The Proterozoic: Dawn of a More Modern World

1
Chapter 9

• The Proterozoic Dawn of a More Modern World

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Proterozoic Eon

• 2.5 billion years to 542 million years ago
• Comprises 42 of Earth history
• Divided into three eras
• Paleoproterozoic Era (2.5 - 1.6 by ago)
• Mesoproterozoic Era (1.6 to 1.0 by ago)
• Neoproterozoic Era (1.0 by ago to the beginning
  of the Paleozoic, 542 my ago)

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The Beginning of the Proterozoic Marks the
Beginning of

• More modern style of plate tectonics
• More modern style of sedimentation
• More modern global climate with glaciations
• Establishment of the beginnings of an oxygen-rich
  atmosphere
• Emergence of eukaryotes

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Precambrian Provinces in North America

• Precambrian provinces were welded (or sutured)
  together to form a large continent called
  Laurentia during the early Proterozoic.

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Precambrian Provinces in North America

• Oldest (Archean) rocks are shown in orange.
• Younger (Proterozoic) rocks are shown in green.

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Precambrian Provinces in North America

• Suturing occurred along mountain belts or
  orogens.
• Provinces were assembled by about 1.7 b.y. ago.
• Laurentia continued to grow by accretion
  throughout the Proterozoic.

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Proterozoic Plate Tectonics

• Late in the Proterozoic, the continents became
  assembled into a supercontinent called Rodinia.

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Proterozoic Sedimentation

• Sedimentation on and around the craton consisted
  of shallow water clastic and carbonate sediments
  deposited on broad continental shelves and in
  epicontinental seas.

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Proterozoic Climate

• Proterozoic glaciations occurred during the
• Paleoproterozoic, about 2.4-2.3 b.y. ago
  (Huronian glaciation)
• Neoproterozoic, 850-600 m.y. ago (Varangian
  glaciation)

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• Overview
• of the
• Precambrian

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• Overview of Proterozoic Events

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Paleoproterozoic Era

• The oldest part of the Proterozoic
• Ranges from about 2.5 b.y. to 1.6 b.y.
• Covers 900 million years

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Major Events of the Paleoproterozoic

1. Active plate tectonics
2. Major mountain building on all major continents
3. Earth's first glaciation
4. Widespread volcanism (continental flood basalts)
5. Rise in atmospheric oxygen (great oxidation
   event)

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Major Events of the Paleoproterozoic

1. Accumulation of high concentrations of organic
   matter in sediments (Shunga event) 2000 m.y. ago,
   and generation of petroleum
2. Oldest known phosphorites and phosphate
   concretions

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• Orogenic belts developed around margins of the
  Archean provinces.
• Wopmay belt in NW Canada
• Trans-Hudson belt, SW of Hudson Bay

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Wopmay orogenic belt contains evidence of

1. Rifting and opening of an ocean basin (with
   normal faults, continental sediments, and lava
   flows)
2. Sedimentation along new continental margins (with
   shallow marine quartz sandstones and carbonate
   deposition)
3. Closure of the ocean basin (with deep water
   clastics overlain by deltaic and fluvial sands),
   followed by folding and faulting.

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Wilson Cycle

• This sequence of events in the Wopmay orogenic
  belt is called a Wilson Cycle, and is a result of
  plate tectonics.
• Rifting and opening of an ocean basin
• Sedimentation along new continental margins
• Closure of the ocean basin
• The sequence of events in the Wopmay belt is
  similar to that in the Paleozoic of the
  Appalachians.

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Trans-Hudson orogenic belt

• Trans-Hudson belt contains the sedimentary record
  of a Wilson Cycle, with evidence of
• Rifting
• Opening of an ocean basin
• Deposition of sediment
• Closure of the ocean basin along a subduction
  zone, associated with folding, metamorphism, and
  igneous intrusions.
• This closure welded the Superior province to the
  Hearne and Wyoming provinces to the west.

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Paleoproterozoic Glaciation - Earth's First Ice
Age?

