Fossil Evidence and Dating Flashcards
1
Q
What are fossils?
A
- Fossils are crucial pieces of evolutionary evidence as they show the gradual changes in the characteristics of organisms over time
- Fossil = any preserved trace left by an organism that once lived
- Footprints - trace fossils
- Burrows - trace fossils
- Faeces - chemical fossils
- Impressions of parts of animal or plants
- Bones, shells or teeth
2
Q
Fossil Formation
A
- In order to become a fossil, hard structures need to be turned into rocks = petrified
How?
- minerals such as lime or iron oxide are deposited into bones and replace the organic matter and the bones original minerals
4 Steps to becoming a fossil:
- Choose an appropriate place to die
- Die there
- Get buried quickly
- Get discovered!
3
Q
Fossil Formation - Step 1 = Choosing an appropiate place to die
A
- The nature of the soil is very important for fossiliation of bone (turning bone into rock)
- The pH and moisture levels need to be considered so that the minerals in bones are not dissolved
- drier more alkaline soils work best to preserve minerals in bone
- in soils with low levels of oxygen (e.g. peat) complete preservation of soft tissues and bones may occur
- high chance of rapid burial to avoid decay by micro-organisms. (near rivers, lakes, volcano, limestone caves consisting of calcium carbonate)
4
Q
Fossil Formation - Step 3 = Get buried quickly
A
- There are several ways that a dead organism can be buried including: silt/sediment from rivers, lakes and ocean, cave collapses, falling volcanic ash (as long as its not too hot otherwise the bone will disintegrate)
- It is important that it happens quickly to:
- to prevent the organisms remains being eaten by other animals
- to slow down decay as it minimises oxygen availabel for decomposers
5
Q
Fossil Formation - Step 4 = Get discovered many years later
A
- Many fossils are deep within the Earth’s crust
- to be found these fossils need to be brough to the surface
- this can be achieved by erosion, or movements of the Earth’s crust
6
Q
Dating of Fossils
A
- Dating = determines the age of the material excavated e.g. fossil or artefact (objects made by humans)
- The age is usually given in years before present e.g. 45,000 BP
- Allow scientists to sequence events
- There are various methods of dating fossils which provide:
- Absolute dating = actual age of the specimen in years
- Relative dating = tells us if one sample if older or younger than another
7
Q
Relative Dating
A
- Used to determine the age of a fossil in comparison to other samples
- Such dating enables a sequence of events to be established
- Stratigraphy - study of layers, or strata
- Fluorine dating - studying the amount of fluorine ions in a fossil
8
Q
Stratigraphy
A
- It is the study of layers. It works on the Principle of Superposition
- In sedimentary rock it assumes the layers on top are younger than the layers underneath
- Problems arise with distortions in the earth’s crust which can disturb layers, or the burial of samples after deposition of soil, making it younger than the layers above it
9
Q
Stratigraphy - Index fossils
A
- Stratigraphy also considers the correlation of rock strata. It involves matching layers of rock from different areas.
- Uses rock characteristics and index fossils to match the age of the strata
- Index fossils are animals/plants/pollen grains that when alive were distibuted widely over a short period of time
- Can determine the order in which the rocks were formed across different areas and thus determine which fossils are older/younger (despite how close to the surface they are)
10
Q
Stratigraphy - Index fossil example (pollen grains)
A
- Preserved pollen grains in rock samples can enable botanists to determine the type and amount of vegetation existing in the area at the point of deposition
- this enables an idea of the climactic conditions of the time
- this data can be used to confirm or refute relative dates arrived by other methods
11
Q
Fluorine Dating
A
- Compares the accumulation of fluorine in bones from the same site
- When bones are left in the soil, the fluoride ions in the water in the soil slowly replace some of the ions in the bone
- the longerthe bone is in the soil, the more fluoride it will contain
- Two bones deposited at the same time in the same soil should contain the same amount of fluoride. If they don’t, the older bone will contain more fluoride.
12
Q
Fluorine dating 2 - Piltdown skull
A
- The absolute age of a fossil can’t be determined using this method as the concentration of fluoride varies between different locations
- For this reason, you can’t compare fossils across different locations
Famous Example: The Piltdown skull
- Bone fragments were presented as fossils from a previously unknown human
- Was set as a missing link in human evolution
- Ended up being a hoax (lasted 41 years)
- It was an old human skull with an orangutan jaw
- This was determined as the fluoride levels didn’t match
13
Q
Absolute Dating
A
- Used to determine the actual age of a fossil in years
- Radiocarbon dating - based on the radioactive decay of carbon isotope carbon 14
- Potassium-Argon dating - based on the radioactive decay of potassium
- Dendrochronology (Tree ring dating) - observing the concentric rings in trees
14
Q
Radiocarbon Dating
A
- This method is based on the decay of the radioactive isotope carbon-14 into nitrogen
How does C-14 get into an organism?
- The atmosphere has one C-14 molecule for every million million C-12 (trillion) molecules
- Ratio 1 : 1 000 000 000 000
- Plants use carbon when photosynthesising so incorporate some C-14 atoms in their cells
- When an animal eats the plant its C-14 is incorporated into its body
- When an organism dies it’s C-14 decays into nitrogen at a fixed rate
15
Q
Radiocarbon Dating - How do we estimate an age from the carbon decay
A
- The carbon-14 decays at a fixed rate. It has a half life of 5730 years.
- This means that in 5730 years, half of the original C-14 quantity will have decayed into nitrogen
- E.g. starting ratio C-14 to C-12 was 1 : trillion. After 5730 years it will be 0.5 : trillion. After another 5730 years it would be 0.25 : trillion.
- The carbon 12 quantity doesn’t decay and so it stays the same.
- We can then compare the quantity of C-14 to the C-12
- If we had a starting quantity of 6 x 10^12 C-12 atoms then the organism would’ve had 6 C-14 atoms present at death.
- If there are only 3 C-14 atoms present in the sample then we know the fossil must be 5730 years old.
- This technique requires minimum 3g of organic material
