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 probably pieces of the skeletons of larger animals.
A few localities around the world that preserve soft-bodied fossils of the Cambrian show that the "Cambrian radiation" generated many unusual forms not easily comparable with anything today. The best-known of these sites is the legendary Burgess Shale in the British Columbian Rocky Mountains. Sites in Utah, southern China, Siberia, and north Greenland are also noted for their unusually good preservation of non-mineralized fossils from the Cambrian. One of these "weird wonders", first documented from the Burgess Shale, is Wiwaxiat. Wiwaxia was an inch-long, creeping, scaly and spiny bottom dweller that may have been a relative of the mollusks, the annelids, or possibly an extinct animal group that combined features of both phyla.
Rocks of Cambrian age are distributed in the Great Basin of the western United States, parts of the northeastern United States, Wales, Scandinavia and the Baltic region, Siberia, and China, among other places. These localities were not where they are now: the position of the continents was very different. It may seem strange to imagine California on the equator, or Venezuela near the South Pole, but that's how things were!
The Cambrian follows the Vendian period, during which time the continents had been joined in a single super continent called Rodinia (from the Russian word for "homeland", rodina). As the Cambrian began, Rodinia began to fragment into smaller continents, which did not always correspond to the ones we see today. The Cambrian world was concentrated in the southern hemisphere. The largest landmass was Gondwana. The second largest continent, Laurentia includes most of North America. Between Gondwana and Laurentia lie Siberia. The rest of Europe and much of what is today Asia lay in fragments along the north coast of Gondwana.
These landmasses were scattered as a result of the fragmentation of the super continent Rodinia that had existed in the Late Proterozoic. Laurentia straddled the equator, while Baltica and Siberia were southeast of that continent. Tectonism affected regions of Gondwana is what are presently Australia, Antarctica, and Argentina the most. This is evident because of the presence of volcanic island arcs, which show that seafloor spreading and crustal subduction occurred.
The Cambrian world was bracketed between two ice ages, one during the late Late Proterozoic and the other during the Ordovician. During these ice ages, the decrease in global temperature led to mass extinctions. Cooler conditions eliminated many warm water species, and glaciation lowered global sea level. However, during the Cambrian there was no significant ice formation. With the beginning of the Cambrian at the retreat of Proterozoic ice, the sea level rose significantly. Lowland areas such as Baltica were flooded and much of the world was covered by epeiric seas. This event opened up new habitats where marine invertebrates, such as the trilobites, radiated and flourished.
Plants had not yet evolved, and the terrestrial world was therefore devoid of vegetation and inhospitable to life as we know it. Photosynthesis and primary production were the monopoly of bacteria and algal protists that populated the world's shallow seas.
Also during the Cambrian, oxygen first mixed into the world's oceans in significant quantity. Although there was plentiful atmospheric oxygen by the opening of the Cambrian, only in the Cambrian did the numbers of oxygen-depleting bacteria reduce in numbers sufficiently to permit the high levels we know today. This made dissolved oxygen available to the diversity of animals, and may have triggered the "Cambrian Explosion". This was when most of the major groups of animals, especially those with hard shells, first appear in the fossil record.