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The Cambrian Period Essay

1063 words - 4 pages

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.Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. Some of the odd fossils of the "Ediacara biota" from the Vendian may also have been animals in or near living phyla. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years. The animals of the Cambrian were developing new ecological niches and strategies -- such as active hunting, burrowing deeply into sediment, and making complex branching burrows. Finally, the Cambrian saw the appearance of the coralline red algae and the dasyclad green algae.This does not mean that life in the Cambrian seas would have been perfectly familiar to a modern-day scuba diver! Although almost all of the living marine phyla were present, most were represented by classes that have since gone extinct or faded in importance. The Brachiopoda for example, was present, but greatest diversity was shown by inarticulate brachiopods. The articulate brachiopods, which would dominate the marine environment in the later Paleozoic, were still relatively rare and not especially diverse. Cambrian echinoderms were predominantly unfamiliar and strange-looking types such as early edrioasteroids, eocrinoids, and helicoplacoids. The more familiar starfish, brittle stars, and sea urchins had not yet evolved. And while jawless vertebrates were present in the Cambrian, it was not until the Ordovician that armored fish became common enough to leave a rich fossil record.Other dominant Cambrian invertebrates with hard parts were trilobites; archaeocyathids, and problematic conical fossils known as hyolithids. Many Early Cambrian invertebrates are known only from "small shelly fossils" - tiny plates and scales and spines and tubes and so on. Many of these were...

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