• A Paleoproterozoic ice age is recorded in rocks
  north of Lake Huron in southern Canada (called
  the Huronian glaciation).
• Gowganda Formation.
• Age of Huronian glaciation 2450-2220 m.y.
• Apparent rapid onset of global glaciations from
  what had been relatively stable climatic
  conditions.

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Evidence for glaciation includes

• Mudstones with laminations or varves - fine
  laminations indicating seasonal deposition in
  lakes adjacent to ice sheets.
• Glacial dropstones (dropped from melting
  icebergs) in varved sedimentary rocks.
• Tillites or glacial diamictites (poorly sorted
  conglomerates of glacial debris).
• Scratched and faceted cobbles and boulders in
  tillite, due to abrasion as ice moved.

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Widespread Glaciation

• Age of global glaciations 2.6 - 2.1 b.y. ago
  (2600-2100 m.y.).
• Widespread glaciation at this time as indicated
  by glacial deposits found in
• Europe
• southern Africa
• India

22
Banded iron formations and prokaryote fossils

• Extensive banded iron formations (BIF's) on the
  western shores of Lake Superior, indicate that
  photosynthesis was occurring and oxygen was being
  produced.

23
Banded iron formations and prokaryote fossils

• Some BIF deposits are gt1000 m thick, and extend
  over 100 km.
• Animikie Group.
• Rich iron deposits were foundation of steel
  industry in Great Lakes region (Illinois,
  Indiana, Ohio, Pennsylvania).
• Mining has declined because U.S. imports most of
  its iron ore and steel.

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Banded iron formations and prokaryote fossils

• The Gunflint Chert, within the BIF sequence,
  contains fossil remains of prokaryotic organisms,
  including cyanobacteria.Age 1.9 b.y.

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Labrador Trough

• East of the Superior province are rocks deposited
  on a continental shelf, slope, and rise.
• Rocks are similar to those of the Wopmay orogenic
  belt.
• These rocks were folded, metamorphosed, and
  thrust-faulted during the Hudsonian orogeny,
  which separates the Paleoproterozoic from the
  Mesoproterozoic.

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Mesoproterozoic Era

• The Mesoproterozoic (or middle Proterozoic)
  ranges from about 1.6 b.y. - 1.0 b.y.

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Highlights of the Mesoproterozoic

• The Midcontinent rift, an abandoned oceanic rift
  in the Lake Superior region with massive basaltic
  lava flows
• Copper mineralization in the Lake Superior region
  
• Continental collisions producing the Grenville
  orogeny in eastern North America
• The assembly of continents to form the
  supercontinent, Rodinia.

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Midcontinent Rift and the Keweenawan Sequence

• Midcontinent rift extends southward from Lake
  Superior region.
• Overlies Archean crystalline basement rocks and
  Paleoproterozoic Animikian rocks (Animikie Group
  BIF).

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Midcontinent Rift and the Keweenawan Sequence

• Large volumes of basaltic rock indicate presence
  of an old abandoned rift zone called the
  Midcontinent rift.
• This was the first stage of a Wilson Cycle.
• Rift developed 1.2 b.y. - 1.0 b.y. ago.
• Extended from Lake Superior to Kansas.
• Rifting ceased before the rift reached the edge
  of the craton, or the eastern U.S. would have
  drifted away from the rest of North America.

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Midcontinent Rift and the Keweenawan Sequence

• The Keweenawan Sequence consists of
• Clean quartz sandstones
• Arkoses
• Conglomerates
• Basaltic lava flows more than 25,000 ft thick
  (nearly 5 mi) with native copper
• Basaltic rock beneath the surface crystallized as
  the Duluth Gabbro, 8 mi thick and 100 mi wide.

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Copper Mineralization

• Native copper fills vesicles (gas bubbles) in the
  Keweenawan basalt, and joints and pore spaces in
  associated conglomerates.
• Native Americans mined the copper as early as
  3000 BC.
• Copper was mined extensively from 1850 to 1950,
  but copper production ceased in the 1970's.

32
Grenville Province and Grenville Orogeny

• The Grenville province in eastern North America
  extends from northeastern Canada to Texas.

