HG study guide #2

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Continental Drift

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Continental Drift

  • The potential to fit the continents together like puzzle pieces had been noted before (as early as 1596!), but…

  • Alfred Wegener (1912, 1915) marshaled many lines of evidence that the continents had moved

  • Many Permian-Triassic aged fossils are found across the southern continents

  • Easy to explain if they were once joined (Gondwana)

  • Distribution of Carboniferous-Permian Glacial Deposits 

  • glaciers will be in cold areas where it will be ice all year round

  • continents were located closer together and centered around the South Pole

  • Distribution of older rocks matches up too

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Boundary Types

  1. Divergent

  2. Transform

  3. Convergent

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Divergent

  • pulling apart

  • faulting/rift forming leads to volcanic and earthquake activity

  • continental rift: faulting forms a rift valley and parallel valleys along it

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Transform

  • sliding past

  • Can be ocean-ocean or continental

  • Plates slide past on strike-slip faults

  • Lithosphere isn’t created or destroyed

  • Earthquakes common

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Convergent

  • one plate pushed under another at subduction zone

  • Ocean-ocean: colder and denser plate subducts, oceanic trench forms

    • Volcanic arc/orogeny on plate that isn’t subducted

  • Continent-ocean: oceanic plate subducts

    • Volcanic arc/orogeny on continental plate

  • Continent-continent: neither plate wants to subduct

    • Lithosphere shortens via crumbling and thickening

    • Leads to faulting and folding

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Wegener’s “Continental Drift”

  • Wegener had no plausible explanation for how the continents moved – he proposed that they plowed across the ocean floor

  • Most geologists rejected his theory for this reason

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Continental Crust

  • low density

  • felsic (feldspar-rich)

  • underlies continental shelves as well as land

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Oceanic Crust

  • high density

  • mafic (iron-rich)

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Mantle

  • very high density

  • ultramafic (very iron-rich)

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Crust-Mantle Boundary

  • = Moho (Mohorovicic discontinuity)

  • marks an increase in rock density

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Lithosphere

  • = crust and upper mantle

  • firmly attaches, forming a rigid layer

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Asthenosphere

  • = slushy, partially molten layer below the lithosphere

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Most Earthquakes and Volcanoes Occur at Plate Boundaries

  • sinking blocks of crust

  • earthquakes are going to happen when there is pressure, variety of depths

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Strike and Dip

  • sed rocks = most likely to have layers

  • strike line = horizontal line

  • tick mark records that it is going downhill

  • number = magnitude of dip

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Syncline

  1. Concave up

  2. Layers dip toward each other

  3. Youngest rock in the middle

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Anticline

  1. Concave down

  2. Layers dip away from each other

  3. Oldest rock in the middle

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Strike and Dip? Why should we care?

  • landslide risk

  • oil exploration

  • mineral resources

  • groundwater resources

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Overturned folds

  • Axial plane – boundary between two plates

  • Syncline and anticline

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Relative Size of Different Water Reservoirs

  • (saltwater) Oceans ~ 97.5%

  • (freshwater)

  • Ice caps and glaciers ~79%

  • Groundwater ~20%

  • Water in lakes ~52%

  • Water in rivers

  • Water in living organisms

  • Water vapor in atmosphere

  • Water in soil ~38%

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Water Cycle

  • Evaporation, precipitation, groundwater, pools (reservoirs) and fluxes, evapotranspiration

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Evaporation and Evapotranspiration

  • Water enters the atmosphere (air) by:

  • Evaporation: conversion to water vapor from bodies of surface water (lakes, oceans, etc.)

  • Evapotranspiration:

    • Water vapor is lost to the air by plants

    • Delivers groundwater into the atmosphere

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Evapotranspiration

  • water enters plants through roots

  • flows up vascular (tubelike) tissues

  • leafe stomata:

    • Openings for gas exchange

    • water loss through these openings

    • draw additional water up the vascular tissues

  • this process delivers water from soil (groundwater) to atmosphere (air)

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Desertification

  • drying up of an area due to lack of plants

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Deforestation

  • no mechanism (plants) to get groundwater into atmosphere —> area dries up

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Carbon Cycle - Earth’s “Thermostat”

  • determines Earth’s climate over very long timescales

  • volcanism releases carbon dioxide

  • rock weathering absorbs (fixes) carbon dioxide

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Carbon Cycle - Volcanic Outgassing

  • Hydrogen Sulfide

  • Methane (carbon gas — greenhouse gas)

  • Carbon Dioxide (carbon gas — greenhouse gas)

