Precambrian Earth and Life History

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Precambrian Earth and Life History

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Title: Precambrian Earth and Life History

1
Chapter 9
Precambrian Earth and Life HistoryThe
Proterozoic Eon
2
Proterozoic Rocks, Glacier NP

Proterozoic sedimentary rocks
in Glacier National Park, Montana
The angular peaks, ridges and broad valleys
were carved by Pleistocene and Recent glaciers

3
How long was the Proterozoic?

at 1.955 billion years long,
accounts for 42.5 of all geologic time
yet we review this long episode of Earth and life
history in a single section

4
How is the Archean-Proterozoic Boundary defined?

arbitrarily placed
the Archean-Proterozoic boundary
at 2.5 billion years ago
because it marks the approximate time
of changes in the style of crustal evolution

5
Different Style of Crustal Evolution?

Archean crust-forming processes generated
granite-gneiss complexes
and greenstone belts
that were shaped into cratons
same rock associations
continued during the Proterozoic,
BUT at a considerably reduced rate

6
Contrasting Metamorphism?

Unlike Archean rocks, vast exposures of
Proterozoic rocks show
little or no effects of metamorphism,
and in many areas they are separated
from Archean rocks by a nonconformity

7
Other Differences with Archean rocks?

the Proterozoic is characterized
by widespread rock assemblages
that are rare or absent in the Archean,
by a plate tectonic style essentially the same as
that of the present
by important evolution of the atmosphere and
biosphere
by the origin of some important mineral resources

8
Proterozoic Evolution of Oxygen-Dependent
Organisms!

During the Proterozoic
oxygen-dependent organisms
made their appearance
and the first cells evolved

9
When did Continents evolve?

Proterozoic accretion at Archean island arcs and
minicontinents margins thereby forming much
larger landmasses

10
Proterozoic Greenstone Belts

Most greenstone belts formed
during the Archean
between 2.7 and 2.5 billion years ago
not as common after the Archean,
and differed in one important detail
the near absence of ultramafic rocks
which resulted from Earth's decreasing amount of
radiogenic heat

11
What is Laurentia?

a large landmass that consisted of what is now
North America,
Greenland,
parts of northwestern Scotland,
and perhaps some of the Baltic shield of
Scandinavia

12
When and how did Laurentia come into existence?

originated and underwent important growth
between 2.0 and 1.8 billion years ago
collisions among various plates formed several
orogens
linear or arcuate deformation belts in which
rocks have been
metamorphosed
and intruded by magma
thus forming plutons, especially batholiths

13
Proterozoic Evolution of Laurentia

Archean cratons were sutured
along deformation belts called orogens,
thereby forming a larger landmass
By 1.8 billion years ago,
much of what is now Greenland, central Canada,
and the north-central United States existed

Laurentia grew along its southern margin
by accretion

14
Craton-Forming Processes

the Trans Hudson orogen
in Canada and the United States,
where the Superior, Hearne, and Wyoming cratons
were sutured
The southern margin of Laurentia
is the site of the Penokian orogen

15
Wilson Cycle

Rocks of the Wopmay orogen
in northwestern Canada are important
because they record the opening and closing
of an ocean basin
or what is called a Wilson cycle
A complete Wilson cycle,
named for the Canadian geologist J. Tuzo Wilson,
involves
fragmentation of a continent,
opening followed by closing
of an ocean basin,
and finally reassembly of the continent

16
Wopmay Orogen

Some of the rocks in Wopmay orogen
are sandstone-carbonate-shale assemblages,
a suite of rocks typical of passive continental
margins
that first become widespread during the
Proterozoic

17
Early Proterozoic Rocks in Great Lakes Region

Early Proterozoic sandstone-carbonate-shale
assemblages are widespread near the Great Lakes

18
Outcrop of Sturgeon Quartzite

The sandstones have a variety of sedimentary
structures
such as
ripple marks
and cross-beds
Northern Michigan

19
Outcrop of Kona Dolomite

Some of the carbonate rocks, now mostly
dolostone,
such as the Kona Dolomite,
contain abundant bulbous structures known as
stromatolites
NorthernMichigan

20
Penkean Orogen

These rocks of northern Michigan
have been only moderately deformed
and are now part of the Penokean orogen

21
When did the southern portion of the continent
accrete?

From 1.8 to 1.6 billion years ago,
as successively younger belts were sutured to
Laurentia,
forming the Yavapai and Mazatzal-Pecos orogens

