1. 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

2. The Length of the Proterozoic
the Proterozoic Eon alone,
at 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

3. The Phanerozoic Yet the Phanerozoic, consisting of
Paleozoic,
Mesozoic,
Cenozoic eras,
lasted a comparatively brief 545 million years
is the subject of the rest of the course

4. Disparity in Time Perhaps this disparity but we know far more
between the coverage of the Proterozoic and the Phanerozoic
seems disproportionate,
but we know far more
about Phanerozoic events
than we do for either of the Precambrian eons

5. Archean-Proterozoic Boundary
Geologist have rather 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
However, we must emphasize "approximate,"
because Archean-type crustal evolution
was largely completed in South Africa
nearly 3.0 billion years ago,
whereas in North America the change took place
from 2.95 to 2.45 billion years ago

6. Style of Crustal Evolution
Archean crust-forming processes generated
granite-gneiss complexes
and greenstone belts
that were shaped into cratons
Although these same rock associations
continued to form during the Proterozoic,
they did so at a considerably reduced rate

7. Contrasting Metamorphism
In addition, Archean and Proterozoic rocks
contrast in metamorphism
Many Archean rocks have been metamorphosed,
although their degree of metamorphism
varies and some are completely unaltered
However, vast exposures of Proterozoic rocks
show little or no effects of metamorphism,
and in many areas they are separated
from Archean rocks by a profound unconformity

8. Other Differences the Proterozoic is characterized
In addition to changes in the style of crustal evolution,
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

9. Proterozoic Evolution of Oxygen-Dependent Organisms
It was during the Proterozoic
that oxygen-dependent organisms
made their appearance
and the first cells evolved
that make up most organisms today

10. Evolution of Proterozoic Continents
Archean cratons assembled during collisions
of island arcs and minicontinents,
providing the nuclei around which
Proterozoic crust accreted,
thereby forming much larger landmasses
Proterozoic accretion at craton margins
probably took place more rapidly than today
because Earth possessed more radiogenic heat,
but the process continues even now

11. Proterozoic Greenstone Belts
Most greenstone belts formed
during the Archean
between 2.7 and 2.5 billion years ago
They also continued to form
during the Proterozoic and at least one is known
from Cambrian-aged rocks in Australia
They were not as common after the Archean,
and differed in one important detail
the near absence of ultramafic rocks
which no doubt resulted from
Earth's decreasing amount of radiogenic heat

12. Focus on Laurentia
Our focus here is on the geologic evolution of 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

13. Early Proterozoic History of Laurentia
Laurentia originated and underwent important growth
between 2.0 and 1.8 billion years ago
During this time, collisions
among various plates formed several orogens,
which are linear or arcuate deformation belts
in which many of the rocks have been
metamorphosed
and intruded by magma
thus forming plutons, especially batholiths

14. 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

15. Craton-Forming Processes
Examples of these craton-forming processes
are recorded in rocks
in the Thelon orogen in northwestern Canada
where the Slave and Rae cratons collided,

16. 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

17. Wilson Cycle Rocks of the Wopmay orogen A complete Wilson cycle,
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

18. 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

19. Early Proterozoic Rocks in Great Lakes Region
Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes

20. Outcrop of Sturgeon Quartzite
The sandstones have a variety of sedimentary structures
such as
ripple marks
and cross- beds
Northern Michigan

21. Outcrop of Kona Dolomite
Some of the carbonate rocks, now mostly dolostone,
such as the Kona Dolomite,
contain abundant bulbous structures known as stromatolites
Northern Michigan

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

23. Accretion along Laurentia’s Southern Margin
Following the initial episode
of amalgamation of Archean cratons
2.0 to 1.8 billion years ago
accretion took place along Laurentia's southern margin
From 1.8 to 1.6 billion years ago,
continental accretion continued
in what is now the southwestern and central United States
as successively younger belts were sutured to Laurentia,
forming the Yavapai and Mazatzal-Pecos orogens

