The Proterozoic Eon: Top Ten Significant Events

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The Proterozoic EonTop Ten Significant Events

• Plate tectonics occurred similar to modern rates
• Accretion at continental boundaries
• Assembly of Laurentia and two super continents
• Rifting and widespread sandstone, carbonate,
  shale deposits (continental shelf deposits)
• Extensive continental glaciation
• Mid-continent rift formed in North America
• Widespread occurrence of stromatolites
• Formation of banded iron and other mineral
  resources (gold, copper, platinum, nickel)
• Free oxygen in atmosphere
• Evolution of eukaryotic cells

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The Length of the Proterozoic

• the Proterozoic Eon alone,
• 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

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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

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Style of Crustal Evolution

• Archean
• granite-gneiss complexes
• and greenstone belts
• cratons
• During the Proterozoic, these formed at a
  considerably reduced rate and cooler temperatures

5
Contrasting Metamorphism

• Proterozoic rocks
• show little or no effects of metamorphism,
• and in many areas they are separated
• from Archean rocks by a profound unconformity

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Evolution of Proterozoic Continents

• Archean cratons assembled from collisions of
  island arcs and minicontinents,
• Proterozoic accretion at craton margins
• probably took place more rapidly than today
• forming much larger landmasses
• Earth possessed more radiogenic heat,
• but the process continues even now

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Proterozoic Greenstone Belts

• Not as common after the Archean,
• near absence of ultramafic rocks
• WHY would this happen?

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Focus on Laurentia

• 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

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Early Proterozoic History of Laurentia

• Laurentia originated 2.0 billion years ago
• collisions called orogens
• formed linear deformation belts
• rocks have been
• metamorphosed
• and intruded by magma
• thus forming plutons, especially batholiths

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Proterozoic Evolution of Laurentia

• Archean cratons were sutured
• along deformation belts called orogens,
• 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

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What is the evidence?Craton-Forming Processes

• Recorded in rocks
• In northwestern Canada
• where the Slave and Rae cratons collided

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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

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Wilson Cycle

• A complete Wilson cycle,
• named for the Canadian geologist J. Tuzo Wilson,
• involves
• fragmentation of a continent (rifting)
• opening of an ocean basin
• followed by closing of an ocean basin,
• and finally reassembly of the continent
• Example Wopmay Orogeny (Canada) complete
  Wilson Cycle

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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

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Early Proterozoic Rocks in Great Lakes Region
Evidence of continental shelf

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

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Where? N. MichiganOutcrop of Sturgeon Quartzite

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

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Outcrop of Kona Dolomitewarm shallow marine

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

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Penokean Orogen

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

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Southern Margin Accretion

• Laurentia grew along its southern margin
• Central Plains, Yavapai, and Mazatzal orogens

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

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BIF, Red Beds, Glaciers

• This was also the time during which
• most of Earths 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
• provide excellent evidence for widespread
  glaciation

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Middle Proterozoic Orogeny and Rifting

• The only Middle Proterozoic event in Laurentia
• 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

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Grenville Orogeny

• A final episode of Proterozoic accretion
• occurred during the Grenville orogeny

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75 of North America formed by 900 million years
ago

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

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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 orogenWhat is the relative age?

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Location of the Midcontinent Rift

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

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Midcontinental Rift

• Most of the rift is buried beneath younger rocks
• except in the Lake Superior region
• with various igneous and sedimentary rocks
  exposed
• The Evidence
• numerous overlapping basalt lava flows
• forming a volcanic pile several kilometers thick

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Portage Lake Volcanics

• Michigan

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Proterozoic Sedimentary 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

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Proterozoic Mudrock

• Outcrop of red mudrock in Glacier National Park,
  Montana

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Proterozoic Limestone

• Outcrop of limestone with stromatolites in
  Glacier National Park, Montana

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Grand Canyon Super-group

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

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Proterozoic Supercontinents

• A supercontinent consists of all
• Or much of the present-day continents,
• so other than size it is the same as a continent
• The supercontinent Pangaea,
• existed MUCH LATER but few people are aware of
  earlier supercontinents

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Early Supercontinents

• Rodinia
• assembled between 1.3 and 1.0 billion years ago
• and then began fragmenting (rifting apart) 750
  million years ago (THE Proterozoic ends at 545my
  ago)

