Everything about the Cambrian totally explained
The
Cambrian is a
geologic period and system that began about
Ma (million years ago) at the end of the
Proterozoic eon and ended about Ma with the beginning of the
Ordovician period . It was the first period of the
Paleozoic era of the
Phanerozoic eon. The Cambrian takes its name from
Cambria, the classical name for
Wales, the area where rocks from this time period were first studied.
The Cambrian is the earliest period in whose rocks are found numerous large, distinctly fossilizable
multicellular organisms. This sudden appearance of hard body fossils is referred to as the
Cambrian explosion. Despite the long recognition of its distinction from younger
Ordovician rocks and older
Precambrian rocks it wasn't 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
Trichophycus pedum assemblage. This assemblage is distinct from anything in the Precambrian as it has ecologically tiered vertical burrows which are absent from the Precambrian.
Cambrian subdivisions
The Cambrian period follows the
Ediacaran and is followed by the
Ordovician period. The Cambrian is divided into three
epochs — the
Early Cambrian (Caerfai or Waucoban),
Middle Cambrian (St Davids or Albertian) and
Furongian (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 epochs are divided into several stages. Only one, the
Paibian, has been recognized by the
International Commission on Stratigraphy, and others are still unnamed. However, the Cambrian is divided into several regional
faunal stages of which the Russian-Kazakhian system is most used in international parlance:
Cambrian dating
The time range for the Cambrian has classically been thought to have been from about 500 mya to about 570 mya. The lower boundary of the Cambrian was traditionally set at the earliest appearance of early
arthropods known as
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 mya 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 mya. This leaves 21 Ma for the other two stages of the Cambrian.
A more precise date of 542 ± 0.3 mya 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's a marked fall in the abundance of
carbon-13, a "reverse spike" that
paleontologists call an
excursion. It is so widespread that it's 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.
Cambrian paleogeography
Cambrian
continents are thought to have resulted from the breakup of a
Neoproterozoic supercontinent called
Pannotia. The waters of the Cambrian period appear to have been widespread and shallow. Gondwana remained the largest supercontinent after the breakup of
Pannotia. It is thought that Cambrian
climates were significantly warmer than those of preceding times that experienced extensive
ice ages discussed as the
Varanger glaciation. Also there was no glaciation at the poles.
Continental drift rates in the Cambrian may have been anomalously high.
Laurentia,
Baltica and
Siberia remained independent continents since the break-up of the supercontinent of Pannotia. Gondwana started to drift towards the
South Pole.
Panthalassa covered most of the southern hemisphere, and minor oceans included the
Proto-Tethys Ocean,
Iapetus Ocean, and
Khanty Ocean, all of which expanded by this time.
Cambrian fauna
Of those modern
animal phyla that fossilize easily, all save the
bryozoans have claimed representatives in the Cambrian. 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". Also, the first possible tracks on land, such as
Protichnites and
Climactichnites, dating to about 530 mya and found in
Ontario,
Canada, and northern United States, appeared at this time. The
conodonts, small predatory primitive chordates known from their fossilised teeth, also appeared during the Furongian epoch of the Cambrian period. The conodonts thrived throughout the Paleozoic and the early Mesozoic until they completely disappeared during the Late Triassic period when the first mammals were evolving.
The best studied sites where the 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 the
People's Republic of China's
Yunnan Province (see
Maotianshan shales). Fairly extensive Precambrian
Ediacaran faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.
Cambrian flora
Generally it's accepted that there were no
land plants at this time although molecular dating suggests that land plant ancestors diverged from the algae much earlier, in the
Neoproterozoic about 700
ma, and that
fungi diverged from the animals about 1 billion years ago. The land at this time was barren, mostly
desert and
badlands.
Further Information
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