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Grenville Province and Grenville Orogeny

• Grenville rocks were originally sandstones and
  carbonate rocks.
• Grenville Province was the last Precambrian
  province to experience a major orogeny.
• Grenville orogeny 1.2 b.y. to 1.0 b.y. ago

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Grenville Province and Grenville Orogeny

• Orogeny occurred when Eastern North America
  (Laurentia) collided with western South America
  (Amazonia).
• Orogeny was associated with formation of the
  supercontinent, Rodinia.
• Later, during the Paleozoic Era, Grenville rocks
  were metamorphosed and intruded during the three
  orogenies involved in the building of the
  Appalachians.

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The Supercontinent, Rodinia

• The supercontinent, Rodinia, as it appeared
  about 1.1 b.y. ago.The reddish band down the
  center of the globe is the location of
  continental collisions and orogeny, including the
  Grenville orogeny.

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The Supercontinent, Rodinia

• Rodinia formed as the continents collided during
  the Grenville Orogeny.
• Rodinia persisted as a supercontinent for about
  350 million years.
• It was surrounded by an ocean called Mirovia.

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Rifting in Rodinia

• Rodinia began to rift and break up about 750
  million years ago, forming the proto-Pacific
  Ocean, Panthalassa, along the western side of
  North America.

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Rifting in Rodinia

• An early failed attempt at rifting began in
  eastern North America about 760 m.y. ago, with
  the deposition of sediments of the Mount Rogers
  Formation in a fault-bounded rift valley.
• Felsic and mafic volcanic rocks are interlayered
  with the sedimentary rocks of the Mount Rogers
  Formation.

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Neoproterozoic Era

• The Neoproterozoic (or new Proterozoic) ranges
  from about 1.0 b.y. to 0.542 b.y. (542 m.y.).

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Highlights of the Neoproterozoic

• Extensive continental glaciations
• Sediments deposited in basins and shelf areas
  along the eastern edge of the North American
  craton.
• Most of these rocks were deformed during the
  Paleozoic orogenies.

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Glacial deposits in the Neoproterozoic

• Glacial deposits formed roughly 600 - 700 m.y.
  ago.
• Evidence for glaciation
• Glacial striations (scratched and grooved pebbles
  and boulders)
• Tillites (lithified, unsorted conglomerates and
  boulder beds) found nearly worldwide
• Glacial dropstones (chunks of rocks released from
  melting icebergs)
• Varved clays from glacial lakes

42
Rifting in Rodinia

• Around 570 million years ago, rifting began
  again, and South America began to separate from
  North America, forming the Iapetus Ocean (or
  proto-Atlantic Ocean).
• The rift ran along what is now the Blue Ridge
  province. Basaltic lava flows formed the Catoctin
  Formation.
• As the Iapetus Ocean opened, sands and silts were
  deposited in the shelf areas.

43
Glacial deposits in the Neoproterozoic

• This time is referred to as "snowball Earth
  because glacial deposits are so widespread.
• Varangian glaciation (named after an area in
  Norway).
• The late Proterozoic ice age lasted about 240
  m.y.

44
Plate Tectonics and Glaciation

• Plate tectonics may have had a role in cooling
  the planet.
• Continents were located around the equator about
  600 to 700 m.y. ago.
• No tropical ocean.

45
Plate Tectonics and Glaciation

• Heat lost by reflection from the rocks on the
  surface of the continents may have caused global
  cooling. (Land plants had not yet appeared.)
• As continental glaciers and ice caps formed,
  reflectivity of snow and ice caused further
  temperature decrease.

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Atmospheric Gases and Glaciation

• Glaciation was associated with
• Decrease in CO2 and
• Increase in O2.
• CO2 causes the greenhouse effect and global
  warming. Decrease in CO2 may have caused cooling.
  
• Decrease in CO2 was probably caused by increase
  in the number of photosynthetic organisms
  (cyanobacteria, stromatolites).

47
Limestones and Glaciations

• Limestones are associated with glacial deposits,
  which is unusual, since limestones generally form
  in warm seas, not cold ones.
• Association of limestones with glacial deposits
  suggests that times of photosynthesis and CO2
  removal alternated with times of glaciation.
• Limestones (made of CaCO3) are a storehouse of
  CO2, which was removed from the atmosphere.

