Cambrian
The Cambrian is a major division of the geologic timescale that begins about 542 million years ago (Ma) at the end of the Proterozoic eon and ended about 488 Ma with the beginning of the Ordovician period (ICS, 2004). Some put the Cambrian from 542 million years ago to 505 Ma. It is the first period of the Paleozoic era of the Phanerozoic eon. The Cambrian period is often divided into the Early, or Lower, Cambrian Epoch, the Middle Cambrian Epoch, and Late Cambrian Epoch or Furongian. The Cambrian is the earliest period in whose rocks are found numerous large, distinctly-fossilizable multicellular organisms that are more complex than sponges or medusoids.
During this time, there was an unparalleled explosion of life, as many of the major groups of organisms or "phyla" (a phylum defines the basic body plan of some group of modern or extinct animals) emerged suddenly, in most cases without evident precursors. Various research recognized between 20 and 50 major phyla appeared, possibly over a period of only a few million years. This radiation of animal phyla is referred to as the Cambrian explosion. In the 500 million years since the Cambrian, no fundamentally new body plan has emerged (Mayr 2001).
| Paleozoic era (542 - 251 mya) | |||||
|---|---|---|---|---|---|
| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian |
Cambrian subdivisions
The Cambrian period follows after the Neoproterozoic and is followed by the Ordovician period. The Cambrian is divided into three epochs — the Early Cambrian (Lower Cambrian, Caerfai or Waucoban), Middle Cambrian (St Davids or Albertian) and Furongian (a.k.a. Late/Upper Cambrian, Merioneth or Croixan).
Each of the epochs are divided into two faunal stages. Only one, Paibian has been recognized by the International Commission on Stratigraphy. Others are still unnamed. However, the Cambrian is divided into several regional faunal stages:
| Chinese | North American | Russian-Kazakhian | Australian | Regional | |
|---|---|---|---|---|---|
| Furongian | Ibexian (part) | Ayusokkanian | Idamean | Dolgellian | |
| Sunwaptan | Sakian | Mindyallan | Festiniogian | ||
| Steptoan | Aksayan | Payntonian | Maentwrogian | ||
| Marjuman | Batyrbayan | ||||
| Middle Cambrian | Maozhangian | Mayan | Boomerangian | ||
| Zuzhuangian | Delamaran | Amgan | Undillian | ||
| Zhungxian | Florian | ||||
| Templetonian | |||||
| Dyeran | Ordian | ||||
| Early Cambrian | Longwangmioan | Toyonian | Lenian | ||
| Changlangpuan | Montezuman | Botomian | |||
| Qungzusian | Atdabanian | ||||
| Meishuchuan | Tommotian | ||||
| Nemakit-Daldynian |
Cambrian dating
The time range for the Cambrian has classically been thought to have been from about 500 Ma to about 570 Ma. The lower boundary of the Cambrian was traditionally set at the earliest appearance of early arthropods known as trilobites and of primitive reef-forming animals known as archeocyathids. The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radioactive dating in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 million years are common between authors. Framing dates of ca. (approximately) 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 first stage of the Cambrian around 511 Ma. This leaves 21 million years for the other two stages of the Cambrian.
A more precise date 542 Ma (plus or minus 300,000 years) 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 palaeological deductive reasoning. Exactly at the Cambrian boundary there is a marked fall in the abundance of carbon-13, a 'reverse spike' that palaeontologists 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 pre-Cambrian 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 Palaeozoic era and the Cambrian period.
Cambrian palaeogeography
Cambrian continents are thought to have resulted from the breakup of a neoproterozoic supercontinent called Rodinia. The waters of the Cambrian period appear to have been widespread and shallow. It is thought that Cambrian climates were significantly warmer than those of preceding times that experienced extensive ice ages discussed as the Varanger glaciation. Continental drift rates in the Cambrian may have been anomalously high. Because of their complexity, it is difficult to describe continental motions in text. Time-sequenced maps of paleo-continents and other major geologic features are called paleomaps and are available at several Internet sites (see below).
Cambrian fauna
Aside from a few enigmatic forms that may or may not represent animals, all modern animal phyla with any fossil record to speak of except bryozoa appear to have representatives in the Cambrian, and of these most except sponges seem to have originated just after or just before the start of the period. However, several modern phyla, primarily those with small and/or soft bodies, have no fossil record, in the Cambrian or otherwise. Many extinct phyla and odd animals that have unclear relationships to other animals also appear in the Cambrian. The apparent "sudden" appearance of very diverse faunas over a period of no more than a few tens of millions of years is referred to as the "Cambrian Explosion".
The best studied sites where soft parts of organisms have fossilized are in the Burgess shale of British Columbia. They represent strata from the middle Cambrian and provide us with a wealth of information on early animal diversity. Similar faunas have subsequently been found in a number of other places — most importantly in very early Cambrian shales in China's Yunnan Province (see Maotianshan shales). Fairly extensive pre-Cambrian Ediacarian faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.
Cambrian explosion
The Cambrian explosion refers to the geologically sudden appearance of complex multi-cellular macroscopic organisms between roughly 542 and 530 million years ago (mya). This period marks a sharp transition in the fossil record with the appearance of the earliest members of many phyla of metazoans (multicellular animals). The "explosive" appearance of this adaptive radiation results both from rapid evolutionary change and the limits of previous technology to appreciate microfossils which formed the foundation of the fossil record before this time.
