Cambrian

The Cambrian Period ( /ˈkæm.bri.ən, ˈkm-/ KAM-bree-ən, KAYM-; sometimes symbolized ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon.[5] The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya.[6] Its subdivisions, and its base, are somewhat in flux. The period was established as "Cambrian series" by Adam Sedgwick,[5] who named it after Cambria, the Latin name for 'Cymru' (Wales), where Britain's Cambrian rocks are best exposed.[7][8][9] Sedgwick identified the layer as part of his task, along with Roderick Murchison, to subdivide the large "Transition Series", although the two geologists disagreed for a while on the appropriate categorization.[5] The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells. As a result, our understanding of the Cambrian biology surpasses that of some later periods.[10]

Cambrian
541.0 ± 1.0 – 485.4 ± 1.9 Ma
Earth in the middle of Cambrian period
Chronology
Etymology
Name formalityFormal
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitPeriod
Stratigraphic unitSystem
First proposed byAdam Sedgwick, 1835
Time span formalityFormal
Lower boundary definitionAppearance of the Ichnofossil Treptichnus pedum
Lower boundary GSSPFortune Head section, Newfoundland, Canada
47.0762°N 55.8310°W / 47.0762; -55.8310
GSSP ratified1992[2]
Upper boundary definitionFAD of the Conodont Iapetognathus fluctivagus.
Upper boundary GSSPGreenpoint section, Green Point, Newfoundland, Canada
49.6829°N 57.9653°W / 49.6829; -57.9653
GSSP ratified2000[3]
Atmospheric and climatic data
Mean atmospheric O
2
content
c. 12.5 vol %
(63 % of modern)
Mean atmospheric CO
2
content
c. 4500 ppm
(16 times pre-industrial)
Mean surface temperaturec. 21 °C
(7 °C above modern)
Sea level above present dayRising steadily from 4m to 90m[4]

The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian Charnia being exceptional. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized—hence readily fossilized—organisms became common.[11] The rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla. Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa (animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.[12]

Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust[13] and a few molluscs that emerged to browse on the microbial biofilm.[14] Most of the continents were probably dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia. The seas were relatively warm, and polar ice was absent for much of the period.