24. 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

25. BIF, Red Beds, Glaciers This was also the time during which
most of Earth’s banded iron formations (BIF)
were deposited
The first continental red beds
sandstone and shale with oxidized iron
were deposited about 1.8 billion years ago
We will have more to say about BIF
and red beds in the section on “The Evolving Atmosphere”
In addition, some Early Proterozoic rocks
and associated features provide excellent evidence
for widespread glaciation

26. Early and Middle Proterozoic Igneous Activity
During the interval
from 1.8 to 1.1 billion years ago,
extensive igneous activity took place
that seems to be unrelated to orogenic activity
Although quite widespread,
this activity did not add to Laurentia’s size
because magma was either intruded into
or erupted onto already existing continental crust

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

28. Igneous Activity However, the igneous rocks are deeply buried
by younger rocks in most areas
The origin of these
granitic and anorthosite plutons,
Anorthosite is a plutonic rock composed
almost entirely of plagioclase feldspars
calderas and their fill,
and vast sheets of rhyolite and ash flows
are the subject of debate
According to one hypothesis
large-scale upwelling of magma
beneath a Proterozoic supercontinent
produced the rocks

29. Middle Proterozoic Orogeny and Rifting
The only Middle Proterozoic event in Laurentia
was the Grenville orogeny
in the eastern part of the continent
1.3 to 1.0 billion years old
Grenville rocks are well exposed
in the present-day northern Appalachian Mountains
as well as in eastern Canada, Greenland, and Scandinavia

30. Grenville Orogeny A final episode of Proterozoic accretion
occurred during the Grenville orogeny

31. Grenville Orogeny Many geologists think the Grenville orogen
resulted from closure of an ocean basin,
the final stage in a Wilson cycle
Others disagree and think
intracontinental deformation or major shearing
was responsible for deformation
Whatever the cause of the Grenville orogeny,
it was the final stage
in the Proterozoic continental accretion of Laurentia

32. 75% of North America By this final stage, about 75% The remaining 25%
of present-day North America existed
The remaining 25%
accreted along its margins,
particularly its eastern and western margins,
during the Phanerozoic Eon

33. Midcontinent Rift Grenville deformation in Laurentia
was accompanied by the origin
of the 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

34. Location of the Midcontinent Rift
Rocks filling the rift
are exposed around Lake Superior
but are deeply buried elsewhere

35. Midcontinental Rift Most of the rift is buried beneath younger rocks
except in the Lake Superior region
where 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
In fact, the volume of volcanic rocks,
between 300,000 and 1,000,000 km3,
is comparable in volume although not areal extent
to the great outpourings of lava during the Cenozoic

36. Midcontinental Rift Along the rift's margins In the vertical section
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

37. Cooper Harbor Conglomerate
Michigan

38. Portage Lake Volcanics
Michigan

39. Middle and Late Proterozoic Sedimentation
Remember the Grenville orogeny
took place 1.2 billion – 900 million years ago,
the final episode of continental accretion
in Laurentia until the Ordovician Period
Nevertheless, important geologic events
were taking place,
such as 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

40. 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

41. Sedimentary Rocks Middle to Late Proterozoic sedimentary rocks
are exceptionally well exposed
in the northern Rocky Mountains
of Montana and Alberta, Canada
Indeed, their colors, deformation features,
and erosion by Pleistocene and recent glaciers
have yielded some fantastic scenery
Like the rocks in the Great Lakes region
and the Grand Canyon,
they are mostly sandstones, shales,
and stromatolite-bearing carbonates

42. Proterozoic Mudrock
Outcrop of red mudrock in Glacier National Park, Montana

43. Proterozoic Limestone
Outcrop of limestone with stromatolites in Glacier National Park, Montana

44. Proterozoic Sandstone
Proterozoic rocks
of the Grand Canyon Super-group lie
unconformably upon Archean rocks
and in turn are overlain unconformably
by Phanerozoic-age rocks
The rocks, consisting mostly
of sandstone, shale, and dolostone,
were deposited in shallow-water marine
and fluvial environments
The presence of stromatolites and carbonaceous
impression of algae in some of these rocks
indicate probable marine deposition