48
Limestones and Glaciations

• Glacial conditions may have inhibited
  photosynthesis by stromatolites.
• As a result, CO2 may have accumulated
  periodically and triggered short episodes of
  global warming.
• This produces the paradox of glaciers causing
  their own destruction.

49
Proterozoic Rocks South of the Canadian Shield

• Extensive outcrops of
• Precambrian rocks are
• present in the
• Canadian Shield.
• Precambrian rocks are also present in other
  areas,
• including
• Rocky Mountains
• Colorado Plateau (Grand Canyon)

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Events Recorded in Proterozoic Rocks

1. Collision of an Archean terrane with volcanic
   island arc, 1.7 or 1.8 b.y.a. (Wyoming and
   western Colorado)
2. Extensive magma intrusion in Mesoproterozoic,
   1.5-1.4 b.y.a. (California to Labrador)
3. Widespread rifting
4. Rifts with thick sequences of shallow water
   Neoproterozoic sedimentary rocks, 1.4 - 0.85
   b.y.a. Belt Supergroup (Glacier National Park,
   Montana, Idaho, and British Columbia).

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Precambrian rocks of the Grand Canyon

• Vishnu Schist metasediments and gneisses,
  intruded by Zoroaster Granite about 1.4 b.y. to
  1.3 b.y.a. during the Mazatzal orogeny.
• Top of Vishnu Schist is an unconformity.

52
Precambrian rocks of the Grand Canyon

• Grand Canyon Supergroup overlies unconformity.
  Neoproterozoic sandstones, siltstones, and
  shales. Correlates with Belt Supergroup.
• Unconformably overlain by Cambrian rocks.

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Proterozoic Life
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Life at the beginning of the Proterozoic was
similar to that in the Archean

1. Archaea in deep sea hydrothermal vents
2. Planktonic prokaryotes floated in seas and lakes
3. Anaerobic prokaryotes in oxygen-deficient
   environments
4. Photosynthetic cyanobacteria (prokaryotes)
   constructed stromatolites (algal filaments)
5. Eukaryotes (as indicated by molecular fossils)

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Other forms of life appeared during the
Proterozoic

1. More diverse eukaryotes including acritarchs
2. Metazoans or multicellular animals with soft
   bodies
3. Metazoans with tiny calcium carbonate tubes or
   shells
4. Metazoans that left burrows in the sediment

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Microfossils of the Gunflint Chert

• First definitive Precambrian fossils to be
  discovered (in 1953) were in the 1.9 b.y. old
  Gunflint Chert, NW of Lake Superior
  (Paleoproterozoic).

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Microfossils of the Gunflint Chert

• The fossils are well-preserved, abundant and
  diverse and include
• String-like filaments
• Spherical cells
• Filaments with cells separated by septae
  (Gunflintia)
• Finely separate forms resembling living algae
  (Animikiea)
• Star-like forms resembling living iron- and
  magnesium-reducing bacteria (Eoastrion)

58
Microfossils of the Gunflint Chert

• A Eoastrion ( dawn star), probably iron- or
  magnesium-reducing bacteriaB Eosphaera, an
  organism or uncertain affinity, about 30
  micrometers in diameterC Animikiea (probably
  algae)D Kakabekia, an organism or uncertain
  affinity

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Microfossils of the Gunflint Chert

• Gunflint fossil organisms resemble photosynthetic
  organisms.
• The rock containing these organisms contains
  organic compounds that are regarded as the
  breakdown products of chlorophyll.
• The Gunflint Chert organisms altered the
  composition of the atmosphere by producing
  oxygen.

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The Rise of Eukaryotes

• The appearance of eukaryotes is a major event in
  the history of life.
• Eukaryotes have the potential for sexual
  reproduction, which increases variation through
  genetic recombination.

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The Rise of Eukaryotes

• Genetic recombination provides greater
  possibilities for evolutionary change.
• Diversification of life probably did not occur
  until after the advent of sexual reproduction, or
  until oxygen levels reached a critical threshold.

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Eukaryotic cells can be differentiated from
prokaryotic cells on the basis of size.

