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9.3 Environments through time: 3. The Cambrian event
Extract from Earth and Environmental Science Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
Prior Learning: Preliminary Module 8.2 (subsection 3) Stage 5 (subsection
Science Stages 4-5 syllabus: Outcome 5.9 (content: 5.9.4 - natural
Background: The Principle of Uniformitarianism, proposed by James Hutton
(1726-97) and the Principle of Superposition, proposed by Nicolas Steno
(1638-86), along with the principles of inclusions, cross-cutting relationships
and fossil succession allow geologists to date rock sequences. Investigating
fossil assemblages in stratigraphic sequences of rocks allows the development
of a picture of the environment of a fossil organism and an idea of the dangers
faced by the fossil organism.
use available evidence,
including computer simulations, models and photographs to examine the changes
in life forms that occurred during what is commonly referred to as the 'Cambrian
To examine the changes in life forms that occurred during Cambrian event you will need to
refer to a diverse range of resources.
The following web sites can provide background to assist your understanding
of the 'Cambrian event'.
Cambrian explosion Earth Sciences, University of Bristol, UK
You need to gather evidence from at least one computer simulation. Following is a computer simulation done in a lab in the UK.
Robot models of trace fossils Tony, J. Prescott & Carl Ibbotson, Department of Psychology, University of Sheffield, UK.
- Models could be reconstructions of actual fossils or reconstructions
to show what scientists believe Cambrian life forms may have looked like.
Some science equipment suppliers or museum shops may have such models available
for sale. Some museums have on display models depicting the "tree of
life", which you may be able to examine for the Cambrian period.
- Photographs are abundant on the Internet of fossils and animals and
plant reconstructions for the period. Search using the following fossils common
during the Cambrian: brachipods, echinoderms, molluscs, nautiloids, trilobites.
The following sites are worthy of review.
photographs of Cambrian fossils and reconstructions
- From the available evidence, develop a coherent summary of the changes
in life forms that occurred during the 'Cambrian event'.
possible importance of the development of hardened body parts in explaining the apparent
explosion of life in the Cambrian period
- Hard shells provided support for soft-bodied organisms and allowed a degree of
protection from environmental influences and predators.
- Skeletons allowed the various animal groups to adopt new ways of life and to spread into
previously unoccupied areas on the seafloor.
- The hard shells or armour developed by organisms of the early Cambrian period are easily
fossilised, as moulds, casts or altered by chemical means.
- The fossil record increases in the number and type of fossils found due to the increase
in organisms with hard parts, so an apparent explosion of life seems to take place in the
advantages that hard shells and armouring would have given these life-forms in comparison
with the soft-bodied Ediacara metazoans of the late Proterozoic, in terms of predation,
protection and defence
- Hard shells provide protection. As the organisms were no longer confined to natural
shelter, such as rock crevices, they could move over a wider area of their environment in
search of food.
- They would also have greater manoeuvrability in marine environments, either by using
their exoskeletons to glide in currents or by using limbs for locomotion against currents.
This would allow them to predate on many free-swimming Ediacara metazoans.
- Shells afford an organism protection from predators. They can retreat into the shell as
a predator approaches.
- Shells provide protection from environmental influences such as the pounding by sand and
rock particles in a marine environment or from drying in the event of being stranded out
of water during low tides.
- Armour deters predators. Hard parts afford an extra protective layer from injury by
Armour plated Animals BBC Science and Nature, UK
examine at least one example of a stratigraphic sequence and describe any
fossils found in
- The type of data that you need to collect for this syllabus requirement
is a stratigraphic sequence and notes to indicate the fossils present in
each part of the
sequence. For an example of a stratigraphic sequence, refer to Stratigraphic Column , The Paleontological Research
- If you are able to study a stratigraphic sequence first-hand, you should:
- Draw a scale diagram of the stratigraphic sequence.
- Indicate the location of any fossils in the sequence.
- Identify the fossils and draw or photograph them.
- Include the diagrams or photos alongside the diagram of the stratigraphic sequence.
- If you need to study a stratigraphic sequence using secondary data, consider the
Localities of the Vendian: Ediacara Hills, Australia from
the Museum of Palaeontology, University of California, Berkeley USA.
A Mini-Lesson A peek
at the past; Fossil patterns: gradualism vs punctuated equilibria, Original by
William F. McComas and Brian J. Alters; modifications by Larry Flammer, Evolution &
the Nature of Science Institutes, Indiana University, USA. This lesson is made up of two
sets of simulated fossils (caminalcules), provided as cut-outs. Students arrange them on
two time scales. One set produces a visual example of "gradualism", the other
shows "punctuated equilibria".
relative age of a fossil from a stratigraphic sequence
- Sedimentary strata are laid down horizontally one on top of the other, to form a
stratigraphic sequence. The Law of Superposition states that, in a stratigraphic sequence
that has not been overturned, the oldest layer is at the bottom and the youngest is at the
top. Because the organisms that become the fossils are trapped in strata as they form, the
age of the fossil matches the age of the strata. Therefore, fossils higher in the sequence
are younger than those in lower strata.
between relative and absolute dating
- Relative dating refers to the process of determining the age of a rock, fossil or
event with respect to the standard geological time scale. There is no reference to actual
ages, say in years. Such dating relies on comparison with other the rock layers, fossils
or events such as intrusions, folding and faulting.
- Radiometric dating is based on the decay of naturally occurring radiogenic
isotopes in rocks to determine the age of rocks in years. The age of a sample can be
calculated from the measured proportions of parent to daughter isotopes. Radiometric
dating is a form of absolute dating where actual times are determined.
dating Earth Science Australia
uses of relative and
absolute dating methods in determining sequences in the evolution of life forms
- When we examine a
geological column showing the different life forms over geological time we can note the
evolutionary changes in the animals and plants described and how they develop more complex
structures as we move up the column.
- According to stratigraphic
rules the oldest layers are at the bottom and the youngest are at the top. This means that
more complex structures belong to the younger organisms found in the upper layers.
- Within a sequence of rocks
some igneous rocks may occur. These can be ash and lava beds from volcanoes or intrusive
rocks from deep in the earth. It is these rocks that are chosen for absolute dating
- The position of the
igneous rocks helps in defining the relative ages of layers above and below and gives them
an absolute age.
- The use of fossils and
similar stratigraphic sequences is part of the relative dating method.
- Geologists have been able to use this process to
accurately place the rocks of the Earth in order of age. The fossils within the rocks are
also placed from oldest to youngest and the trends in evolutionary changes follow the