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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.
[Edit: 24Jul08]

Prior Learning: Preliminary Module 8.2 (subsection 3) Stage 5 (subsection 5.9.4)
Science Stages 4-5 syllabus: Outcome 5.9 (content: 5.9.4 - natural events)

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 event'

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'.

The Cambrian explosion , Earth Sciences, University of Bristol, UK

The Cambrian Bioradiation Event , Technische Universitat, Berlin, Germany Coordination: Prof. Bernd-Dietrich Erdtmann & Dr. Michael Steiner

You need to gather evidence from:

• At least one computer simulation. Following are some suggestions for software that may be appropriate to this investigation.
  
  
  
  Computer simulation downloadable from the Internet
  
  Computer models of selection, evolution and punc-eq , by Glenn R Morton, USA. (click on Download the programs). Although the simulation is about Cambrian fauna, the actual simulation is of abstract shapes.
  
  
  
• 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.
  
  
  Sites containing photographs of Cambrian fossils and reconstructions
  
  

  • Burgess Shale fossils The Department of Geology and Geophysics, The University of Calgary, Alberta, Canada
  • A Guide to the Orders of Trilobites , An amazing site devoted to understanding trilobites developed by Dr. Sam Gon III, The Nature Conservancy of Hawaii, USA
  
  
  
• From the available evidence, develop a coherent summary of the changes in life forms that occurred during the 'Cambrian event'.

discuss the 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 Cambrian period.

deduce possible 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

Predation

• 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.

Protection

• 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.

Defence

• Armour deters predators. Hard parts afford an extra protective layer from injury by predators.

Armour plated Animals BBC Science and Nature, UK

process information to examine at least one example of a stratigraphic sequence and describe any fossils found in this sequence

• 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 Institution, USA
  
  
• If you are able to study a stratigraphic sequence first-hand, you should:
  1. Draw a scale diagram of the stratigraphic sequence.
  2. Indicate the location of any fossils in the sequence.
  3. Identify the fossils and draw or photograph them.
  4. 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 following possibilities:
  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".

interpret the 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.

distinguish 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.
  Radioactive dating Earth Science Australia

compare 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 methods.
• 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 stratigraphic sequences

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