22
Southern Margin Accretion

Laurentia grew along its southern margin
by accretion of the Central Plains, Yavapai, and
Mazatzal orogens

Also notice that the Midcontinental Rift
had formed in the Great Lakes region by this time

23
What else happened during the Proterozoic?

Deposition of most of Earths banded iron
formations (BIF)
First deposition of continental red beds at 1.8
billion years ago
sandstone and shale with oxidized iron
excellent evidence for widespread glaciation

24
Other events?

Extensive igneous activity
from 1.8 to 1.1 billion years ago unrelated to
orogenic activity
Although quite widespread,
this activity did not add to Laurentias size
because magma was either intruded into
or erupted onto already existing continental
crust

25
Igneous Activity

These igneous rocks are exposed
in eastern Canada, extend across Greenland,
and are also found in the Baltic shield of
Scandinavia

26
Cause of Igneous Activity?

According to one hypothesis
large-scale upwelling of magma
beneath a Proterozoic supercontinent
produced the rocks

27
Middle Proterozoic Orogeny and Rifting

The only Middle Proterozoic event in Laurentia
was the Grenville orogeny
in the eastern and southern part of the continent
1.3 to 1.0 billion years old
Grenville rocks are well exposed
in the the northern Appalachian Mountains
eastern Canada, Greenland, and Scandinavia

28
Grenville Orogeny

A final episode of Proterozoic accretion
occurred during the Grenville orogeny

29
With what was the Grenville Orogeny associated?

1) closure of an ocean basin
the final stage in a Wilson cycle
2) intracontinental deformation or major shearing
Whatever the cause,
it was the final Proterozoic stage of Laurentia
continental accretion

30
How much of North American continent was in
existence by the end of the Proterozoic?

about 75 of present-day North America existed
The remaining 25
accreted during the Phanerozoic Eon

31
Whats the Midcontinent Rift?

Grenville-age extension, volcanism and
sedimentation in Laurentia
Midcontinent rift
a long narrow continental trough bounded by
faults,
extending from the Lake Superior basin southwest
into Kansas,
and a southeasterly branch extends through
Michigan into Ohio
It cuts through Archean and Early Proterozoic
rocks
and terminates in the east against rocks
of the Grenville orogen

32
Location of the Midcontinent Rift

Rocks filling the rift
are exposed around Lake Superior
but are deeply buried elsewhere

33
Midcontinental Rift

Most of the rift is buried
except in the Lake Superior region
various igneous and sedimentary rocks are well
exposed
The central part of the rift contains
numerous overlapping basalt lava flows
forming a volcanic pile several kilometers thick

34
Midcontinental Rift

Along the rift's margins
coarse-grained sediments were deposited
in large alluvial fans
that grade into sandstone and shale
with increasing distance
from the sediment source
In the vertical section
Freda Sandstone overlies
Cooper Harbor conglomerate,
which overlies Portage Lake Volcanics

35
Cooper Harbor Conglomerate

Michigan

36
Portage Lake Volcanics

Michigan

37
Middle and Late Proterozoic Sedimentation

sediment deposition in what is now
the eastern United States and Canada,
in the Death Valley region of California and
Nevada,
and in three huge basins in the west

38
Sedimentary Basins in the West

Map showing the locations of sedimentary Basins
in the western United States and Canada
Belt Basin
Uinta Basin
Apache Basin

39
Proterozoic Mudrock

Outcrop of red mudrock in Glacier National Park,
Montana

40
Proterozoic Limestone

Outcrop of limestone with stromatolites in
Glacier National Park, Montana

41
Proterozoic Sedimentary Rocks

Proterozoic rocks
of the Grand Canyon Super-group lie
unconformably upon Archean rocks
The rocks consist mostly
of sandstone, shale, and dolostone,
deposited in shallow-water marine
and fluvial environments
The presence of stromatolites and carbonaceous
impression of algae in some of these rocks
also indicate probable marine deposition

42
Grand Canyon Super-group

Proterozoic Sandstone of the Grand Canyon
Super-group in the Grand Canyon Arizona

43
When did the current style of Style of Plate
Tectonics come into play?

almost certainly by the Early Proterozoic
the oldest complete ophiolite
is the Jormua mafic-ultramafic complex in Finland
It is about 1.96 billion years old,
but nevertheless compares closely in detail
with younger well-documented ophiolites

44
Jormua Complex, Finland

Reconstruction
of the highly deformed
Jormua mafic-ultramafic complex
in Finland
This sequence of rock
is the oldest known complete ophiolite
at 1.96 billion years old

45
Jormua Complex, Finland

Metamorphosed basaltic pillow lava

12 cm
46
Jormua Complex, Finland

Metamorphosed gabbro between mafic dikes

65 cm
47
Proterozoic Supercontinents?

