|- Art Gallery -||
The Cambrian is the first geological period of the Paleozoic Era, lasting from 542 ± 0.3 million years ago to 488.3 ± 1.7 million years ago(ICS, 2004,; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the classical name for Wales, where Britain's Cambrian rocks are best exposed.
The Cambrian is unique in its unusually high proportion of lagerstätten. These are sites of exceptional preservation, where 'soft' parts of organisms are preserved as well as their more resistant shells. This means that our understanding of the Cambrian biota surpasses that of later periods.
The Cambrian Period marked a profound change in life on Earth. Before the Cambrian, life was on the whole small and simple. Complex organisms became gradually more common in the millions of years immediately preceding the Cambrian, but it wasn't until this period that mineralised — hence readily fossilised — organisms became common. This diversification of lifeforms was relatively rapid, and is termed the Cambrian explosion. This explosion produced the first representatives of most modern phyla, but on the whole, most Cambrian animals look alien to today's eyes, falling in the evolutionary stems of modern groups. While life prospered in the oceans, the land was barren — with nothing more than a microbial 'crud' known as soil crust gracing the soils. Apart from tentative evidence suggesting that some animals floundered around on land, most of the continents resembled deserts spanning from horizon to horizon. Shallow seas flanked the margins of several continents, which had resulted from the relatively recent breakup of the preceding supercontinent Pannotia. The seas were relatively warm, and polar ice was absent.
Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks it was not until 1994 that this time period was internationally ratified. The base of the Cambrian is defined on a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. Nevertheless, the Treptichnus pedum, a reference ichnofossil for the lower boundary of the Cambrian, its usage for the stratigraphic detection of this boundary is always risky because of occurrence of very similar trace fossils belonging to the Treptichnids group well below the T. pedum in Namibia, Spain and Newfoundland, and possibly, in the western USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.
The Cambrian period follows the Ediacaran and is followed by the Ordovician period. The Cambrian is divided into four epochs or series and ten ages or stages. Currently only two series and four stages are named and have a GSSP.
Since the international stratigraphic subdivision is not yet complete, many local subdivisions are still widely used. In some of these subdivisions the Cambrian is divided into three epochs with locally differing names — the Early Cambrian (Caerfai or Waucoban, 542 ± 0.3 million years ago to 513 ± 2 million years ago), Middle Cambrian (St Davids or Albertian, 513 ± 2 million years ago to 499 ± 2 million years ago) and Furongian (499 ± 2 million years ago to 488.3 million years ago million years ago ; also known as Late Cambrian, Merioneth or Croixan). Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.
Each of the local epochs are divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance:
*In Russian tradition the lower boundary of the Cambrian is suggested to be defined at the base of the Tommotian Stage which is characterized by diversification and global distribution of organisms with mineral skeletons and appearing of first Archaeocyath bioherms.
The time range for the Cambrian has classically been thought to have been from about 570 Ma to about 500 Ma. The lower boundary of the Cambrian was traditionally set at the earliest appearance of trilobites and also unusual forms known as archeocyathids (literally 'ancient cup') that are thought to be the earliest sponges and also the first non-microbial reef builders.
The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radiometric dating in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 Ma are common between authors. Framing dates of ca. 545 to 490 Ma were proposed by the International Subcommission on Global Stratigraphy as recently as 2002.
A radiometric date from New Brunswick puts the end of the Lower Cambrian around 511 Ma. This leaves 21 Ma for the other two series/epochs of the Cambrian.
A more precise date of 542 ± 0.3 Ma for the extinction event at the beginning of the Cambrian has recently been submitted. The rationale for this precise dating is interesting in itself as an example of paleological deductive reasoning. Exactly at the Cambrian boundary there is a marked fall in the abundance of carbon-13, a "reverse spike" that paleontologists call an excursion. It is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age. One of the places that this well-established carbon-13 excursion occurs is in Oman. Amthor (2003) describes evidence from Oman that indicates the carbon-isotope excursion relates to a mass extinction: the disappearance of distinctive fossils from the Precambrian coincides exactly with the carbon-13 anomaly. Fortunately, in the Oman sequence, so too does a volcanic ash horizon from which zircons provide a very precise age of 542 ± 0.3 Ma (calculated on the decay rate of uranium to lead). This new and precise date tallies with the less precise dates for the carbon-13 anomaly, derived from sequences in Siberia and Namibia. It is presented here as likely to become accepted as the definitive age for the start of the Phanerozoic eon, and thus the start of the Paleozoic era and the Cambrian period.
Laurentia, Siberia, Baltica, Gondwana
Reconstructions of Cambrian geography contain relatively large sources of error. They suggest that a global supercontinent, Pannotia, was in the process of breaking up, with Laurentia (North America) and Siberia having separated from the main mass of the Gondwana supercontinent to form isolated landmasses. Most continental land mass was clustered in the southern hemisphere. Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian.
With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth were long melted – the sea level was high, which led to large areas of the continents being flooded in warm, shallow seas ideal for thriving life. The sea levels fluctuated somewhat, suggesting that there were 'ice ages', associated with pulses of expansion and contraction of a south polar ice cap.
While the Cambrian period was, on the whole, rather warm, it was not entirely without glaciation.
The Cambrian marked a steep change in the diversity and composition of Earth's biosphere. The incumbent Ediacaran biota suffered a mass extinction at the base of the period, which corresponds to an increase in the abundance and complexity of burrowing behaviour. This behaviour had a profound and irreversible effect on the substrate which transformed the seabed ecosystems. Before Cambrian, the sea floor was covered by microbial mats. By the end of the period, burrowing animals had destroyed the mats through bioturbation, and gradually turned the seabeds into what they are today. As a consequence, many of those organisms who were dependent on the mats went extinct, while the other species adapted to the changed environment who now offered new ecological niches. Around the same time, the Cambrian explosion saw the seemingly rapid appearance of representatives of all the mineralized phyla. There are also suggestions that some Cambrian organisms ventured onto land, producing the trace fossils Protichnites and Climactichnites.
Many modes of preservation are unique to the Cambrian period, resulting in an unusually high proportion of lagerstätte; see the list at the end of the article.
Generally it is accepted that there were no land plants at this time, although it is likely that a microbial "scum" comprising fungi, algae, and possibly lichens covered the land.
* End Botomian extinction event—circa 517 m.y.a.
v • d • e
1. ^ Image:Sauerstoffgehalt-1000mj.svg
* Amthor, J. E.; Grotzinger, John P.; Schröder, Stefan; Bowring, Samuel A.; Ramezani, Jahandar; Martin, Mark W.; Matter, Albert (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology 31: 431–434. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2.