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  Rsc2SYs Rsc2SYs

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 to assist you