A supercontinent consists of all
or at least much of the present-day continents,
The supercontinent Pangaea,
existed at the end of the Paleozoic Era,

48
Pre-Pangean Supercontinents?

Supercontinents may have existed
as early as the Late Archean,
but if so we have little evidence of them
The first that geologists recognize
with some certainty, known as Rodinia
assembled between 1.3 and 1.0 billion years ago
and then began fragmenting 750 million years ago

49
How did Rodinia look?

Possible configuration
of the Late Proterozoic supercontinent Rodinia
before it began fragmenting about 750 million
years ago

50
Pannotia The next supercontinent

Rodinia's separate pieces reassembled
and formed Pannotia
about 650 million years ago
Fragmentated
by the latest Proterozoic, about 550 million
years ago,

51
Was there glaciation during the Proterozoic?

The most recent glacial was during the
Pleistocene is certainly the best known.
BUT several major episodes of Proterozoic
glaciation

52
How can we be sure that there were Proterozoic
glaciers?

After all, their most common deposit
called tillite is simply a type of conglomerate
that may look much like conglomerate
that originated by other processes
Tillite or tillite-like deposits are known
from at least 300 Precambrian localities,

53
Glacial Evidence

But the extensive geographic distribution
of conglomerates and their associated glacial
features is distinctive,
such as striated and polished bedrock

54
Proterozoic Glacial Evidence

Bagganjarga tillite in Norway
overlies striated bedrock surface
on sandstone of the Veidnesbotn Formation

55
Geologists Convinced

Geologists are now convinced
based on this kind of evidence
that widespread glaciation
took place during the Early Proterozoic
The occurrence of tillites of about the same age
in Michigan, Wyoming, and Quebec
indicates that North America may have had
an Early Proterozoic ice sheet centered southwest
of Hudson Bay

56
Early Proterozoic Glaciers

Deposits in North America
indicate that Laurentia
had an extensive ice sheet
centered southwest of Hudson Bay

57
One or More Glaciations?

Tillites of about this age are also found
in Australia and South Africa,
but dating is not precise enough to determine
if there was a single widespread glacial episode
or a number of glacial events at different times
in different areas
One tillite in the Bruce Formation in Ontario,
Canada
may date from 2.7 billion years ago,
thus making it Late Archean

58
Glaciers of the Late Proterozoic

Tillites and other glacial features
dating from between 900 and 600 million years ago
are found on all continents except Antarctica
Glaciation was not continuous during this entire
time
but was episodic with four major glacial episodes
so far recognized

59
Late Proterozoic Glaciers

The approximate distribution of Late Proterozoic
glaciers

60
Most Extensive Glaciation in Earth History

The map shows only approximate distribution
of Late Proterozoic glaciers
The actual extent of glaciers is unknown
Not all the glaciers were present at the same
time
Despite these uncertainties,
this Late Proterozoic glaciation
was the most extensive in Earth history
In fact, Late Proterozoic glaciers
seem to have been present even
in near-equatorial areas

61
The Evolving Atmosphere

Geologists agree that the Archean atmosphere
contained little or no free oxygen so the
atmosphere
was not strongly oxidizing as it is now
Even though processes were underway
that added free oxygen to the atmosphere,
the amount present
at the beginning of the Proterozoic
was probably no more than 1 of that present now
In fact, it might not have exceeded
10 of present levels even
at the end of the Proterozoic

62
Cyanobacteria and Stromatolites

Remember from our previous discussions
that cyanobacteria,
also known as blue-green algae,
were present during the Archean,
but stromatolites
the structures they formed,
did not become common until about 2.3 billion
years ago,
that is, during the Early Proterozoic
These photosynthesizing organisms
and to a lesser degree photochemical dissociation
added free oxygen to the evolving atmosphere

63
Oxygen Versus Carbon Dioxide

Earth's early atmosphere
had abundant carbon dioxide
More oxygen became available
whereas the amount of carbon dioxide decreased
Only a small amount of CO2
still exists in the atmosphere today
It is one of the greenhouse gases
partly responsible for global warming
What evidence indicates
that the atmosphere became oxidizing?
Where is all that additional the carbon dioxide
now?

