کامبرین
Cambrian
An interval of time in Earth history
(Cambrian Period) and its rock record (Cambrian System). The Cambrian Period
spanned about 60 million years and began with the first appearance of marine
animals with mineralized (calcium carbonate, calcium phosphate) shells. The
Cambrian System includes many different kinds of marine sandstones, shales,
limestones, dolomites, and volcanics. Apart from the occurrence of an alkaline
playa containing deposits of trona (hydrated basic sodium carbonate) in the
Officer Basin of South Australia, there is very little provable record of
nonmarine Cambrian environments.
The concept that great systems of
rocks recorded successive periods of Earth history was developed in
Geography
Knowledge of Cambrian geography and
of the dynamic aspects of evolution and history in Cambrian time is derived from
rocks of this age that have been exposed by present-day erosion or penetrated by
borings into the Earth's surface. Despite the antiquity of Cambrian time, a
surprisingly good record of marine rocks of Cambrian age has been preserved at
many localities throughout the world. Each of the different rock types contains
clues about its environment of deposition that have been derived from analogy
with modern marine environments. From this information, together with knowledge
gained from fossils of about the same age within the Cambrian and information
about the present geographic distribution of each Cambrian locality, a general
picture of world geography and its changes through Cambrian time is available.
Plate
tectonics
The theory of plate tectonics has
provided criteria whereby ancient continental margins can be identified. By
using these criteria and the spatial information about marine environments
derived from study of the rocks, the Cambrian world can be resolved into at
least four major continents that were quite different from those of today (Fig.
1). These were (1) Laurentia, which is essentially North America, minus a narrow
belt along the eastern coast from eastern
Fig. 1 Reconstruction of the Lower Cambrian
world. (After W. S. McKerrow, C. R. Scotese, and M. D. Brasier, Early Cambrian
continental reconstructions. J. Geol. Soc., 149:599–606,
1992)
Time
divisions
For most practical purposes, rocks
of Cambrian age are recognized by their content of distinctive fossils. On the
basis of the successive changes in the evolutionary record of Cambrian life that
have been worked out during the past century, the Cambrian System has been
divided globally into three or four series, each of which has been further
divided on each continent into stages, each stage consisting of several zones
(Fig. 2). Despite the amount of work already done, precise intercontinental
correlation of series and stage boundaries, and of zones, is still difficult,
especially in the Early Cambrian due to marked faunal provinciality. Refinement
of intercontinental correlation of these ancient rocks is a topic of research.
Fig. 2 North American divisions of the Cambrian
S ystem. Asterisks denote levels of major trilobite
extinctions.
Life
The record preserved in rocks
indicates that essentially all Cambrian plants and animals lived in the sea. The
few places where terrestrial sediments have been preserved suggest that the land
was barren of major plant life, and there are no known records of Cambrian
insects or of terrestrial vertebrate animals of any kind.
Plants
The plant record consists entirely
of algae, preserved either as carbonized impressions in marine black shales or
as filamentous or blotchy microstructures within marine buildups of calcium
carbonate, called stromatolites, produced by the actions of these organisms.
Cambrian algal stromatolites were generally low domal structures, rarely more
than a few meters high or wide, which were built up by the trapping or
precipitation of calcium carbonate by one or more species of algae. Such
structures, often composed of upwardly arched laminae, were common in regions of
carbonate sedimentation in the shallow Cambrian seas. See also: Stromatolite
Animals
The animal record is composed almost
entirely of invertebrates that had either calcareous or phosphatic shells (Fig.
3). The fossils of shell-bearing organisms include representatives of several
different classes of arthropods, mollusks, echinoderms, brachiopods, and
poriferans. Coelenterates, radiolarians, and agglutinated foraminiferans are
extremely rare, and bryozoans are unknown from Cambrian rocks. Rare occurrences
of impressions or of carbonized remains of a variety of soft-bodied organisms,
including worms and a group of soft-bodied trilobites, indicate that the fossil
record, particularly of arthropods, is incomplete and biased in favor of
shell-bearing organisms. Some widespread fossil groups, such as Archaeocyatha,
are known only from Cambrian rocks, and several extinct groups of Paleozoic
organisms, such as hyolithids and conodo nts, first appear in Cambrian rocks.
Conodonts are thought by some specialists to have affinity with vertebrates, but
others prefer to relate them to cephalochordates. Dermal plates recovered from
the Late Cambrian of North America and Australia are considered to represent the
earliest fish remains. See also:
Arthropoda; Conodont; Porifera
Fig. 3 Representative Cambrian fossils: (a–c)
trilobites; (d–f) brachiopods; (g) hyolithid; (h–i) mollusks; (j–l) echinoderms;
and (m, n) archaeocyathids.