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Early Supercontinent

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

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• Rodinia's separate pieces reassembled
• and formed another supercontinent
• Pannotia
• about 650 million years ago
• Fragmentation was underway again,
• about 550 million years ago,
• giving rise to the continental configuration
• that existed at the onset of the Phanerozoic Eon
  the Cambrian

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Recognizing Glaciation

• extensive geographic distribution
• conglomerates and tillites
• and their associated glacial features
• striated and polished bedrock

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Proterozoic Glacial Evidence

• Bagganjarga Tillite in Norway
• Over bedrock

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Geologists Convinced

• The occurrence of tillites
• in Michigan, Wyoming, and Quebec
• indicates that North America may have had
• an Early Proterozoic ice sheet centered southwest
  of Hudson Bay

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Early Proterozoic Glaciers

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

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Late Proterozoic Glaciers

• The approximate distribution of Late Proterozoic
  glaciers

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• Late Proterozoic glaciers
• seem to have been present even
• in near-equatorial areas!!
• Geologists have recently named this phenomenon
• SNOWBALL EARTH

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The Evolving Atmosphere

• Archean little or no free oxygen
• At beginning of the Proterozoic was probably no
  more than 1 of that present now
• Stromatolitesnot common until
• 2.3 billion years ago,
• that is, during the Early Proterozoic
• There is evidence of increasing oxygen.

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Early Proterozoic Banded Iron Formation

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

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Banded Iron Formations (BIF)

• Banded iron formations (BIFs),
• consist of alternating layers of
• iron-rich minerals
• and chert
• about 92 of all BIFs
• formed during the interval
• from 2.5 to 2.0 billion years ago

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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

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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

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Formation of BIFs

• Depositional model for the origin of banded iron
  formation

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Source of Iron and Silica

• 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
• 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

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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
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Red Beds

• Red beds first appear
• in the geologic records about 1.8 billion years
  ago,
• and are quite common in rocks of Phanerozoic age
• coincides with the introduction of free oxygen
• into the Proterozoic atmosphere
• may have had only 1 - 2 of present levels

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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

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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

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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),
  
• Little diversification
• all organisms were single-celled prokaryotes,
• until about 2.1 billion years ago
• when more complex eukaryotic cells evolved

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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
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Prokaryote and Eukaryotes

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

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Lack of Organic Diversity

• 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

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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

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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
• How do they differ from their predecessors,
• the prokaryotic cells?
• All prokaryotes are single-celled,
• but most eukaryotes are multicelled,

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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

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Prokaryotic Cell

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

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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

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Eukaryotic Fossil Cells

• The Negaunee Iron Formation in Michigan
• 2.1 billion years old
• Fossils are oldest known eukaryotic cells
• Bitter Springs Formation
• of Australia --1 billion yrs old
• fossils of single-celled eukaryotes
• evidence of meiosis and mitosis,
• processes carried out only by eukaryotic cells

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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

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Acritarchs

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

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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

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Endosymbiosis and the Origin of Eukaryotic Cells

• Formed from several prokaryotic cells
• In 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

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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

68
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

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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

70
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

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Multicelled Algae?

• Carbonaceous impressions
• in Proterozoic rocks, Montana
• These may be impressions of multicelled algae
• Skip next slide

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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

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The Multicelled Advantage?

• 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

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The Multicelled Advantage?

• 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

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Ediacaran Fauna

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

Spriggina floundersi, a possible ancestor of
trilobites
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Ediacaran Fauna

• Pavancorina minchami

• Restoration of the Ediacaran Environment

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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 certainly increased our knowledge
• Representatives of jellyfish, corals, worms,
  insects, spider crabs

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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,
• --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

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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

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Wormlike Fossils from China

• Wormlike fossils from Late Proterozoic rocks in
  China

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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

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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

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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
• 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
  Cambrian age

84
Proterozoic Mineral Resources

• Proterozoic banded iron formations
• large deposits of these rocks
• in the US Lake Superior region
• and in eastern Canada
• Nickel, Sudbury basin Canada
• platinum, chromium, S. Africa

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Iron Mine

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

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Oil and Gas

• Economically recoverable oil and gas
• have been discovered in Proterozoic rocks in
  China and Siberia,
• Dunton pegmatite in Maine gemstones
  tourmaline, micas, quartz, aquamarine, feldspars,
  garnets, topaz, lepidolite (lithium)
• Also Black Hills, S.D. pegmatite vein