64
Evidence from Rocks

Much carbon dioxide is now tied up
in various minerals and rocks
especially the carbonate rocks
limestone and dolostone,
and in the biosphere
For evidence that the Proterozoic atmosphere was
evolving
from a chemically reducing one
to an oxidizing one
we must discuss types
of Proterozoic sedimentary rocks, in particular
banded iron formations
and red beds

65
Banded Iron Formations (BIF)

Banded iron formations (BIFs),
consist of alternating layers of
iron-rich minerals
and chert
Some are found in Archean rocks,
but about 92 of all BIFs
formed during the interval
from 2.5 to 2.0 billion years ago

66
Early Proterozoic Banded Iron Formation

At this outcrop in Ishpeming, Michigan
the rocks are alternating layers of
red chert
and silver-colorediron minerals

67
Typical BIF

A more typical outcrop of BIF near Nagaunee,
Michigan

68
BIFs and the Atmosphere

How are these rocks related to the atmosphere?
Their iron is in iron oxides, especially
hematite (Fe2O3)
and magnetite (Fe3O4)
Iron combines with oxygen in an oxidizing
atmosphere
to from rustlike oxides
that are not readily soluble in water
If oxygen is absent in the atmosphere, though,
iron easily dissolves
so that large quantities accumulate in the
world's oceans,
which it undoubtedly did during the Archean

69
Formation of BIFs

The Archean atmosphere was deficient in free
oxygen
so that little oxygen was dissolved in seawater
However, as photosynthesizing organisms
increased in abundance,
as indicated by stromatolites,
free oxygen,
released as a metabolic waste product into the
oceans,
caused the precipitation of iron oxides along
with silica
and thus created BIFs

70
Formation of BIFs

One model accounting for the details
of BIF precipitation involves
a Precambrian ocean with an upper oxygenated
layer
overlying a large volume of oxygen-deficient
water
that contained reduced iron and silica
Upwelling,
that is transfer of water from depth to the
surface,
brought iron- and silica-rich waters
onto the shallow continental shelves
and resulting in widespread precipitation of BIFs

71
Formation of BIFs

Depositional model for the origin of banded iron
formation

72
Source of Iron and Silica

A likely source of the iron and silica
was submarine volcanism,
similar to that now talking place
at or near spreading ridges
Huge quantities of dissolved minerals are
also discharged at submarine hydrothermal vents
In any case, the iron and silica
combined with oxygen
thus resulting in the precipitation
of huge amounts of banded iron formation
Precipitation continued until
the iron in seawater was largely used up

73
Continental Red Beds

Obviously continental red beds refers
to red rocks on the continents,
but more specifically it means red sandstone or
shale
colored by iron oxides,
especially hematite (Fe2O3)

Red mudrock in Glacier National Park, Montana
74
Red Beds

Red beds first appear
in the geologic records about 1.8 billion years
ago,
increase in abundance throughout the rest of the
Proterozoic,
and are quite common in rocks of Phanerozoic age
The onset of red bed deposition
coincides with the introduction of free oxygen
into the Proterozoic atmosphere
However, the atmosphere at that time
may have had only 1
or perhaps 2 of present levels

75
Red Beds

Is this percentage sufficient to account
for oxidized iron in sediment?
Probably not,
but no ozone (O3) layer existed in the upper
atmosphere
before free oxygen (O2) was present
As photosynthesizing organisms released
free oxygen into the atmosphere,
ultraviolet radiation converted some of it
to elemental oxygen (O) and ozone (O3),
both of which oxidize minerals more effectively
than O2

76
Red Beds

Once an ozone layer became established,
most ultraviolet radiation failed
to penetrate to the surface,
and O2 became the primary agent
for oxidizing minerals

77
Important Events in Life History

Archean fossils are not very common,
and all of those known are varieties
of bacteria and cyanobacteria (blue-green algae),
although they undoubtedly existed in profusion
Likewise, the Early Proterozoic fossil record
has mostly bacteria and cyanobacteria
Apparently little diversification
had taken place
all organisms were single-celled prokaryotes,
until about 2.1 billion years ago
when more complex eukaryotic cells evolved