  • Water Vapor

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Biological Carbon

  • Atmospheric carbon enters biological systems via photosynthesis

  • CO2+H2O => C6H12)6(sugar)+O2

  • Carbon is then repurposed into a wide variety of organic molecule

    • fats/oils (lipids)

    • Proteins

    • Nucleic acids (DNA & RNA)

  • ex.) “Blue-Green Algae” (photosynthesizing bacteria)

  • ex.) Tree photosynthesizing

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Fossil Fuels

  • Ancient Organic Material

  • Generally the result of photosynthesis

  • Contain many high-energy bonds → burn readily

  • Major contributor to climate change

  • Include:

    • Coal

    • Oil

    • Natural Gas

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Future Coal

  • Coal:

    • Accumulated plant matter

    • Start as peat bogs

    • Anoxic conditions prevent decay

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Future Oil

  • Oil:

    • Accumulation of algai biomass

    • Accumulate at bottoms of oceans and large lakes

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Fossil Fuel Formation

  • Organisms live and die

  • Organic material is buried

  • Heat and pressure alter molecule structure (not unlike metamorphic rock formation)

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Why Continental Weathering Draws Down CO2

  • Rocks exposed to air and water are susceptible to chemical weathering

  • Produces:

    • Clastic sediments

    • Dissolved ions (including Ca+)

  • Ca+ & CO2 can combine to form CaCO3 → limestone

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Eutrophication

  • Can be caused by an excess of nitrogenous nutrients

  • Results in anoxia

  • Healthy Water living organisms: fresh water and natural balance between water organisms, oxygen, algae and nutrients: enough sunlight for water plants

  • Eutrophication dead zone: explosive algal bloom due to excess nutrients coming from nitrogen (N) and phosphorus (P) rich pollutants; oxygen (O2) consumption by algae, reduced by sunlight, breakdown of natural chemical cycles

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Human Causes of Eutrophication

  • Nitrate and Phosphate fertilizers

  • Concentrated animal feeding operations

  • Direct sewage and industrial waste discharge into water

  • Improperly managed aquaculture

  • Natural events

    • Stream discharge

    • flood events

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Rules for air flow

  • Flows from high pressure to low pressure

  • Hot air rises, cold air sinks

  • Locations that produce hot air, produce low pressure zones

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Global air circulation and precipitation

  • Warms air rises

  • Dry air descends, warms, and becomes even drier

  • Warm air rises, cools and loses moisture

  • Dry belts at ~30º N/S and at the poles

  • Can be altered by local geography

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Greenhouse Gases

  • Gases like O2 and O3 absorb Ultraviolet radiations

  • Blocked from Earth’s Energy Budget

  • O2 and O3 let through visible and some shortwave radiation

  • Greenhouse gases also let through visible and shortwave radiation

  • Earth absorbs visible and shortwave radiation

  • Earth re-radiates near-and IR radiation

  • Greenhouse gases absorb and re-radiate and near-and IR radiation

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Albedo

  • reflectiveness of a surface

  • controls how much energy is absorbed (as heat) vs reflected

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Different Surfaces with Different Albedos

  • land vs water

  • forest

  • deserts

  • glacial ice

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Continental Position Influences Albedo

  • Availability of ice

  • Bare rock vs foliage cover

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Proxy Data

  • Information accessible in the present that indirectly indicates conditions in the past

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Climate Proxy Data: Recent Past

  • How do we get temperature info from the distant past? → PROXY DATA:

  • Tree ring thickness and density

  • Coral reef annual layers

  • Lake sediment – snowmelt and pollen

  • δ18O in mollusk shells and ice cores

  • Various metrics agree that current average temperatures are higher than they have ever been in the past 1000 years.

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Proxies for Condition of the More Distant Past

  • Rocks specific to climate:

    • E.g. Glacial deposits

    • E.g. Bauxite, only forms in tropical climates

  • Fossil organisms present

  • Geochemical data

    • Stable isotopes

    • Chemically equivalent, but have different masses

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Fossils: Proxy Data

  • some organisms live in specific environments. Their fossils indicate climate

  • Cycads and crocodilians prefer tropical climates today but lived closer to the poles in the past – therefore, mid-to-high latitudes must have had warmer climates than today

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Lead Margin Analysis: Proxy Data

  • cold-adapted floras have more jagged leaves; warm-adapted ones have more smooth leaves.

  • The percentage of smooth leaves in a fossil flora can be used to determine temperature.

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Stomatal Density: Proxy Data

  • plants respire through openings called stomata

  • In woody plants, the number of stomata increases as CO2 decreases

  • Low CO2 correlates with low temperatures, so more stomata suggest low temperatures too

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Oxygen Isotope Ratios

  • at low temperatures, heavy isotopes move slower

    • this is fractionation

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What are Isotopes?

