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by others. Also, the fossil record bears the evidence of evolution, so that the evolution of fossils can be used to chart a continuous succession of former living things through time. This is the key to seeing that fossils can be used to build up a worldwide geological time scale. (For more information on this topic see pages 50 to 59 and the book Geological Time in the Earth Science set.)
Locality 1
Locality 2
Ammonite
Trilobite
(Above) Ammonites are free swimming and can be found worldwide. They are good indicators of time, even when continents are separated.
(Above) This diagram shows the principle of how fossils can be used to make a geological record from rocks of different textures (facies). In the lower part of the diagram, an ancient form of life called
a trilobite is found in both shale and limestone, indicating that they are both rocks of the same age. But, the trilobite is not found in the upper rocks because, by the time these rocks were laid down,
it had become extinct. Here, a different fossil, an ammonite, is found in both shale and limestone rocks, and so it, too, can be used to relate rocks of different textures. Rocks are placed in their correct order in time, using fossils that have overlapping time ranges.
(Above) Trilobites were bottom dwellers and found close to coasts. They could not cross large oceans. They are good indicators of the isolation of continents (or of their previous grouping).
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