Relative dating of fossils

• Relative Dating


  Technique that compares age of one fossil or rock with another to determine which is older
• Absolute Dating


  Process of determining how old rock is by looking at radioactive material in the rocks and how it has decayed
• Law of Fossil Succession

  • Sedimentary rock is formed by accumulation of sedimentary layers on top of each other, so fossils closer to surface must be younger than those found below them
  • Fossils are same age as rocks in which they are found
• Sometimes the Law of Fossil Succession is not true because layers of rock have been turned upside down by movement of Earth's crust
• Strata (Stratum for single layer)


  Layers in which sedimentary rock form
• Index Fossils


  • Most species only existed on Earth for a relatively short time
  • Each fossil species is only found in narrow band of rock strata in any location
  • All rocks containing a particular species of fossil should be same age as each other even if places long way apart
• Index Fossils


  Fossils of a known age which are used to easily determine age of unknown fossils by comparing ages of strata in different locations
• Index Fossils


  • Have been fairly widespread
  • Have lived in a fairly narrow period of time
  • Have been abundant
  • Be easy to identify
• Index Fossils


  • Ammonites (extinct group of molluscs)
  • Trilobites (extinct group of arthropods with external skeleton and jointed limbs, same phylum as current day insects, crabs and spiders)
• Known ages of different types of Trilobites that existed in narrow periods of time, so they can be used as Index fossils
• Stratigraphy


  Relative dating technique of comparing strata in different locations to determine their relative ages
• Fossil is labelled by the locality letter and stratum number that it is found in (e.g. A1, C1)
• Strata with same types of fossils are same age
• Stratigraphy: Try it yourself

  1. Compare different types of fossils found in different localities to determine oldest and youngest strata overall

  2. Oldest stratum will be at the bottom, youngest will be at the top