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

46. Style of Plate Tectonics
The present style of plate tectonics
involving opening and then closing ocean basins
had almost certainly been established by the Early Proterozoic
In fact, the oldest known complete ophiolite
providing evidence for an ancient convergent plate boundary
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

47. 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

48. Jormua Complex, Finland
Metamorphosed basaltic pillow lava
12 cm

49. Jormua Complex, Finland
Metamorphosed gabbro between mafic dikes
65 cm

50. Proterozoic Supercontinents
You already know that a continent
is one of Earth's landmasses
consisting of granitic crust
with most of its surface above sea level
A supercontinent consists of all
or at least much of the present-day continents,
so other than size it is the same as a continent
The supercontinent Pangaea,
which existed at the end of the Paleozoic Era,
is familiar,
but few people are aware of earlier supercontinents

51. Early 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

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

53. Pannotia Rodinia's separate pieces reassembled
and formed another supercontinent
this one known as Pannotia
about 650 million years ago
judging by the Pan-African orogeny
the large-scale deformation that took place
in what are now the Southern Hemisphere continents
Fragmentation was underway again,
by the latest Proterozoic, about 550 million years ago,
giving rise to the continental configuration
that existed at the onset of the Phanerozoic Eon

54. Ancient Glaciers Very few times of widespread glacial activity
have occurred during Earth history
The most recent one during the Pleistocene
1.6 million to 10,000 years ago
is certainly the best known,
but we also have evidence for Pennsylvanian glaciers
and two major episodes of Proterozoic glaciation

55. Recognizing Glaciation
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,
and some of these are undoubtedly not glacial deposits

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

57. Proterozoic Glacial Evidence
Bagganjarga tillite in Norway
overlies striated bedrock surface
on sandstone of the Veidnesbotn Formation

58. 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

59. Early Proterozoic Glaciers
Deposits in North America
indicate that Laurentia
had an extensive ice sheet
centered southwest of Hudson Bay

60. 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

61. 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

62. Late Proterozoic Glaciers
The approximate distribution of Late Proterozoic glaciers

63. 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

64. 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

65. 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

66. 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?

67. 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

68. 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

69. Early Proterozoic Banded Iron Formation
At this outcrop in Ishpeming, Michigan
the rocks are alternating layers of
red chert
and silver- colored iron minerals

70. Typical BIF
A more typical outcrop of BIF near Nagaunee, Michigan

71. 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

72. 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

73. Formation of BIFs One model accounting for the details Upwelling,
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

74. Formation of BIFs
Depositional model for the origin of banded iron formation

75. 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

76. 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

77. Red Beds Red beds first appear The onset of red bed deposition
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

78. Red Beds Is this percentage sufficient to account Probably not,
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

79. 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

80. 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

81. Gunflint Microfossils
Even in well-known Early Proterozoic fossils assemblages, only fossils of bacteria are recognized
Photomicrograph of spheroidal and filamentous microfossils from the Gunflint Chert of Ontario Canada

82. 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

83. 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

84. 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

85. 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 the tempo of evolution increased

86. 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

87. 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

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

89. 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

90. 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 yrs old
it has some remarkable fossils of single-celled eukaryotes
that show evidence of meiosis and mitosis,
processes carried out only by eukaryotic cells

91. Evidence for Eukaryotes
Prokaryotic cells are mostly rather simple
spherical or platelike structures
Eukaryotic cells
are 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

92. 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

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

94. 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

95. 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

96. Endosymbiosis In a symbiotic relationship, This may have been the case
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

97. 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

98. 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

99. 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

100. 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

101. Multicelled Algae? Carbonaceous impressions
in Proterozoic rocks, Montana
These may be impressions of multicelled algae
Skip next slide