Diversity
Although the record of marine life
in the Cambrian seems rich, one of the dramatic differences between Cambrian
marine rocks and those of younger periods is the low phyletic diversity of most
fossiliferous localities. The most diverse faunas of Cambrian age have been
found along the ocean-facing margins of the shallow seas that covered large
areas of the Cambrian continents. Because these margins were often involved in
later geologic upheavals, their rich record of Cambrian life has been largely
destroyed. Only a few localities in the world remain to provide a more accurate
picture of the diversity of organisms living in Cambrian time. These are known
as Konservat Lagerstätten—conservation deposits containing occurrences of
extraordinary preservation, particularly of soft body parts. Globally, they are
known from more than 35 localities to date if the “Orsten”-type preservation in
the Swedish Alum Shale and elsewhere are considered as Lagerstätten. Orsten is
an organic-rich, anthraconitic, concretionary limestone in which phosphatized
cuticle-bearing organisms are exquisitely preserved in three dimensions. In
Laurentia, the richest localities are in the Kinzers Formati on of southeastern
Pennsylvania, the Spence Shale of northern Utah, the Wheeler Shale and Marjum
Formation of western Utah, the Buen Formation of northern Greenland, and the
Burgess Shale of British Columbia. The last is the largest such deposit,
containing about 152 mostly monospecific genera of Middle Cambrian age. Equally
spectacular is the Chengjiang fauna found at Maotianshan in Yunnan, southwest
China, which contains in excess of 70 arthropod-dominated species of Early
Cambrian age. However, for extremely fine morphological detail, Orsten-type
preservation in the Lower Cambrian of England, the Middle Cambrian of Russia and
Australia, and the Upper Cambrian of Poland and Sweden is unsurpassable.
Trilobites
The most abundant remains of
organisms in Cambrian rocks are of trilobites (Fig. 3a–c). They are present in
almost every fossiliferous Cambrian deposit and are the principal tools used to
describe divisions of Cambrian time and to correlate Cambrian rocks. These
marine arthropods ranged from a few millimeters to 20 in. (50 cm) in length, but
most were less than 4 in. (10 cm) long. Although some groups of trilobites such
as the Agnostida (Fig. 3a) were predominantly pelagic in habitat, most
trilobites seem to have been benthic or nektobenthic and show a reasonably close
correlation with bottom environments. For this reason, there are distinct
regional differences in the Cambrian trilobite faunas of the shallow seas of
different parts of the Cambrian world.
See also: Trilobita
Brachiopods
The next most abundant Cambrian
fossils are brachiopods (Fig. 3d–f). These bivalved animals were often
gregarious and lived on the sediment surface or on the surfaces of other
organisms. Brachiopods with phosphatic shells, referred to the Acrotretida (Fig.
3f), are particularly abundant in many limestones and can be recovered in nearly
perfect condition by dissolving these limestones in acetic or formic acids.
Upper Cambrian limestones from Texas, Oklahoma, and the Rocky Mountains yield
excellent silicified shells of formerly calcareous brachiopods when they are
dissolved in dilute hydrochloric acid.
See also: Brachiopoda
Archaeocyathids
Limestones of Early Cambrian age may
contain large reeflike structures formed by an association of algae and an
extinct phylum of invertebrates called Archaeocyatha (Fig. 3m and n). Typical
archeocyathids grew conical or cylindrical shells with two walls separated by
elaborate radial partitions. The walls often have characteristic patterns of
perforations. See also:
Archaeocyatha
Mollusks and
echinoderms
The Cambrian record of mollusks and
echinoderms is characterized by many strange-looking forms (Fig. 3g–l). Some
lived for only short periods of time and left no clear descendants.
