fossils - COMPLETE

• Dating fossils:

  • absolute dates - actual age of specimen in years
  • relative dates - comparison of fossils to determine which sample is older
• Potassium-argon dating:

  (technique is based on the decay of radioactive potassium to form calcium and argon)

  • **potassium-40 (radioactive isotope) which decays to form calcium-40 and argon-40 which occurs at a constant rate
  • determining the amounts of potassium-40 and argon-40 enables the rock’s age to be calculated
  • as rock ages, potassium-40 decreases while argon-40 increases
  • can only date rocks older than 100k to 200k years
  • not all rock types are suitable for this method
  • half-life of potassium-40 is 1250 billion years
• Carbon-14 dating: (radiocarbon dating)

  (method is based on the decay of the carbon-14 to nitrogen)

  • carbon-14 incorporated in plant tissues → becomes part of animal’s tissue if eaten
  • carbon-14 decays at a fixed rate
  • materials over 60k years old cannot be dated (carbon-14 is negligible)
  • material must contain organic compounds (from living things containing carbon)
  • carbon-14 half-life is 5730 ± 40 years
• Stratisgraphy:

  • principle of superposition - assumption that sedimentary rock layers on top are younger than those below
  • must be applied with care as distortions of the Earth’s crust do occur and sequence of rock layers may be turned upside down
  • possible for fossils to be buried by animals after deposition of sediment
  • correlation of rock strata - matching layers of rock from different areas
  • index fossils are widely distributed and were present on Earth for a limited time → makes dating more precise
• Problems with the fossil record:

  • fossilisation is a chance occurence → several gaps in the fossil records
  • only a very small proportion of fossils existing have been discovered
• Fossil:

  • preseved trace left by an orgranism
  • bones, shells and teeth are most common
  • can build up sequence of evolutionary change for a particular plant or animal
• Effect of soil type on fossilisation:

  • wet, acific soils → bones in minerals dissolves → no fossilisation
  • oxygen-less soils → complete preservation of soft tissues and bones may occur
  • alkaline soils → minerals are not dissolved
• Location of fossils:

  • often found at edges of ancient lakes and river systems, caves or volcanically active areas
  • sediment built up when flooding occurs or when water flow slows rapidly
  • caves consists of calcium carbonate which can be deposited around dead organisms
• Conditions for fossilisation to occur:

  • quick burial of material
  • presence of hard body parts
  • absence of decay organisms
  • long period of stability