The CAMBRIAN Period ( /ˈkæmbriən/ or /ˈkeɪmbriən/ ) was the
first geological period of the
Paleozoic Era, of 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. Its subdivisions, and its base, are
somewhat in flux. The period was established (as “Cambrian
Adam Sedgwick , who named it after
Cambria , the
Latinised form of Cymru, the Welsh name for
Wales , where Britain's
Cambrian rocks are best exposed. 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
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
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. The rapid diversification of
lifeforms 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
metazoa (animals ) evolved monophyletically from a single common
ancestor: flagellated colonial protists similar to modern
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 and a few molluscs that emerged to
browse on the microbial biofilm known to have been present. 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.
* 1 Stratigraphy
* 1.1 Subdivisions
* 1.2 Dating the
* 2 Paleogeography
* 3 Climate
* 4 Flora
* 5 Oceanic life
* 6 Symbol
* 7 Gallery
* 8 See also
* 9 References
* 10 Further reading
* 11 External links
Stratigraphy of the Cambrian
Stratigraphy of the Cambrian
Despite the long recognition of its distinction from younger
Ordovician Period rocks and older
Precambrian Supereon rocks, it was
not until 1994 that this time period was internationally ratified. The
base of the
Cambrian lies atop a complex assemblage of trace fossils
known as the
Treptichnus pedum assemblage. The use of Treptichnus
pedum, a reference ichnofossil to mark the lower boundary of the
Cambrian, is difficult as the occurrence of very similar trace fossils
belonging to the Treptichnids group are found well below the T. pedum
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.
Cambrian Period followed the
Ediacaran Period and was followed by
Ordovician Period. The
Cambrian is divided into four epochs
(series ) and ten ages (stages ). Currently only two series and five
stages are named and have a
Because 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,
541 ± 1.0 to 509 ± 1.7 mya),
Middle Cambrian (St Davids or Albertan,
509 ± 1.0 to 497 ± 1.7 mya) and
Furongian (497 ± 1.0 to 485.4 ±
1.7 mya; also known as Late Cambrian, Merioneth or Croixan). Rocks of
these epochs are referred to as belonging to the Lower, Middle, or
Trilobite zones allow biostratigraphic correlation in the Cambrian.
Each of the local epochs is 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:
Trempealeauan , Fengshanian)
Ffestiniogian (Franconian , Changshanian)
Cambrian Series 3
Cambrian Series 2
*In Russian scientific thought the lower boundary of the
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 the appearance of the first Archaeocyath
DATING THE CAMBRIAN
Archeocyathids from the
Poleta formation in the Death Valley
International Commission on Stratigraphy list the
as beginning at 541 million years ago and ending at 485.4 million
The lower boundary of the
Cambrian was originally held to represent
the first appearance of complex life, represented by trilobites . The
recognition of small shelly fossils before the first trilobites, and
Ediacara biota substantially earlier, led to calls for a more
precisely defined base to the
After decades of careful consideration, a continuous sedimentary
sequence at Fortune Head,
Newfoundland was settled upon as a formal
base of the
Cambrian period, which was to be correlated worldwide by
the earliest appearance of
Treptichnus pedum . Discovery of this
fossil a few metres below the
GSSP led to the refinement of this
statement, and it is the T. pedum ichnofossil assemblage that is now
formally used to correlate the base of the Cambrian.
This formal designation allowed radiometric dates to be obtained from
samples across the globe that corresponded to the base of the
Cambrian. Early dates of 570 million years ago quickly gained favour,
though the methods used to obtain this number are now considered to be
unsuitable and inaccurate. A more precise date using modern
radiometric dating yield a date of 541 ± 0.3 million years ago. The
ash horizon in Oman from which this date was recovered corresponds to
a marked fall in the abundance of carbon-13 that correlates to
equivalent excursions elsewhere in the world, and to the disappearance
Ediacaran fossils (Namacalathus, Cloudina).
Nevertheless, there are arguments that the dated horizon in Oman does
not correspond to the Ediacaran-
Cambrian boundary, but represents a
facies change from marine to evaporite-dominated strata — which
would mean that dates from other, more suitable sections, ranging from
544 or 542 Ma, are more suitable.
Plate reconstructions suggest a global supercontinent,
Pannotia , was
in the process of breaking up early in the period, with Laurentia
Baltica , and Siberia having separated from the main
Gondwana to form isolated land masses. Most
continental land was clustered in the Southern Hemisphere at this
time, but was drifting north. Large, high-velocity rotational
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 sea life. The sea levels fluctuated somewhat, suggesting
there were 'ice ages', associated with pulses of expansion and
contraction of a south polar ice cap .
Baltoscandia a Lower
Cambrian transgression transformed large
swathes of the
Sub-Cambrian peneplain into a epicontinental sea .
The Earth was generally cold during the early Cambrian, probably due
to the ancient continent of
Gondwana covering the
South Pole and
cutting off polar ocean currents. However, average temperatures were 7
degrees Celsius higher than today. There were likely polar ice caps
and a series of glaciations, as the planet was still recovering from
Snowball Earth . It became warmer towards the end of the
period; the glaciers receded and eventually disappeared, and sea
levels rose dramatically. This trend would continue into the
Although there were a variety of macroscopic marine plants no land
plant (embryophyte ) fossils are known from the Cambrian. However,
biofilms and microbial mats were well developed on
flats and beaches 500 mya., and microbes forming microbial Earth
ecosystems, comparable with modern soil crust of desert regions,
contributing to soil formation.