From the modern point-of-view, the apparently explosive radiation from obscure beginnings was partly an artifact of disregarding microfossils, which were scarcely detectable with 19th-century technology, and concentrating solely on the hard-shelled macrofossils that defined the phyla well established by 19th-century biologists, all of which were multiple-celled metazoa. Apparently abruptly, many kinds of fossils appearing in the Burgess Shale were seen showing obvious skeletal body features, whereas the traces of the hard-to-analyze "small shelly fauna" of Cambrian beginnings were ignored.
With time, advanced microscopy has gradually revealed the range of earlier microfossils. Prior to the discovery in 1909 of the Burgess Shale—incompletely published at the time and largely forced into existing categories as "precursors"— no fossilizations of early soft-bodied organisms had been published, and the vast reach of undiscovered earlier life was consigned to an enormous space of time— the "Pre-Cambrian" of old-fashioned schoolbooks.
More recent microfossil finds have shown "Pre-Cambrian" life consisting of more than single- celled organisms or simple diploblastic fauna. In 1994, phosphatized triploblastic embryos were discovered in rocks from southern China (Xiao et al. 1998). Evidence for Ediacaran triploblasts was available long before this discovery.
Fossils
This period of evolution is source to some of the most unusual fossils ever recovered. A single formation, the Burgess shale, has provided some of the best insights into this period of dramatic evolutionary change and experimentation that laid the foundation for most major modern animal body plans. Also appearing at this time are a wide variety of enigmatic and exotic configurations that appear to be unrelated to any modern animals. Fossils from the Chengjiang biota, about 10 million years older than the Burgess Shale, have given both a broader and sharper view of early Cambrian life.
Before the explosion, the fossil record is dominated by single-celled organisms with only the rare soft-bodied Ediacaran fauna and certain microfossils showing that multi-cellular life forms had arisen roughly 30 million years earlier (Xiao et al. 1998).
With the Cambrian explosion came the evolution of shells and other hard body parts. As shells are more easily preserved in sediment than soft body parts, this makes life forms of this and subsequent periods much easier to study in the fossil record than their Precambrian counterparts. This also contributes to the perception of an abrupt change in the fossil record.
Causes of the Cambrian explosion
The Cambrian explosion may have been precipitated by several environmental changes occurring in and just before this period. First the Varangian glaciation gave rise to a Snowball Earth in which all, or nearly all, of the oceans are covered entirely with ice. This was followed by a deglaciation and rapid global warming just before the beginning of the explosion itself.
In modern Arctic environments, single-celled organisms often form mats on the underside of ice sheets in order to maximize their exposure to sunlight. It is possible that adaptations useful to the maintenance of such colonies also assisted in the formation of the first triploblastic animals estimated to be 570 million years of age (Xiao et al. 1998). In addition, the Snowball Earth environment would have given rise to relatively few ecological niches, so the subsequent deglaciation and global warming may have provided to impetus for rapid evolution to fill many new environments.
Diversification
Of the 20 metazoan phyla with extensive fossil records, at least 11 first appeared in the Cambrian. Of the remainder, 1 is known to Precambrian and the other 8 first appear more recently (Collins 1994). An additional 12 soft-bodied phyla have poorly defined fossil records, but it is speculated that a significant number of these may also be Cambrian in origin.
Though this period is definitely of special significance in terms of rapid diversification and the emergence of new forms, some of that significance is likely to be overstated by the focus on macroscopic forms in the ways phyla are observed and defined. Molecular evidence suggests that at least six animal phyla had established themselves as distinct evolutionary paths during the Precambrian (Wang et al. 1999).
The sheer variety of forms found in the Burgess shale and other sites, has made some skeptical that single period of ~10-15 million years could have been long enough to give rise to such diversity. An emerging view is that the Cambrian explosion is the macroscopic conclusion to a prolonged period of evolution begun ~30 million years earlier with the innovation of multi-cellular organisms.
The Cambrian "explosion" continues to fascinate the non-specialist. Stephen Jay Gould's Wonderful Life (1989) provides the best general account for the layman of the paleontological analysis of the Burgess Shale; the book was the 1991 winner of The Aventis Prizes for Science Books.
ReferencesISBN links support NWE through referral fees
- Collins, Allen G. "Metazoa: Fossil record". Retrieved Dec. 14, 2005.
- Wang, D. Y.-C., S. Kumar and S. B. Hedges (1999). "Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi." Proceedings of the Royal Society of London, Series B, Biological Sciences 266, 163-71.
- Xiao, S., Y. Zhang, and A. Knoll (1998). "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite." Nature 391, 553-58.
External links and references
- Weird Life on the Mats
- Amthor, J. E., et al.. 2003. Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman. Geology, 31: pp 431-434.
- Report on the web on Amthor et al. from Geology vol. 31.
- Paleomap Project.
- International Commission on Stratigraphy (ICS). Home Page. URL accessed on Sept 19, 2005.
| Cambrian period | ||
|---|---|---|
| Early Cambrian | Middle Cambrian | Furongian |
| Stage 1 | Stage 2 | Stage 3 | Stage 4 | Stage 5 Stage 6 | Stage 7 |
Paibian | Stage 9 Stage 10 |
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