NAME:
Fossils: Evidence For Evolution
Much of the evidence for the way that life on Earth has changed over time comes from the fossil
record.
Fossils are the preserved remains, impressions or traces of past life
on Earth.
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Watch the BRAINPOP "Fossils" and record some notes in the
space below.
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Formation of Fossils
When most organisms die, scavengers (eg some birds, ants, worms) and decomposers (such as
bacteria or fungi) feed on the dead material. After a relatively short period of time, very little of the
organism remains, apart from things such as bones, teeth or shell. Therefore, there is only a very
small chance that a dead organism can become a fossil.
To form a typical fossil, the following steps must occur:
1. The dead organism must be covered quickly with sediment such as mud, silt or sand. This
slows down or stops the action of scavengers and decomposers. This explains why organisms
living in or near water are more likely to become fossils.
2. The top layers of sediment are compressed by the weight of new sediment and water on top of
them. Gradually this pressure turns the sediment into sedimentary rock, trapping the fossil
inside their layers.
These steps are shown below:
Petrification of Fossils
The chemicals making up the bones, shells and other hard parts of an organism are often replaced
by minerals in the surrounding rock. This process takes millions of years and is called petrification.
Types of Fossils
Mould and Cast Fossils
The remains of an organism trapped in sediment decompose slowly over time. This may leave a
cavity inside the rock called a mould. In certain conditions, minerals from the surrounding rock
seep into the mould and refill it, harden and form a cast. An impression can also form a cast. For
example, a dinosaur may form a footprint in sand, which dries out and hardens in the sun. It is then
filled up with sand or mud, and under pressure over time, this hardens to form a cast of the print.
Carbonised Fossils
The action of extreme heat and pressure on dead organisms can result in chemical changes that
cause all of the materials in an organism to convert to gases, leaving only carbon remaining. This
produces a carbonised fossil, which is usually black.
Unaltered fossils
Sometimes an organism's remains may be preserved entirely by the environment in which it dies.
Insects may be trapped and die in amber, preserving their bodies entirely. Animals may fall into tar
pits or may die inside deep, dry caves and become 'mummified', because decomposers cannot live
in such environments.
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View some fossil samples or use the Internet to find pictures of different types of fossils.
Dating Fossils and Rocks
Scientists use two methods to date rock layers and the fossils they contain.
1.
Relative dating
Palaeontologists apply the following rules to infer the relative age of fossils (relative = in
comparison to other things) from the rock layers in which they occur.
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Principle of superposition = rock layers (or strata) at the bottom are older than
those at the top (providing no faults have occurred).
Principle of Cross-Cutting Relations: any geological feature is younger than
anything else it cuts across
Read and complete the activity over the page.
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Complete the following Relative Dating Questions.
1.
The following diagram represents rock core samples obtained by geologists exploring an
area within a sedimentary basin.
Which core sample contains the youngest rock?
(A) I
(B)
II
(C) III
(D)
IV
2.
(a)
What is the youngest rock? Give the column letter AND the rock name.
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(b)
List the 5 oldest rocks (rock type and column letter) in order of increasing age.
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(c)
Which fossil is useless for correlation purposes?
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2.
Absolute dating (or radiometric dating)
Radioactive materials are present in rocks and fossils.
Absolute dating uses the principle of radioactive decay
to determine the actual age of a rock or fossil. This
method relies on the concept of half-life. All
radioisotopes (forms of elements which are radioactive)
decay (forming other atoms) by a rate determined by
their half-life. The half-life of a radioisotope is the time it
takes for one half of its atoms to decay into different
atoms).
For example, fossils contain small levels of the
radioisotope carbon-14. When an organism dies, it stops
taking in any carbon (from photosynthesis) and so the
carbon-14 it contains decays and its levels slowly
decrease. Its half-life is around 5700 years. That is, ½ (50%) of the carbon-14 level is
present after 1 half-life of 5700 years. After another half-life (ie
in total 11,400 years), only ¼ (25%) of the carbon-14 will be
present.
Therefore, when scientists find a fossil, they can analyse its
carbon-14 content, and from this determine the age of the fossil.
For example, if its carbon-14 content was 1/8th the level that it
would be if it was living, this represents 3 half-lives (ie 3 x 5700
= 17,100 years old).
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Watch the BRAINPOP “Carbon Dating"
Other forms of radioactive dating can be used to measure the
absolute age of rocks, depending on how old the rock sample is
estimated to be. For example, rocks which are thought to be very
old would be dated using uranium dating, because uranium has a
much longer half-life (of 4500 million years) than carbon-14.
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Complete the following questions.
1.
Calculate the age of a fossil which contained 6.25% of the
normal carbon-14 levels present in living material. Take the
half-life of carbon-14 as 5730 years.
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2.
What are some of the positive and negative aspects of using
absolute dating of fossils instead of relative dating?
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