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The Cambrian Period
The Cambrian Period marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears.
A lot can happen in 40 million years, the approximate length of the Cambrian period. Animals showed dramatic change and diversity during this period of Earth's history. When the fossil record is studied closely, it turns out that the fastest growth in the number of major new animal groups took place during the Tommotian and Atdabanian stages of the Early Cambrian, a period of time which may have been as short as five million years!
Stratigraphic boundaries are determined by the occurrences of fossils. In fact, much paleontological work is concerned with questions surrounding when and where stratigraphic boundaries should be defined. At first glance, this may not seem like important work, but consider this. If you wanted to know about the evolution of life on Earth, you would absolutely have to keep track of time.
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