Earth Science

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Chapter 13

• Earth Science

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The Precambrian

• The Precambrian
• Begun 4.56 billion years ago (the birth of Earth
  until the start of the Cambrian Over 4 billion
  years. This is 88 of the age of Earth.
• Most Precambrian rocks do not contain fossils
• Many rocks of this age are metamorphosed and
  deformed or extremely eroded.
• Lots of missing layers and missing data.

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• Most Precambrian rock is buried deep under
  younger strata.
• Large continental cores composed of Precambrian
  rock exist on every continent.
• These are called shields

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The evolution of Earths atmosphere

• The evolution of Earths atmosphere
• The atmosphere we breathe today consists of N, O2
  and a small amount of Ar and CO2 and H2O.
• Earths original atmosphere was similar to the
  gases released in volcanic eruptions, CO2, H2O, N
  and other trace gases but NO O2.

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• Because the planet was cooling, the water vapor
  condensed to form clouds.
• It began to rain, the rain continued to evaporate
  but as Earth cooled more, the water began to fill
  low areas, forming the oceans.
• This reduced the water vapor in the air. It also
  reduced the CO2 because it became dissolved in
  the water.
• The first life forms did not need O2.

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• Later, plants evolved that used photosynthesis
  and released O2.
• Plants put O2 in our atmosphere.
• At first, the Fe on earth began to react with the
  free O2. once the free Fe finished reacting,
  oxygen began to accumulate. This happened about
  2.5 BYA. We know this because of fossils of
  ocean organisms that require oxygen to live.

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• The most common Precambrian fossils are
  stromatolites.
• These are algae colonies that layered calcium
  carbonate and grew in the uppermost layer. There
  are organisms living today that are similar to
  stromatolites.
• Most Precambrian organisms preserved in rock are
  microscopic. These organisms are similar to
  bacteria existing today.

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• Precambrian plants date back to about 3.5 BYA.
  Precambrian animals date back to the late
  Precambrian, about 2 BYA.
• Most animal fossils are trace fossils, like
  trails and worm holes.

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• Modern, living stromatolites

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• The Paleozoic
• This era comprises the most recent 540 million
  years.
• The Paleozoic is the longest Era after the
  Precambrian.
• Earth began to be home to many animals with hard
  body parts. Therefore there are lots of fossils
  dating from the Paleozoic.

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Before Pangaea

• The continents were arranged as two major
  continents with Gondwanna (Africa, South America,
  Antarctica) around the South Pole, and North
  America, Europe and what is now Siberia clustered
  together, forming Laurasia, somewhere at the
  equator. There were no living things on
  N.America during this time. An orogeny occurred,
  which eventually produced the Appalachian
  Mountains.

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• Trilobites lived during the Cambrian period.
• There were more than 600 types of these animals.
• During the Ordovician period, brachiopods were
  the dominant animals.
• Brachiopods are now extinct except for one group.
• Cephalopods began to appear during the
  Ordovician.
• These included squid and octopus ancestors

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• Cephalopods

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• Late Paleozoic
• This is when Ancestral North America collided
  with Africa to form Pangaea.
• The Appalachian Mountains were the result.
• Plants adapted to survive at the waters edge,
  then to move farther from the water.
• Early plants had no leaves.

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• By the late Devonian, fish had developed lungs to
  assist their breathing with gills.
• There were lobe finned fish that lived on mud
  flats.
• By the end of the Devonian there were true
  amphibians that existed.
• The amphibians quickly diversified because they
  had no competition from other land animals.
• Tropical swamps existed in the late Pennsylvanian
  which created our coal deposits.

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• The Great Paleozoic Extinction
• The creation of Pangaea, the distribution of land
  and water had an effect on the world climate.
• Vast areas of the northern continents became
  elevated above sea level
• Vast areas became drier
• 75 of land amphibians disappeared

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• Much of the marine life did not adapt and survive
  with 80-95 disappearing. This was the mother
  of all extinctions. The cause is uncertain. We
  know at least 2 million cubic km of lava flowed
  across Siberia. Sunlight could have been
  blocked, high concentrations of S gas forming
  H2SO4 could have poisoned life on land and in the
  sea. Life almost died out altogether.

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• Trilobites became extinct
• Most brachiopods
• Some corals
• This set the stage for the survivors forming new
  biological communities that were more diverse
  than their predecessors.

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• The Mesozoic
• Lasted 183 million years. Dicvided into three
  periods, Triassic, Jurassic and Cretaceous.
• Marked the beginning of the breakup of Pangaea
• Dinosaurs became dominant on land.
• They were the major animal group for 100 million
  years.
• Very few marine fossils found from the Triassic.
• Shallow seas covered much of western North
  Amerida. The shallow seas created swamps which
  produced our coal deposits.

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• The breakup of Pangaea
• A rift developed between eastern US and west
  coast of Africa.
• This developed the Atlantic Ocean
• New animals included reptiles with a shelled egg.
• Water dwelling stage (like tadpole) was
  eliminated.

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• Some dinosaurs were carnivores, some herbivores
• One group of reptiles began to fly (pterosaurs)
• One group became very proficient (we have fossil
  remains of feathered Archaeopteryx) led to the
  evolution of birds.
• Another group became sea dwellers (plesiosaurs
  and ichthyosaurs)
• At the end of the Mesozoic there was another
  extinction event. The dinosaurs and the marine
  pleiosaurs and the pterosaurs died out.

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• Scientists believe that this was caused by a
  large meteorite colliding with Earth which caused
  huge quantities of dust to block out the sun.
• Plants died.
• Herbivores died.
• Carnivores that ate the herbivores died.
• Smaller animals probably survived because they
  needed less food than huge dinosaurs.

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• The Cenozoic
• This means era of recent life
• Dominant animals were the mammals.
• Dominant plants were flowering plants.
  Angiosperms replaced gymnosperms

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• The evolution of flowering plants strongly
  influenced the development of birds and mammals.
• Birds fed on seeds and fruits
• Grasses developed during the middle Tertiary
  which allowed the success of Grazing animals that
  fed on the grasses.

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• Mammals had developed about the same time that
  dinosaurs emerged but they were small and
  primitive.
• By the end of the Mesozoic, dinosaurs and other
  reptiles were gone.
• The fossil record shows the replacement of the
  reptiles by mammals.

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• Mammals are different from reptiles.
• They are born alive (except platypus and spiny
  anteater babies who hatch from eggs. These are
  very primitive mammals called monotremes)
• They are warm blooded and can survive in cold
  regions. Hair insulates their bodies.
• They can search for food anytime or any season
• Their hearts and lungs are better developed.

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• Because there were many open niches, mammals
  developed rapidly.
• They increased in size
• Brain size increased
• Teeth specialized to accommodate a particular
  diet
• Limbs specialized to adapt to life in a
  particular environment.

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• Many large mammals became extinct at the end of
  the Pleistocene.
• These included Mastodon, mammoth, giant beavers,
  saber toothed cats, large ground sloths, some
  horses, camels, giant bison.
• The reason is not clear, these groups had
  survived several major glacial advances and
  interglacial periods.
• One hypothesis is that humans hunted these
  mammals, selecting large forms. This is not
  accepted by all scientists.

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