78
Gunflint Microfossils

Even in well-known Early Proterozoic fossils
assemblages,
such as the Gunflint Iron Formation of Canada,
only fossils of bacteria are recognized

Photomicrograph of spheroidal and filamentous
microfossils from the Gunflint Chert of Ontario
Canada
79
Prokaryote and Eukaryotes

An organism made up of prokaryotic cells is
called a prokaryote
whereas those composed of eukaryotic cells are
eukaryotes
In fact, the distinction between prokaryotes and
eukaryotes
is the basis for the most profound distinction
between all living things

80
Lack of Organic Diversity

Actually, the lack of organic diversity
during this early time in life history
is not too surprising
because prokaryotic cells reproduce asexually
Most variation in
sexually reproducing populations comes from
the shuffling of genes,
and their alleles,
from generation to generation
Mutations introduce new variation into a
population,
but their effects are limited in prokaryotes

81
Genetic Variation in Bacteria

A beneficial mutation would spread rapidly
in sexually reproducing organism,
but have a limited impact in bacteria
because they do not share their genes with other
bacteria
Bacteria usually reproduce by binary fission
and give rise to two cells
having the same genetic makeup
Under some conditions,
they engage in conjugation during
which some genetic material is transferred

82
Sexual Reproduction Increased the Pace of
Evolution

Prior to the appearance of cells capable of
sexual reproduction,
evolution was a comparatively slow process,
thus accounting for the low organic diversity
This situation did not persist
Sexually reproducing cells probably
evolved by Early Proterozoic time,
and thereafter the tempo of evolution
increased markedly

83
Eukaryotic Cells Evolve

The appearance of eukaryotic cells
marks a milestone in evolution
comparable to the development
of complex metabolic mechanisms
such as photosynthesis during the Archean
Where did these cells come from?
How do they differ from their predecessors,
the prokaryotic cells?
All prokaryotes are single-celled,
but most eukaryotes are multicelled,
the notable exception being the protistans

84
Eukaryotes

Most eukaryotes reproduce sexually,
in marked contrast to prokaryotes,
and nearly all are aerobic,
that is, they depend on free oxygen
to carry out their metabolic processes
Accordingly, they could not have evolved
before at least some free oxygen was present in
the atmosphere

85
Prokaryotic Cell

Prokaryotic cells
do not have a cell nucleus
do not have organelles
are smaller and not nearly as complex as
eukaryotic cells

86
Eukaryotic Cell

Eukaryotic cells have
a cell nucleus containing
the genetic material
and organelles

such as mitochondria
and plastids,
as well as chloroplasts in plant cells

87
Eukaryotic Fossil Cells

The Negaunee Iron Formation in Michigan
which is 2.1 billion years old
has yielded fossils now generally accepted
as the oldest known eukaryotic cells
Even though the Bitter Springs Formation
of Australia is much younger
1 billion years old
it has some remarkable fossils of single-celled
eukaryotes
that show evidence of meiosis and mitosis,
processes carried out only by eukaryotic cells

88
Evidence for Eukaryotes

Prokaryotic cells are mostly rather simple
spherical or platelike structures
Eukaryotic cells
are larger, commonly much larger
much more complex
have a well-defined, membrane-bounded cell
nucleus, which is lacking in prokaryotes
have several internal structures
called organelles such as plastids and
mitochondria
their organizational complexity
is much greater than it is for prokaryotes

89
Acritarchs

Other organisms that were
almost certainly eukaryotes are the acritarchs
that first appeared about 1.4 billion years ago
they were very common by Late Proterozoic time
and were probably cysts of planktonic (floating)
algae

90
Acritarchs

These common Late Proterozoic microfossils
are probably from eukaryotic organisms
Acritarchs are very likely the cysts of algae

91
Late Proterozoic Microfossil

Numerous microfossils of organisms
with vase-shaped skeletons
have been found
in Late Proterozoic rocks
in the Grand Canyon
These too have tentatively been identified as
cysts of some kind of algae

92
Endosymbiosis and the Origin of Eukaryotic Cells

Eukaryotic cells probably formed
from several prokaryotic cells
that entered into a symbiotic relationship
Symbiosis,
involving a prolonged association of two or more
dissimilar organisms,
is quite common today
In many cases both symbionts benefit from the
association
as occurs in lichens,
once thought to be plants
but actually symbiotic fungi and algae

