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Text Introduction to the Early Paleozoic Contents Precambrian-Cambrian Transition Cambrian The PreCambrian-Cambrian Transition To learn more about the Vendian, click here. To learn about Vendian fossils, click here. Placement of Precambrian-Cambrian boundary is problematic; usually below "shelly" faunas. Originally at first trilobites. Return to top of page The Cambrian Period 545 - 495 Ma The Cambrian page from Berkeley. Information on the Cambrian time scale and Cambrian stratigraphy from Berkeley. Cambrian Life Tectonic and climatic setting See paleogeographic reconstruction in text Spend a few minutes to locate the various continents and their names at this time. North America = Laurentia Northern Europe = Baltica Gondwanaland = the southern continents. They are upside down for the most part. You should be able to pick out the outline of Africa (far right), Australia (upper left part of Gondwanaland), Antarctica (to the right of Australia). A glance at the Ordovician map will show you South America upside down on the left, nestled on Africa. As we proceed through the Paleozoic, we will be referring to these paleogeographic reconstructions frequently to see changes as time passes. For the Cambrian, note that the continents are separate (not all assembled into Pangea until the end of the Paleozoic), and that they are primarily located in tropical latitudes. Also note that there are NO CONTINENTS LOCATED AT THE POLES. This will become important later in the Paleozoic because it influences climate. What does this mean? Basically, if there are no continents at the poles, there was no place for glaciers to grow. Global temperatures were probably warm, because tropical waters could not flow far before running into a continent, and being deflected N or S. Also note the distribution of rock types, such as evaporites (E). Today, the desert latitudes are at about 30 degrees N and S of the equator. Note all of the shallow sea deposits. Most are tropical, suggesting the sort of warm, shallow sea conditions needed for the formation of limestones. Indeed, the Cambrian and Ordovician are times of widespread deposition of carbonates, particularly in the Appalachian region. Check the Cambrian Paleogeographic map to see the distribution of sedimentary environments. -------------------------------------------------------------------------------- Cambrian Fossil Record The Cambrian is best described as a time of evolutionary explosion. All marine invertebrate phyla appeared during this time, with the exception of the bryozoa. The dominant marine invertebrates with hardparts were the trilobites, brachiopods, and archaeocyathids. Tommotian (lowermost Cambrian) archaeogastropods, brachiopods, sponges, archaeocyathids, hyolithids, worm-like taxa First diverse biotas of animals with skeletons. Between 570 - 520 my, nearly all coelomate phyla with skeletons appeared rapidly. Origin of skeletons (shells) Function: Support for muscles, etc. Protection against environment & other organisms, predators Aid in locomotion Skeleton served as an adaptive breakthrough allowing preferential survival and niche exploitation. Adaptive radiation occurred; rapid diversification of forms. Possible reasons for the advent of skeletonization: Increasing oxygen levels (Toewe, 1970) Synthesis of collagen, a fibrous connective tissue in shells, cuticles, carapaces, etc. directly or indirectly requires collagen. Oxygen priorities Low oxygen levels: respiration & tissue synthesis High oxygen levels: oxygen can be used for lower priority things - skeletons, shells, etc. Oxygen reached critical threshold at this time, eliminating the need for priorities. Evidence: modern low O2 environments have only small, soft-bodied forms. Problems? Muscles require collagen, suggesting that early muscles were weak, BUT early evidence for metazoans shows burrows which would require good muscles. Stanley (1976) disagrees. Says Cambrian was a time of the initial proliferation of organisms and life in general. Says skeletonization was a major evolutionary experiment, which succeeded, leading to rapid adaptive radiation and survival of the fittest. The earliest organisms with soft bodies may have had respiration by diffusion across body wall. When O2 content of atm. increased, it allowed them to develop respiratory systems, circulatory systems, etc. No longer needed to use O2 diffusion. Larger body size possible. Sudden simultaneous appearance of skeletonized taxa. Adaptive radiation. Follows Punctuated Equilibrium model. |
