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Chapter 13 Earth Science 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

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chapter 13

Chapter 13

Earth Science

the precambrian
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.
Most Precambrian rock is buried deep under younger strata.
  • Large continental cores composed of Precambrian rock exist on every continent.
    • These are called shields
the evolution of earth s atmosphere
The evolution of Earth’s atmosphere
  • The evolution of Earth’s atmosphere
    • The atmosphere we breathe today consists of N, O2 and a small amount of Ar and CO2 and H2O.
    • Earth’s original atmosphere was similar to the gases released in volcanic eruptions, CO2, H2O, N and other trace gases but NO O2.
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.
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.
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.
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.
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.
before pangaea
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.
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
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 water’s edge, then to move farther from the water.
  • Early plants had no leaves.
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.
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
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.
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.

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