102. 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

103. 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

104. 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

105. 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

106. The Ediacaran Fauna In 1947, an Australian geologist, R.C. Sprigg,
in the Pound Quartzite in the Ediacara Hills of South Australia
Additional discoveries by others turned up what appeared to be
discovered impressions of soft-bodied animals
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

107. Ediacaran Fauna The Ediacaran fauna of Australia
Tribrachidium heraldicum, a possible primitive echinoderm
Spriggina floundersi, a possible ancestor of trilobites

108. Ediacaran Fauna Pavancorina minchami
Restoration of the Ediacaran Environment

109. 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

110. 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

111. Distinct Evolutionary Group
However, some scientists think
these Ediacaran animals represent
an early evolutionary group quite distinct from
the ancestry of today’s invertebrate animals
Ediacara-type faunas are known
from all continents except Antarctica,
--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

112. 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

113. Wormlike Fossils from China
Wormlike fossils from Late Proterozoic rocks in China

114. 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

115. 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

116. 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

117. 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

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

119. 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

120. 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

121. 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

122. 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

123. Proterozoic Pegmatites
A number of Proterozoic pegmatites
are important economically
The Dunton pegmatite in Maine,
whose age is generally considered
to be Late Proterozoic,
has yielded magnificent gem-quality specimens
of tourmaline and other minerals
Other pegmatites are mined for gemstones as well as for
tin, industrial minerals, such as feldspars, micas, and quartz
and minerals containing such elements
as cesium, rubidium, lithium, and beryllium

124. Proterozoic Pegmatites
Geologists have identified more than 20,000 pegmatites
in the country rocks adjacent
to the Harney Peak Granite
in the Black Hills of South Dakota
These pegmatites formed ~ 1.7 billion years ago
when the granite was emplaced as a complex of dikes and sills
A few have been mined for gemstones, tin, lithium, micas,
and some of the world's largest known
mineral crystals were discovered in these pegmatites

125. Summary The crust-forming processes
that yielded Archean granite-gneiss complexes
and greenstone belts
continued into the Proterozoic
but at a considerably reduced rate
Archean and Proterozoic greenstone belts
differed in detail
Early Proterozoic collisions
between Archean cratons formed larger cratons
that served as nuclei
around which Proterozoic crust accreted

126. Summary One such landmass was Laurentia Important events
consisting mostly of North America and Greenland
Important events
in the evolution of Laurentia were
Early Proterozoic amalgamation of cratons
followed by Middle Proterozoic igneous activity,
the Grenville orogeny, and the Midcontinent rift
Ophiolite sequences
marking convergent plate boundaries
are first well documented from the Early Proterozoic,
indicating that a plate tectonic style similar
to that operating now had been established

127. Summary Sandstone-carbonate-shale assemblages
deposited on passive continental margins
are known from the Archean
but they are very common by Proterozoic time
The supercontinent Rodinia
assembled between 1.3 and 1.0 billion years ago,
fragmented,
and then reassembled to form Pannotia about 650 million years ago
Glaciers were widespread
during both the Early and Late Proterozoic

128. Summary Photosynthesis continued
to release free oxygen into the atmosphere
which became increasingly oxygen rich through the Proterozoic
Fully 92% of Earth's iron ore deposits
in banded iron formations were deposited
between 2.5 and 2.0 billion years ago
Widespread continental red beds
dating from 1.8 billion years ago indicate
that Earth's atmosphere had enough free oxygen
for oxidation of iron compounds

129. Summary Most of the known Proterozoic organisms
are single-celled prokaryotes (bacteria)
When eukaryotic cells first appeared is uncertain,
but they may have been present by 2.1 billion years ago
Endosymbiosis is a widely accepted theory for their origin
The oldest known multicelled organisms
are probably algae,
some of which may date back to the Early Proterozoic

130. Summary Well-documented multicelled animals
are found in several Late Proterozoic localities
Animals were widespread at this time,
but because all lacked durable skeletons
their fossils are not common
Most of the world's iron ore produced
is from Proterozoic banded iron formations
Other important resources
include nickel and platinum