning only 15-20 million
years at the end of the Neoproterozoic? Work is
ongoing, in the Mackenzie Mountains and Mexico
where older impressions (pre-Varangian ice age)
have been found. Radiometric dates are rare, so
chemostratigraphy is increasingly used.
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Neoproterozoic (900-550 Ma) is marked by -
decrease in extent of stromatolites (being
grazed on by herbivores?) - increase in burrows
feeding traces (do animals evolve new
feeding/hiding strategies but leave no body
fossils because most are soft-bodied?) -
appearance decline of the Ediacaran fauna
(under pressure from the predators and grazers
who left the trace fossils?)
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After 570 Ma, Neoproterozoic fine-grained rocks
(shales and siltstones) show simple burrows and
feeding traces, probably made by worm-like
organisms.
Were they grazers, responsible for the demise of
the Ediacaran fauna and the decline of
stromatolites?
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Very few Neoproterozoic fossils suggest that life
forms (algal or animal?) were starting to armor
their body with CaCO3 coverings. In Namibia
(Germs, 1972), Canada (British Columbia, Hoffman
and Mountjoy, 2001), Mexico and southwestern
U.S., calcareous tubes and stemmed sheaths have
been found. These enigmatic fossils are known as
Cloudina and Namacalathus.
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Namacalathus
Cloudina
Very thin shells. Overall shape reconstructed by
tomography (from digital photographs taken as the
rock is polished, gradually exposing the
specimen).
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Is the Cambrian explosion for real? Evidence
of life forms becomes more obvious in strata of
Cambrian age. Many different kinds of 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).
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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.
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• 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.

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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 evolutionary steps have
been recognized 1) SSF (small shelly
fossils) 2) Tommotian fauna 3) larger,
skeletalized fauna
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The SSF is succeeded by the richer Tommotian
fauna, first discovered in Siberia. - all are
small hard parts, most lt 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.
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Many of the smaller fragments were probably
sclerites, i.e. scales and spines which covered
small, armored, worm-like animals.
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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 most
abundant type of animal remains in the Cambrian
fossil record are arthropods.
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Proterozoic traces are shallow, simple, often
resting traces rather than deeper meandering
feeding traces.
resting traces of jellyfish?
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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.
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Other Cambrian phyla include benthic animals
(bottom-dwellers)
Edrioasteroid (primitive echinoderm, ancestral to
starfish) on a brachiopod shell.