• Eukaryotes tend to be much larger than
  prokaryotes (larger than 60 microns, as compared
  with less than 20 microns).

63
The Rise of Eukaryotes

• Eukaryotes appeared by Archean time (as
  determined by molecular fossils or biochemical
  remains).
• Larger cells begin to appear in the fossil record
  by 2.7 b.y. to 2.2 b.y.
• Eukaryotes began to diversity about 1.2 to 1.0
  b.y. ago.

64
Acritarchs

• Eukaryotes
• Single-celled, spherical microfossils
• Thick organic covering
• May have been phytoplankton
• First appeared 1.6 b.y. ago (at
  Paleoproterozoic-Mesoproterozoic boundary)
• Some resemble cysts or resting stages of modern
  marine algae called dinoflagellates.

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Acritarchs

1. Reached maximum diversity and abundance 850 m.y.
   ago
2. Declined during Neoproterozoic glaciation
3. Few acritarchs remained by 675 m.y. ago
4. Extinct in Ordovician time
5. Useful for correlating Proterozoic strata

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The First Metazoans (Multicellular Animals)

• Metazoans are multicellular animals with various
  types of cells organized into tissues and organs.
  
• Metazoans first appeared in the Neoproterozoic,
  about 630 m.y. ago (0.63 b.y.). Preserved as
  impressions of soft-bodied organisms in
  sandstones.

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Examples of metazoan fossils in the Proterozoic

• Ediacara fauna - Imprints of soft-bodied
  organisms, first found in Australia in the 1940's
  
• Metazoan eggs and embryos in uppermost
  Neoproterozoic Doushantuo Formation, South China
• Trace fossils of burrowing metazoans in rocks
  younger than the Varangian glaciation.
• Tiny shell-bearing fossils (small shelly fauna)

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• Geologic time scale across the
  Precambrian-Cambrian boundary, showing the
  Ediacaran fauna and other faunas.

69
Ediacara fauna

• Ediacara fauna is an important record of the
  first evolutionary radiation of multicellular
  animals.
• Some were probably ancestral to Paleozoic
  invertebrates.
• Oldest Ediacara-type fossils are from
  China.Youngest Edicara-type fossils are Cambrian
  (510 m.y., Ireland).

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Types of Ediacara fossils

• Discoidal
• Frondlike
• Elongate or ovate

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Ediacara fauna

• Because the Ediacara creatures are not really
  similar to animals that are living today, this
  has led to the suggestion that they be placed in
  a separate taxonomic category or new phylum.
• The name proposed for this new category is
  Vendoza (named after the Vendian, or the latest
  part of the Neoproterozoic in Russia).

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Small Shelly Fauna The Origin of Hard Parts

• Small fossils with hard parts or shells appeared
  in the Neoproterozoic.

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Small Shelly Fauna The Origin of Hard Parts

• Cloudina, an organism with a small, tubular
  shell of calcium carbonate (CaCO3).
• Resembles structures built by a tube-dwelling
  annelid worm.
• Earliest known organism with a CaCO3 shell.
• Found in Namibia, Africa.

74
Small Shelly Fauna The Origin of Hard Parts

• Other latest Proterozoic and earliest Cambrian
  small fossils with shells include
• Possible primitive molluscs
• Sponge spicules,
• Tubular or cap-shaped shells, and
• Tiny tusk-shaped fossils called hyoliths
• Some early shelly material is made of calcium
  phosphate.

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Precambrian Trace Fossils

• Trails, burrows, and other trace fossils are
  found in late Neoproterozoic rocks.
• In rocks deposited after the Neoproterozoic
  Varangian glaciation.
• Mostly simple, shallow burrows.
• Trace fossils increase in diversity, complexity,
  and number in younger (Cambrian) rocks.

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What stimulated the appearance of metazoans?

• May be related to the accumulation of sufficient
  oxygen in the atmosphere to support an
  oxygen-based metabolism.
• Ancestral metazoans may have lived in "oxygen
  oases" of marine plants.
• Ediacaran life may have evolved gradually from
  earlier forms that did not leave a fossil record.

77

• Review of Proterozoic Events

78

• Review
• of the
• Precambrian