Life timeline view • discuss • edit -4500 — – -4000 —
– -3500 — – -3000 — – -2500 — – -2000 — – -1500 —
– -1000 — – -500 — – 0 — WATER Single-celled
life PHOTOSYNTHESIS EUKARYOTES Multicellular
life LAND LIFE DINOSAURS MAMMALS FLOWERS ←
Earliest Earth (−4540 ) ← Earliest water ← Earliest
life ← LHB meteorites ← Earliest oxygen ←
Atmospheric oxygen ←
Oxygen crisis ← Earliest sexual
Ediacara biota ←
← Earliest humans P
n Pongola Huronian
Cryogenian Andean Karoo
Axis scale : millions of years ago
Orange labels: ICE AGES.
Human timeline and Nature timeline Main article: Cambrian
Most animal life during the
Cambrian was aquatic.
Trilobites were once assumed to be the dominant life form, but this
has proven to be incorrect. Arthropods in general were by far the most
dominant animals in the ocean, but trilobites were only a minor part
of the total arthropod diversity. What made them so apparently
abundant was their heavy armor that was reinforced by calcium
carbonate (CaCO3), which fossilized far more easily than the fragile
more purely chitin exoskeletons of other arthropods, leaving behind
numerous preserved remains which give the false impression that they
were the most abundant part of the fauna.
The period marked a steep change in the diversity and composition of
Earth's biosphere. The
Ediacaran biota suffered a mass extinction at
the start of the
Cambrian Period, which corresponded 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 the Cambrian, the sea floor
was covered by microbial mats . By the end of the Cambrian, burrowing
animals had destroyed the mats in many areas through bioturbation ,
and gradually turned the seabeds into what they are today. As a
consequence, many of those organisms that were dependent on the mats
became extinct, while the other species adapted to the changed
environment that now offered new ecological niches. Around the same
time there was a seemingly rapid appearance of representatives of all
the mineralized phyla except the
Bryozoa , which appeared in the Lower
Ordovician . However, many of those phyla were represented only by
stem-group forms; and since mineralized phyla generally have a benthic
origin, they may not be a good proxy for (more abundant)
non-mineralized phyla. A reconstruction of
Margaretia dorus from
Burgess Shale , which were once believed to be green algae , but
are now understood to represent hemichordates.
While the early
Cambrian showed such diversification that it has been
Cambrian Explosion, this changed later in the period, when
there occurred a sharp drop in biodiversity. About 515 million years
ago, the number of species going extinct exceeded the number of new
species appearing. Five million years later, the number of genera had
dropped from an earlier peak of about 600 to just 450. Also, the
speciation rate in many groups was reduced to between a fifth and a
third of previous levels. 500 million years ago, oxygen levels fell
dramatically in the oceans, leading to hypoxia , while the level of
poisonous hydrogen sulfide simultaneously increased, causing another
extinction. The later half of
Cambrian was surprisingly barren and
show evidence of several rapid extinction events; the stromatolites
which had been replaced by reef building sponges known as
Archaeocyatha , returned once more as the archaeocyathids became
extinct. This declining trend did not change until the Great
Ordovician Biodiversification Event .
Cambrian organisms ventured onto land, producing the trace
Climactichnites . Fossil evidence suggests
that euthycarcinoids , an extinct group of arthropods, produced at
least some of the Protichnites. Fossils of the track-maker of
Climactichnites have not been found; however, fossil trackways and
resting traces suggest a large, slug -like mollusc .
In contrast to later periods, the
Cambrian fauna was somewhat
restricted; free-floating organisms were rare, with the majority
living on or close to the sea floor; and mineralizing animals were
rarer than in future periods, in part due to the unfavourable ocean
Many modes of preservation are unique to the Cambrian, and some
preserve soft body parts, resulting in an abundance of Lagerstätten .
The United States
Federal Geographic Data Committee uses a "barred
capital C" ⟨Ꞓ⟩ character to represent the
Cambrian Period. The
Unicode character is U+A792 Ꞓ LATIN CAPITAL LETTER C WITH BAR.
Stromatolites of the Pika Formation (Middle Cambrian) near Helen
Lake, Banff National Park, Canada
Trilobites were very common during this time
Anomalocaris was an early marine predator, among the various
arthropods of the time.
Pikaia was an early chordate from the
Opabinia was a creature with an unusual body plan; it was probably
related to arthropods
Protichnites were the trackways of arthropods that walked Cambrian
Hallucigenia is maybe an early ancestor of the Velvet worms .
Reconstructions of H. sparsa, H. hongmeia, and H. fortis
Size comparison of different
Part of a series on
THE CAMBRIAN EXPLOSION
Small shelly fauna
Evolutionary concepts Trends
Cambrian substrate revolution
Cambrian substrate revolution
* Stem and crown groups
Ordovician extinction event – circa 488 mya
Dresbachian extinction event—circa 502 mya
* End Botomian extinction event—circa 517 mya
List of fossil sites (with link directory)
Type locality (geology) , the locality where a particular rock
type, stratigraphic unit, fossil or mineral species is first
Modes of preservation in the
* Doushantuo type
* Bitter Springs type
Burgess Shale type
Trilobite Bed type
Small shelly fossils
Small shelly fossils
* Trace fossils
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Wikimedia Commons has media related to CAMBRIAN .
Cambrian period on In Our Time at the
Biostratigraphy – includes information on Cambrian