Geology

Cambrian

December 1, 2011, 4:52 pm
Content Cover Image

Cambrian Period continental configuration.

The Cambrian Period (543-490 million years ago) marks an important point in the history of life on Earth; it is the time period 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 biodiversity of lifeforms appears. It was once thought that the Cambrian rocks contained the first and oldest fossil animals, but the earliest fossil records of life are now known to be found in the earlier Vendian strata.

caption Subdivisions of the Cambiran

Subdivisions of the Cambrian

The accompanying chart depicts the major subdivisions of the Cambrian Period for North America (a continent configured as Laurentia during the Cambrian). International ages (subdivisions) have not been established. The size of the bars does not correlate with the length of time for each age. The oldest unnamed age is 543 to 520 million years ago, while the remaining six ages are from 520 to 490 million years ago, each approximately five to six million years in length. This chart is mapped to allow one to travel back to the Vendian, or forward to the Ordovician.

The Cambrian Period is part of the Paleozoic Era.

Stratigraphy of the Cambrian

A lot can happen in 40 million years, the approximate length of the Cambrian period. Animals showed dramatic diversification during this period of Earth's history. This has been called the "Cambrian Explosion". When the fossil record is scrutinized 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! In that time, the first undoubted fossil annelids, arthropods, brachiopods, echinoderms, molluscs, onychophorans, poriferans, and priapulids show up in rocks all over the world. Perhaps we should call this bit of time the "Tommotian Explosion". 

Stratigraphic boundaries are determined by the occurrences of fossils. For instance, the trace fossil Phycodes pedum marks the base of the Cambrian. This boundary is an unusual case, since stratigraphic boundaries are normally defined by the presence or absence of groups of fossils, called assemblages. 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. Questions such as: "how long did something stay the same?" or,"how fast did it change?" can only be assessed in the context of time. 

Tectonics and paleoclimate of the Cambrian

The Cambrian follows the Vendian period, during which time the continents had been joined in a single supercontinent 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 reconstruction below shows the rifting of Rodinia durring the Tommotian. Green represents land above water at this time, red indicates mountains, light blue indicates shallow seas of the continental shelves, and dark blue denotes the deep ocean basins. (For clarity, the outlines of present-day continents have been superimposed on the continental map at the article top.) 

As one can see, the Cambrian world was concentrated in the southern hemisphere. The largest landmass (lower right) was Gondwana (a collection of today's southern continents). The second largest continent, Laurentia is just left of center in the map, and includes most of North America (the southeastern US can be seen wedged between Africa and South America as part of Gondwana). Between Gondwana and Laurentia lie Siberia (just south of the equator) and Baltica (Scandinavia, eastern Europe, and European Russia). 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 supercontinent Rodinia that had existed in the Late Proterozoic. Laurentia stradled 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 continental plate movement and collisions during this period generated pressure and heat between continents. this resulted in the folding, faulting, and crumpling of rock which formed large mountain ranges.

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. None of the continents were located at the poles, and so land temperatures remained mild. In fact, global climate was probably warmer and more unifrom than it is today. 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. This era can also be considered the beginning of the Edenic Period, an epoch which marked rather stable ratios of speciation versus extinction; the Edenic Period is considered ended at the beginning of the Holocene, when man began to exert a massive influence on species extinctions.

Life of the Cambrian

caption Examples of the Cambrian fossil record

Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Vendian period immediately preceding the Cambrian. Some of the odd fossils of the "Ediacara biota" from the Vendian may also have been animals in or near living phyla, although this remains a somewhat controversial topic. 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. Trace fossils made by animals also show increased diversity in Cambrian rocks, showing that 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 and/or diversification of mineralized algae of various types, such as 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 become extinct or faded in importance. The Brachiopoda for example, was present, but greatest diversity was shown by inarticulate brachiopods (like the one in the upper middle, from the Upper Cambrian of Iowa). 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 there is some controversy over whether crinoids (sea lilies) were present or not. Even if present, crinoids were rare in the Cambrian, although they became numerous and diverse through the later Paleozoic. 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 (like the one on the upper left, Nevadella from the Lower Cambrian of southwest Nevada); archaeocyathids (relatives of sponges that were restricted to the Lower Cambrian), and problematic conical fossils known as hyolithids (like the one on the upper right, also from the Lower Cambrian of Nevada). 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 (Middle Cambrian) 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 Wiwaxia, depicted at lower left. Wiwaxia was an inch-long, creeping, scaly and spiny bottom dweller that may have been a relative of the molluscs, the annelids, or possibly an extinct animal group that combined features of both phyla.

Further reading

The text of this article is based upon work authored by Ben Waggoner, Karen Hsu, Myun Kang, Amy Lavarias, Kavitha Prabaker and Cody Skaggs.

Glossary

Citation

Paleontology, U. (2011). Cambrian. Retrieved from http://www.eoearth.org/view/article/150867

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