Fossils

Question 1

What are fossils

Answer 1

• Any preserved trace left by a previously living organism
• May include footprints, burrows, faeces or impressions of all or part of an animal or plant, as well as bones, teeth or shells

Question 2

Fossilisation process

Answer 2

1. Death
2. Rapid burial
3. Sedimentation
4. Mineralisation/petrification

Question 3

Death

Answer 3

Organism dies

Question 4

Rapid burial and sedimentation

Answer 4

Organism quickly covered by sediments and conditions are favourable (alkaline, no air)

Question 5

Mineralisation

Answer 5

Minerals replace organic matter in bone, and bones become fossils.

Question 6

Fossilisation requirements

Answer 6

1. A quick burial of the material by sediment
2. The presence of hard body parts (bone and teeth)
3. Absence of decay organisms
4. Anaerobic conditions - no decay
5. Alkaline soils - wet, acid soils dissolve and destroy bones
6. A long period of stability - organism needs to be left undisturbed

Question 7

Location of fossils

Answer 7

1. Edges of ancient lakes and river systems
2. Caves
3. Volcanically active areas

Question 7

Why are these locations

Answer 7

• Rapid burial by sediments - prevents decomposition
• Limestone caves (calcium carbonate in limestone helps preserve hard parts like bones and teeth.)

Question 8

Discovery of fossils

Answer 8

• Some by chance and/or
• An excavation or dig site is where fossils uncovered at ground level

Question 9

Artefact

Answer 9

An object deliberately made by humans. They include stone tools, beads, carvings, charcoal from cooking fires and cave paintings.

Question 10

Dating a fossil

Answer 10

Once a fossil is discovered it is important to be able to place the fossil in the history of life. This is called dating the fossil.

Question 11

Methods of dating fossils

Answer 11

1. Relative dating
2. Absolute dating

Question 12

Relative dating

Answer 12

Provides an approximate age of a sample based on comparisons to fossils of a known age

Question 13

Absolute dating

Answer 13

Methods provide an actual and quantitative age of the fossil.

Question 14

Methods of relative dating

Answer 14

1. Stratigraphy
2. Fossilised pollen grains
3. Fluorine dating

Question 15

Methods of absolute dating

Answer 15

1. Potassium argon dating
2. Radiocarbon dating
3. Tree ring dating

Question 16

Stratigraphy

Answer 16

The study of rock layers (strata) and the sequence of events they reflect.

Question 17

Principal of superposition

Answer 17

Undisturbed sequence of sedimentary rock layers, the oldest layers are at the bottom, and the youngest layers are at the top.

Question 18

Index fossils

Answer 18

Widely distributed fossils that were only present on Earth for a limited period of time. Used to correlate strata from different regions

Question 19

Fossilised pollen grains

Answer 19

• Used as index fossils
• Enables botanists to analayse the type and amount of vegetation and the climactic conditions in an area at the time the deposit was laid down.

Question 20

Fluorine dating

Answer 20

• The more fluoride ions in a fossil, the older it is
• Fluoride ions in water replace ions in bone
• Can only be used to compare fossils found in the same area.

Question 21

Isotopes

Answer 21

An element with a different number of neutrons. Radioactive, and unstable element that emits radiation.

Question 22

Half-Life

Answer 22

The time taken for half of a radioactive material to decay into a stable form.

Question 23

Potassium argon dating

Answer 23

• Potassium-40 is a radioactive isotope that breaks down over time to form Argon-40 at a slow/constant rate
• By comparing the ratio of K-40 to Ar-40 the age of the rock sample can be calculated
• Can only be used on rocks older than 100,000 – 200,000 years
• The technique works well for almost any igneous or volcanic rock, but not on sedimentary rocks
• Rocks must be same age as fossil
• 1.25-billion-year half-life

Question 24

Radiocarbon dating

Answer 24

ii) Radiocarbon dating - Carbon-14 is a radioactive isotope that breaks down to form Nitrogen-14 - The less C-14 the older the sample is - C-14 has a half-life of 5730 years - Can only be used on samples <70 000 years old and the sample must be organic

Question 25

Limitations of radiocarbon dating

Answer 25

- If remains older than 60/70 000 years, no measurable C14 left - Must be organic/contain organic matter - C14 to C12 has not been constant over past 60 000 years - Assumes that C14 in animals and plants matches that in the environment - Requires a minimum amount of substance.

Question 26

Accelerator Mass Spectrometry

Answer 26

- Normal carbon dating requires 3g of organic material - Accelerator mass spectrometry, radiocarbon dating requires 100 micrograms - Breaks sample into constituent atoms which are counted - Uses: Cave paintings from pigment samples (contain charcoal, honey, milk, oil, seed for binding pigments)

Question 27

Tree ring dating (dendrochronology)

Answer 27

The number of rings accounts for age of tree. Width of growth rings shows growth rate and gives an indication of environmental conditions at different times in the pasts.

Question 28

Dating problems

Answer 28

- Conflicting dates based on different technologies - Limitations of carbon dating - The paucity of the fossil record - Different interpretations of the same evidence - Rare for ideal fossilisation conditions to be perfect - Chance fossil forms - Undiscovered fossils - Partial skeletons or fragments

Question 29

How is carbon 14 used for dating?

Answer 29

i. Absorption of C14 by plants (CO2 – photosynthesis) ii. C14 passed on in the food chain iii. Measure amount of C14 (radiocarbon) in remains iv. C14 decays into N14 v. Establish ratio of C14 to C12 vi. C14 has a half-life of 5730 vii. Amount of C14 in the remains indicates the number of half-lives that have passed viii. Multiply the number of half-lives by 5730 ix. Gives an absolute age