93
Endosymbiosis

In a symbiotic relationship,
each symbiont must be capable
of metabolism and reproduction,
but in some cases one symbiont
cannot live independently
This may have been the case
with Proterozoic symbiotic prokaryotes
that became increasingly interdependent
until the unit could exist only as a whole
In this relationship
one symbiont lived within the other,
which is a special type of symbiosis
called endosymbiosis

94
Evidence for Endosymbiosis

Supporting evidence for endosymbiosis
comes from studies of living eukaryotic cells
containing internal structures called organelles,
such as mitochondria and plastics,
which contain their own genetic material
In addition, prokaryotic cells
synthesize proteins as a single system,
whereas eukaryotic cells
are a combination of protein-synthesizing systems

95
Organelles Capable of Protein Synthesis

That is, some of the organelles
within eukaryotic cells are capable of protein
synthesis
These organelles
with their own genetic material
and protein-synthesizing capabilities
are thought to have been free-living bacteria
that entered into a symbiotic relationship,
eventually giving rise to eukaryotic cells

96
Multicelled Organisms

Obviously multicelled organisms
are made up of many cells,
perhaps billions,
as opposed to a single cell as in prokaryotes
In addition, multicelled organisms
have cells specialized to perform specific
functions
such as respiration,
food gathering,
and reproduction

97
Dawn of Multicelled Organisms

We know from the fossil record
that multicelled organisms
were present during the Proterozoic,
but we do not know exactly when they appeared
What seem to be some kind of multicelled algae
appear
in the 2.1-billion-year-old fossils
from the Negaunee Iron Formation in Michigan
as carbonaceous filaments
from 1.8 billion-year-old rocks in China
as somewhat younger carbonaceous impressions
of filaments and spherical forms

98
Multicelled Algae?

Carbonaceous impressions
in Proterozoic rocks
in the Little Belt Mountains, Montana
These may be impressions of multicelled algae

99
Studies of Present-Day Organisms

How did this important transition taken place?
Perhaps a single-celled organism divided
but the daughter cells formed
an association as a colony
Each cell would have been capable
of an independent existence,
and some cells might have become somewhat
specialized
as are the cells of colonial organisms today
Increased specialization of cells
may have given rise to
comparatively simple multicelled organisms
such as algae and sponges

100
The Multicelled Advantage?

Is there any particular advantage to being
multicelled?
For something on the order of 1.5 billion years
all organisms were single-celled
and life seems to have thrived
In fact, single-celled organisms
are quite good at what they do
but what they do is very limited

101
The Multicelled Advantage?

For example, single celled organisms
can not grow very large, because as size
increases,
proportionately less of a cell is exposed
to the external environment in relation to its
volume
and the proportion of surface area decreases
Transferring materials from the exterior
to the interior becomes less efficient

102
The Multicelled Advantage?

Also, multicelled organisms live longer,
since cells can be replaced and more offspring
can be produced
Cells have increased functional efficiency
when they are specialized into organs with
specific capabilities

103
Late Proterozoic Animals

Biologists set forth criteria such as
method of reproduction
and type of metabolism
to allow us to easily distinguish
between animals and plants
Or so it would seem,
but some present-day organisms
blur this distinction and the same is true
for some Proterozoic fossils
Nevertheless, the first
relatively controversy-free fossils of animals
come from the Ediacaran fauna of Australia
and similar faunas of similar age elsewhere

104
The Ediacaran Fauna

In 1947, an Australian geologist, R.C. Sprigg,
discovered impressions of soft-bodied animals
in the Pound Quartzite in the Ediacara Hills of
South Australia
Additional discoveries by others turned up what
appeared to be
impressions of algae and several animals
many bearing no resemblance to any existing now
Before these discoveries, geologists
were perplexed by the apparent absence
of fossil-bearing rocks predating the Phanerozoic

105
Ediacaran Fauna

The Ediacaran fauna of Australia
Tribrachidium heraldicum, a possible primitive
echinoderm

Spriggina floundersi, a possible ancestor of
trilobites
106
Ediacaran Fauna

Pavancorina minchami

Restoration of the Ediacaran Environment

107
Ediacaran Fauna

Geologists had assumed that
the fossils so common in Cambrian rocks
must have had a long previous history
but had little evidence to support this
conclusion
The discovery of Ediacaran fossils and subsequent
discoveries
have not answered all questions about
pre-Phanerozoic animals,
but they have certainly increased our knowledge
about this chapter in the history of life