  • Forms of an element that have a different number of neutrons than usual

  • Atomic mass is different but chemical properties are the same

  • Each radioisotope has a characteristic

  • Half Life: the time it takes for half the atoms to decay

  • On a linear scale, decay follows a hollow curve

  • On a logarithmic scale, decay follows a straight line

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When is a Particular Isotope Useful?

  • Short half-life good for young samples

  • Long half-life good for old samples

Why?

  • Must have enough parent isotope left to measure accurately

  • Enough time must have past for some daughter isotope to accumulate

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Blocking or Closure Temperature

  • In radiometric dating, we measure the ratio between parent and daughter isotope

    • There is no daughter isotope to begin with

  • Above the blocking temperature, the daughter isotope diffuses out of a mineral

  • Below the blocking tem, the daughter isotope collects

  • Thus, a radiometric age is the age since the mineral last cooled below the blocking temp.

    • Can represent time since cooling from a magma or cooling after metamorphism

  • Blocking temp. depends on the mineral and isotope

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Radiometric Dating Only puts Upper and Lower Boundaries on Date of Sedimentary Rocks

  • Procedure: date igneous rocks and determine relative age of associated sedimentary rocks using:

    • Principle of Superposition

    • Principle of Cross-Cutting Relationships

    • Principle of Inclusions

  • Radiometric dating does not tell you when a sedimentary rock was deposited

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Reconstructing Paleogeography: Methods

  • Linear Magnetic Anomalies: can reverse time by “deleting” stripes of opposite polarity from oceanic ridges

  • Paleomagnetism: magnetic grains are vertical at poles and horizontal at equator (measures paleolatitude)

  • Paleobiogeography: can indicate paleolatitude (based on climate) and proximity of continents

  • Paleoclimatology

  • Regional Geologic and Tectonic History: matching up geologic features to find paleolongitude 

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Paleoclimatology

  • Some rocks are only deposited in certain climatic conditions

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Evaporite

  • arid

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Coal

  • swamp

  • high precipitation

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Most Carbonates

  • warm water

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Our knowledge of paleogeography

  • Cenozoic and Mesozoic — Excellent

    • Magnetic seafloor anomalies allow precise reconstructions

  • Paleozoic — Good

    • lots of paleomagnetic and geologic data

  • Neoproterzoic — topic of current research

    • data are more sparse, but they are being collected

    • there are competing interpretations

  • Earlier in the Precambrian — much less known

    • much less rock on which to work

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Precambrian Geology

  • Cratons

    • large under-formed portions of continents

    • primarily Precambrian

  • Precambrian Shield

    • craton exposed at surface

    • Canadian Shield exposed by glaciation

  • continental crust formed during Archean

  • high heat flow early in Earth’s history required small continents

  • thinner sections of crust

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Oldest Region of Crust

  • the Canadian Shield (northeastern Canada)

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Origin of the Universe

  • stars cluster in galaxies

    • organized in disks

  • Milky Way

    • our galaxy of stars

  • Expanding universe

    • galaxies

      • redshift

    • originally concentrated into a single point

  • Big Bang

    • ~13.7 billion years ago (age of universe)

  • Galactic matter in concentrated

  • stars form

    • our sun

  • supernova

    • exploding star

  • solar nebula

    • dense rotational cloud

  • Fragments of larger rocky bodies that have undergone collision and broken into pieces

    • Provide important information concerning age of Earth

  • Stony meteorites

    • Rocky composition

  • Iron meteorites

    • Metallic composition

  • Stony-iron meteorites

    • Mixture of rocky and metallic

    • Proxy for core composition

  • Most date around 4.6 billion years ago

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Origin of Earth and Moon

  • 4.6 Ba

  • Earth materials differentiated

    • dense at center

    • less dense silicates rose to surface

      • magma ocean

    • cooled to form crust

  • meteorite impacts increased concentrations of some elements in upper Earth

  • Early Atmosphere

    • Degassing from volcanic emission

    • CH4 and NH3 abundant

    • Little O2

      • No photosynthesis

  • Earth’s oceans

    • Volcanic emissions cooled, condensed

    • Salts

      • Carried to sea by rivers and introduced at ridges

      • Approximately constant through time

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Origin of Continents

  • small Archean fragments

    • high heat flow limited continental thickness

  • Zircon crystals

    • 4.1-4.2 B years old

    • weathered from felsic rocks

  • Canadian Shield

    • 3.8-4.0 B years ago

  • Greenstone belts

    • Weakly metamorphosed

    • Previously ancient sedimentary rocks

    • Abundant chlorite

      • Green color

    • Nested in high-grade felsic metamorphic rocks

  • Greenstone belts contain igneous rocks

    • Volcanics contain pillow basalts

      • Underwater extrusion

    • Formation of sediments in deep water

      • Greywackes, mudstones, iron formations, volcanic sediments

  • Continental accretion

    • Deep water sediments accreted to continent

    • Marine sediments form wedge between continental masses

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Prokaryote

  • Smaller and simpler

  • No organelles

  • Many different metabolisms, including anaerobic

  • Bacteria and Archaea

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Eukaryote

  • Larger, more complex

  • Organelles, incl. nucleus

  • Generally require oxygen

  • Some lineages evolved multicellularity

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When did life begin?

