Chapter 2: Geological

Time

Fossils

Fossils provide a window into the past as they provide evidence about the changes that have occurred to the Earth over the 4.5 billion years of existence.

A fossil is the preserved evidence in rocs or soils of organisms that once existed on Earth.

Fossils may be the whole body, part of the body or traces of its activities.

Fossils. To be preserved as a fossil the

organism must decay very slowly. Occurs when the organism is covered in

sediment, which then turns to rock. A palaeontology is the study of past life.

Formation of Fossils Animals that are only

composed of soft parts such as jellyfish, slugs etc. are rarely found as fossils.

Hard objects such as bone, tree bark, shells and teeth are most likely found as fossils.

The following process is how fossils occur in sedimentary rock.

Fossil Formation Fossils are found

only in sedimentary rock not in igneous or metamorphic rock as the heat and pressure used in formation destroys the organism.

Types of Fossils There are different types of fossils due

to the different ways in which they are formed.

Types of fossils:- Original fossils- Replacement fossils - Carbon Film fossils- Indirect fossils

Original Fossils Occur when a part of the organism is preserved and its

composition is almost the same as when it was alive. Most commonly skeletons, bones, teeth, shells; this is

because they are harder to break down and scavengers find them difficult to eat.

Common original fossils include: - Sea creatures that had shells, such as mussles and oysters- Verterbrates, because they had teeth and a bony skeleton, such as megafauna – the three meter tall giant kangaroo, sheep sized echidnas, 3 metre long wombats.

Original Fossils

Replacement Fossils A replacement fossil occurs when a

part of the organism is chemically changed into another mineral.

Commonly occurs when calcium carbonate, from bones and shells, turns into another mineral such as silica.

Carbon film fossils

A carbon film fossil occurs when a dead body partially decays and leaves behind a thin black deposit of carbon.

The traces of carbon left are often in the shape of the organism that decayed and often show fine details.

Indirect fossils An indirect fossil is

not part of the organism but is the preserved remains, such as imprints of the body like footprints, fossilised dung and burrows.

Dung and Burrows are sometimes called trace fossils.

Preserving environments The environment in which the organism

dies can influence the fossil produced. These preserving environments include:

- Permafrost- Amber- Tar- Peat - Dry air

Permafrost Near the artic circle

the land is permanently frozen.

Provides the perfect environment for fossilisation as the decaying process is stopped due to the temperature being below freezing.

This environment is best for original fossils.

Amber Amber is a solid

plant sap or gum Insects, spiders or

small vertebrates such as lizards and frogs get stuck in the sticky sap which seeps out of trees

When it sets it perfectly preserves whatever has become stuck inside it.

Tar Tar pits occur when oil seeps naturally from

the ground into the surface. This preservation is rare, but when it is

achieved, provides fantastic preservation.

Peat Peat is the partly

decomposed remains of plants and is commonly found in swamps and bogs.

The peat contains acids making the hard tissue, such as bone, very soft.

In Europe ancient human bodies have been found preserved in peat.

Dry Air Extremely dry

conditions, reduce the number of bacteria that are able to decay the organism therefore; Dehydrates the soft tissue which fossilises and turns it into a “mummy” (muffication).

Fossil dating techniques Palaeontologists use several techniques

to decide the age of a fossil. This is known as dating a fossil Relative dating is a technique that

compares the age of a fossil or rock with others to determine which is older.

Relative dating relies on two factors: Sedimentary rock layers Fossils are the same age as the rocks they

are found in.

Layers of sedimentary rock Sedimentary rock forms in layers

called stratum So the first layer (base) is the oldest and

the top is the youngest.

Index Fossils Fossil records and rock strata indicate

organisms only existed on Earth for a short period of time.

Each fossil is only found in a narrow band of the rock strata in one location.

The fossils only lived at a certain time in the past, so the rocks containing the same fossils, must be the same age.

Fluorine analysis Fluorine analysis compares the

amount of fluorine in different bones found in the same rock.

Bones absorb the flourine from water over long periods of time

This happens slowly therefore the more fluorine the older the bone.

Radioactive dating Radioactive dating is a method that

uses the natural rate of decay of radioactive isotopes.

The amount of decay allows scientists to estimate how long, before discovery, the rock was laid down.

Tree rings A useful method

for dating wood. This process

involves counting the growth rings in the trunk of the tree.

Many trees grow by adding a woody layer around the stem.

1 ring = 1 year

Geological Time Scale Paleontologists have constructed a time

scale showing the sequence in which the different fossils species are found on the strata around the world.

This time scale of the past is called the geological time scale.

Geological Time Song http://

www.youtube.com/watch?v=7PQURsc2SYs

Creating a geological time scale

Students are to create a geological time scale that shows the history of animal to human habitation of Earth.

This A3 time scale must include: The four era’s from beginning to end The organisms identified through fossils in these

time periods Pictures and explinations of events and specific

discoveries. Use Chapter 2.3 pg 56 - 60 to assist you