Representatives of these phyla, such as cephalopods, clams, and true crinoids,
which are abundant in younger rocks, are rare in Cambrian rocks; but rostroconch
mollusks are known from the Early, Middle, and Late Cambrian at various times in
Laurentia, Australia, Siberia, north China, and Korea. Snails, however, are
found throughout the Cambrian. Discoveries of primitive clams have been made in
Early Cambrian beds, but they are apparently absent from the later record of
life for tens of millions of years until post-Cambrian time. See also: Echinodermata; Mollusca
Corals
Except for rare jellyfish
impressions, the Coelenterata were thought to be unrepresented in Cambrian
rocks. Corals have now been discovered in early Middle Cambrian rocks in Austral
ia. However, like clams, they are not seen again as fossils until Middle
Ordovician time, many tens of millions of years later. See also: Cnidaria
Extinction
The stratigraphic record of Cambrian
life in Laurentia (North America) shows perhaps five major extinctions of most
of the organisms living in the shallow seas. These extinction events form the
boundaries of evolutionary units called biomeres (Fig. 2). Their cause, and
their presence in the Cambrian records of other continents, is under
investigation. At least one of these extinction events, that at the
Marjuman-Steptoean boundary, coincides with a large positive carbon isotope
anomaly in Laurentia, Australia, south China, and Kazakhstan. However, perhaps
it was these periodic disasters that prevented clear continuity in the
evolutionary records of many groups and which led, particularly, to the
discontinuous records of the echinoderms, corals, and mollusks. See also: Animal evolution; Extinction
(biology)
Faunal
origin
One major unsolved problem is the
origin of the entire Cambrian fauna. Animal life was already quite diverse
before Cambrian time. The earliest Cambrian beds contain representatives of more
than 20 distinctly different invertebrate groups. All of these have calcified
shells, but none of the Precambrian organisms have any evidence of shells. There
is still no clear evidence to determine whether shells evolved in response to
predation or to environmental stress, or as the result of some change in oceanic
or atmospheric chemistry. See also:
Precambrian
History
At the beginning of Cambrian time,
the continents were largely exposed, much as they are now. Following some
still-unexplained event, the seas were suddenly populated by a rich fauna of
shell-bearing invertebrates after 3 billion years of supporting only simple
plants and perhaps 100 million years with shell-less invertebrates. See also: Precambrian
Belts of volcanic islands comparable
to those of the western Pacific Ocean today fringed eastern Laurentia, the
Australian and western Antarctic margins of Gondwana, and southern
Volcanism and evaporitic conditions
continued into the Middle Cambrian in Siberia and parts of Gondwana, and
evaporites of this age are also known from northern
In the Late Cambrian, parts of
western Baltica and eastern Laurentia began to show signs of crustal deformation
suggesting that Iapetus, the ocean between Laurentia, Gondwana, and Baltica, was
beginning to close. Crustal deformation was also taking place in southern
Siberia, eastern
Throughout Cambrian time, terrestrial landscapes were stark and barren. Life in the sea was primitive and struggling for existence. Only in post-Cambrian time did the shallow marine environment stabilize and marine life really flourish. Only then did vertebrates evolve and plants and animals invade the land.
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149:559–606, 1992
M. A. McMenamin and D. L.
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A. R. Palmer, Search for
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J. A. Secord, Controversy
in Victorian Geology: The Cambrian-Silurian Dispute, 1986
H. B. Whittington, The
Burgess Shale, 1985
Additional
M. D. Brasier, The basal
Cambrian transition and Cambrian bio-events (from terminal Proterozoic
extinctions to Cambrian biomeres), in O. H. Walliser (ed.), Global Events and
Event Stratigraphy in the Phanerozoic, Springer,
M. D. Brasier, Towards a
carbon isotope stratigraphy of the Cambrian System: Potential of the Great Basin
succession, in E. A. Hailwood and R. B. Kidd (eds.), High Resolution
Stratigraphy, Geol. Soc. Spec. Publ., no. 70, 1992
S. Conway Morris and H.
B. Whittington, The animals of the Burgess Shale, Sci. Amer., 241(1):122–133,
1979
P. J. Cook and J. H.
Shergold (eds.), Phosphate Deposits of the World, 1. Proterozoic and Cambrian
Phosphorites,
J. W. Cowie and M. D.
Brasier (eds.), The Precambrian-Cambrian Boundary,
S. J. Gould, Wonderful
Life, W. W. Norton,
Hou Xianguang, L.
Ramsköld, and J. Bergström, Composition and preservation of the Chengjiang
fauna: A Lower Cambrian soft-bodied biota, Zoologica Scripta, 20(4):395–411,
1991
C. E. Isachsen et al.,
New constraint on the division of Cambrian time, Geology, 22:496–498,
1994
P. Janvier, Vertebrate
origins: Conodonts join the club, Nature, 374:761–762, 1995
E. Landing et al.,
Duration of the Early Cambrian: U-Pb ages of volcanic ashes from Avalon and
D. Walossek and K. J.
Müller, Cambrian “Orsten”-type arthropods and the phylogeny of Crustacea, in R.
A. Fortey and R. H. Thomas (eds.), Arthropod Relationships, Systematics Ass.
Spec. Vol. Ser., no. 55, 1997
H. B. Whittington,
Trilobites, Boydell Press,
G. C. Young and J. R.
Laurie (eds.), An Australian Phanerozoic Time-scale, Oxford University Press,
Melbourne, 1996
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