Fossils & Dating Flashcards
Preservation of Soft Tissue
Only occurs if breakdown of soft tissues is delayed long enough for fossilisation to occur
Occurs when:
Dead organ is mummified (frozen or dried)
Organism is rapidly entombed by a substance that excludes oxygen (resin fine sediment or tar)
Preserved by some other method
Fossilisation steps
More commonly found in fine grained sedimentary rocks eg Linestone, Shale or siltstone
Dead organism is rapidly buried (protection from scavengers and seals off oxygen slowing decay bacteria)
In such circumstances both hard and soft tissue can be preserved
Factors Favouring Fossilisation
Hard body parts
Low energy aquatic environments
Rapid burial
Burial in mud or other fine sediment which excludes oxygen
Alkaline conditions as acidic conditions can dissolve calcium salts in bone and she’ll
Relative Dating: Superposition
sedimentary rocks are laid out in strata (beds or layers)
Observing order of these layers can be use to established which is older or newer
Older beds are usually the lowest beds unless disturbed
Relative Dating: Comparative Stratigraphy
Compare rock types in different areas
Different types of rock form under different circumstances. Because conditions in area change overtime so does the order of rocks
If rocks sequence in different areas are similar = former under similar conditions = same age
Relative Dating: Index Fossils
Fossils with a short and distinctive appearance (confined to a narrow band of rock)
These fossils at particular stratigraphic horizon= snapshot of evolution of that point in time
Useful for correlating specific horizons within larger formations
Relative Dating: fluoride dating
Measure of amount of fluoride taken up by bones.
Bones absorb fluorine from ground water
2 bones:
fluorine same= same place and age
Fluorine different= different location and age
Absolute Dating: Radiocarbon
Radioisotope of carbon-14
(Formed by cosmic radiation and nitrogen)
Is absorbed by animals and humans - contain a constant amount of c-12,13,14.
Animal dies
C14 is unstable and decays into N14 giving off beta radiation
Large amount = young sample
Small amount = old sample
Half life of c14 is 5730 years
By measuring residual c14 left in sample Age can be determined
Radioisotopes
Variation in neuroma affects weight of element.
Some unstable isotopes decay to more stable element
When it decays it gives off radiation
Each radioisotope has its own rate of decay (half life)
Is this Half life is known, the age of sample can be determined by measuring the radioisotope or amount of daughter product.
Absolute Dating: Dendrochronology
Ageing Timber
New ring of wood each year
Count number of rings=age
Trees same age in same area= similar growth conditions therefore matching rings of growth (one year could of been a lot of growth due to environmental conditions)
Fossils
Preserved remains of once-living organisms
Only occurs if suitable conditions exist
When it does occur typically only hard tissues (shells bones) that are preserved.