108
Represented Phyla

Three present-day phyla may be represented
in the Ediacaran fauna
jellyfish and sea pens (phylum Cnidaria),
segmented worms (phylum Annelida),
and primitive members of the phylum Arthropoda
(the phylum with insects, spiders crabs, and
others)
One Ediacaran fossil, Spriggina,
has been cited as a possible ancestor of
trilobites
Another might be a primitive member
of the phylum Echinodermata

109
Distinct Evolutionary Group

However, some scientists think
these Ediacaran animals represent
an early evolutionary group quite distinct from
the ancestry of todays invertebrate animals
Ediacara-type faunas are known
from all continents except Antarctica,
are collectively referred to as the Ediacaran
fauna
were widespread between 545 and 670 million years
ago
but their fossils are rare
Their scarcity should not be surprising, though,
because all lacked durable skeletons

110
Other Proterozoic Animal Fossils

Although scarce, a few animal fossils
older than those of the Ediacaran fauna are known
A jellyfish-like impression is present
in rocks 2000 m below the Ediacara Hills Pound
Quartzite,
Burrows, in many areas,
presumably made by worms,
occur in rocks at least 700 million years old
Wormlike and algae fossils come
from 700 to 900 million-year-old rocks in China
but the identity and age of these "fossils" has
been questioned

111
Wormlike Fossils from China

Wormlike fossils from Late Proterozoic rocks in
China

112
Soft Bodies

All known Proterozoic animals were soft-bodied,
but there is some evidence that the earliest
stages in the origin of skeletons was underway
Even some Ediacaran animals
may have had a chitinous carapace
and others appear to have had areas of calcium
carbonate
The odd creature known as Kimberella
from the latest Proterozoic of Russia
had a tough outer covering similar to
that of some present-day marine invertebrates

113
Latest Proterozoic Kimberella

Kimberella, an animal from latest Proterozoic
rocks in Russia

Exactly what Kimberella was remains uncertain
Some think it was a sluglike creature
whereas others think it was more like a mollusk

114
Durable Skeletons

Latest Proterozoic fossils
of minute scraps of shell-like material
and small tooth like denticles and spicules,
presumably from sponges
indicate that several animals with skeletons
or at least partial skeletons existed
However, more durable skeletons of
silica,
calcium carbonate,
and chitin (a complex organic substance)
did not appear in abundance until the beginning
of the Phanerozoic Eon 545 million years ago

115
Proterozoic Mineral Resources

Most of the world's iron ore comes from
Proterozoic banded iron formations
Canada and the United States have large deposits
of these rocks
in the Lake Superior region
and in eastern Canada
Thus, both countries rank among
the ten leading nations in iron ore production

116
Iron Mine

The Empire Mine at Palmer, Michigan
where iron ore from the Early Proterozoic
Negaunee Iron Formation is mined

117
Nickel

In the Sudbury mining district in Ontario,
Canada,
nickel and platinum are extracted from
Proterozoic rocks
Nickel is essential for the production of nickel
alloys such as
stainless steel
and Monel metal (nickel plus copper),
which are valued for their strength and
resistance to corrosion and heat
The United States must import
more than 50 of all nickel used
mostly from the Sudbury mining district

118
Sudbury Basin

Besides its economic importance, the Sudbury
Basin,
an elliptical area measuring more than 59 by 27
km,
is interesting from the geological perspective
One hypothesis for the concentration of ores
is that they were mobilized from metal-rich rocks
beneath the basin
following a high-velocity meteorite impact

119
Platinum and Chromium

Some platinum
for jewelry, surgical instruments,
and chemical and electrical equipment
is exported to the United States from Canada,
but the major exporter is South Africa
The Bushveld Complex of South Africa
is a layered igneous complex containing both
platinum
and chromite
the only ore of chromium,
United States imports much of the chromium
from South Africa
It is used mostly in stainless steel

120
Oil and Gas

Economically recoverable oil and gas
have been discovered in Proterozoic rocks in
China and Siberia,
arousing some interest in the Midcontinent rift
as a potential source of hydrocarbons
So far, land has been leased for exploration,
and numerous geophysical studies have been done
However, even though some rocks
within the rift are know to contain petroleum,
no producing oil or gas wells are operating

121
Proterozoic Pegmatites