  • Some think on the ocean floor, where hot, nutrient-rich waters are expelled

  • Mid-ocean ridges, hydrothermal vents

  • Not many sedimentary rocks older than ~3.5 Ga (=billions of years ago)

  • Pretty good evidence there was life by at least 3.5-3.2 Ga, although people argue about the fossils and the exact date

  • Hematite tubes: Possible microbial sheathes from 3.77-4.28 Ga, Quebec. Would have lived in a hydrothermal setting

  • Other scientists dispute both the age of the rocks and the biological nature of the structures.

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Multicellular Algae

  • ~580 Ma

  • Fossils of non-mineralized “seaweeds” are relatively rare

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Ediacaran Biota

  • There were animals, but most of them…

    • Did not belong to modern phyla (e.g., echinoderms, arthropods)

    • Did not have legs, fins, etc.

    • Did not have obvious sense organs

    • Did not have obvious body openings (probably fed by absorption)

    • Did not have mineralized skeletons

    • Did not burrow into the sediment, at least deeply

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In the Cambrian

  • Almost all modern phyla with skeletons appeared

  • Animals with legs, fins, body openings, sense organs, and skeletons were abundant

  • Marine ecosystems are relatively modern: herbivory, predation, suspension feedings, etc. were present

  • Burrowing and bioturbation increased

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Cambrian World — Geologic Environments and Events

  • Laurentia (N. America) rifts from Gondwana

  • Laurentia coastlines become passive margins all the way around

  • Sedimentation accommodated by:

    • Sea level rise

    • Thermal subsidence

  • Sediment Sources:

    • Silicate weathering from continent

    • Carbonates precipitating from water

    • Biogenic carbonates

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Sea Level in the Cambrian Period

  • Sea level low at beginning of Cambrian

  • Rises over course of the period

  • Laurentia (N. America) is almost entirely drowned by the end of the period

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Marine Sedimentation Around Laurentia

  • Dominated by terrigenous sands

    • Mature silicates

      • Almost solely quartz

      • Rounded, well-sorted grains

    • Evidence for aeolian (wind-based) weathering

  • Carbonates on shelf edges, far from terrestrial sources

  • Toward end of Cambrian period, continent flooded

    • Way less terrigenous sediment

    • Widespread carbonate deposition

      • Tropical temperatures

      • Agitated water → carbonate precipitation

      • Rise of skeletal carbonate

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Why was there so much highly-processed terrigenous sediment?

  • Continents above sea level

  • NO PLANTS

  • ex.) Still from the opening sequence of the movie Prometheus. This sequence depicts erosion and transportation of sediment on a landscape without plants.

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Mineralized Skeletons, what are they?

  • Minerals for structure and protections

    • calcite/Aragonite

    • Apatite

    • Silica

  • Ediacaran trace fossils: simple horizontal burrows

  • Cambrian trace fossils: some are more complex, branching and/or probing vertically into the sediment

ex.) Treptichnus pedum: defined the base of the Cambrian

  • Ediacaran/Cambrian boundary type section

    • Fortune Head, SE Newfoundland

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Ediacaran

  • Microbial mats cover seafloor

  • Only shallow horizontal burrows

  • Animals can attach to mat

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Later Cambrian

  • Bioturbated “mixed layer” replaces mats

  • More burrowing, some vertical burrowing

  • Sediment surface less stable

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Tectonic Changes to Laurentia

  • Started at high sea level with extensive epicontinental seas

  • Subduction zone forms on east coast of Laurentia

  • Taconic orogeny at the end of the Ordovician

    • Island arc accreted to Laurentia

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Prominent Ordovician Fauna

  • Graptolites: colonial filter feeders

    • Good index fossils, wide geographic range b/c some floated closer to surface

  • Corals: functioned as reef-builders

    • Rugose Corals

    • Tabulate Corals

    • Ecosystem engineers

    • Extinct now

  • Cephalopods: nautiloids (curved walls) and ammonoids (zigzag walls)

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