DEFINITION OF CAMBRIAN

The first geologic period of the Paleozoic Era

and the Phanerozoic Eon, beginning at 544 million years ago

(Ma), and ending 505 million years ago. It is preceded by the

Late Proterozoic Eon and succeeded by the Ordovician Period.

The Cambrian System refers to the rocks deposited during

this period. The Cambrian is named after Cambria, which

was the Roman name for Wales, where the first detailed studies

of rocks of this age were completed.

The Cambrian is sometimes referred to as the age of

invertebrates, and until this century, the Cambrian was

thought to mark the first appearance of life on Earth. As the

oldest period of the Paleozoic Era, meaning “ancient life,”

the Cambrian is now recognized as the short time period in

which a relatively simple pre-Paleozoic fauna suddenly diversified

in one of the most remarkable events in the history of

life. For the 4 billion years prior to the Cambrian explosion,

life consisted mainly of simple single-celled organisms, with

the exception of the remarkable Late Proterozoic soft-bodied

Ediacaran (Vendian) fauna, sporting giant sea creatures that

all went extinct by or in the Cambrian and have no counterpart

on Earth today. The brief 40-million-year-long Cambrian

saw the development of multicelled organisms, as well as

species with exoskeletons, including trilobites, brachiopods,

arthropods, echinoderms, and crinoids.

At the dawn of the Cambrian most of the world’s continents

were distributed within 60°N/S of the equator, and

many of the continents that now form Asia, Africa, Australia,

Antarctica, and South America were joined together in the

supercontinent of Gondwana. These continental fragments

had broken off from an older supercontinent (Rodinia)

between 700 and 600 million years ago, then joined together

in the new configuration of Gondwana, with the final ocean

closure of the Mozambique Ocean between East and West

Gondwana occurring along the East African Orogen at the

Precambrian-Cambrian boundary. Even though the Gondwana

supercontinent had only formed at the end of the Proterozoic,

it was already breaking up and dispersing different

continental fragments by the Cambrian.

Neoproterozoic closure of the Mozambique Ocean

sutured East and West Gondwana and intervening arc and

continental terranes along the length of the East African Orogen.

Much active research in the Earth Sciences is aimed at

providing a better understanding of this ancient mountain

belt and its relationships to the evolution of crust, climate,

and life at the end of Precambrian time and the opening of

the Phanerozoic. There have been numerous and rapid

changes in our understanding of events related to the assembly

of Gondwana. The East African Orogen encompasses the

Arabian-Nubian Shield in the north and the Mozambique

Belt in the south. These and several other orogenic belts are

commonly referred to as Pan-African belts, recognizing that

many distinct belts in Africa and other continents experienced

deformation, metamorphism, and magmatic activity in

the general period of 800–450 Ma. Pan-African tectonic

activity in the Mozambique Belt was broadly contemporaneous

with magmatism, metamorphism, and deformation in the

Arabian-Nubian Shield. The difference in lithology and metamorphic

grade between the two belts has been attributed to

the difference in the level of exposure, with the Mozambican

rocks interpreted as lower crustal equivalents of the rocks in

the Arabian-Nubian Shield.

The timing of Gondwana’s amalgamation is remarkably

coincident with the Cambrian explosion of life, which has

focused the research of many scientists on relating global-scale

tectonics to biologic and climatic change. It is thought that the

dramatic biologic, climatic, and geologic events that mark

Earth’s transition into the Cambrian might be linked to the distribution

of continents and the breakup and reassembly of a

supercontinent. The formation and dispersal of supercontinents

causes dramatic changes in the Earth’s climate and

changes the distribution of environmental settings for life to

develop within. Plate tectonics and the formation and breakup

of the supercontinents of Rodinia and Gondwana set the stage

for life to diversify during the Cambrian explosion, bringing

life from the primitive forms that dominated the Precambrian

to the diverse fauna of the Paleozoic. The breaking apart of

supercontinents creates abundant shallow and warm water

inland seas, as well as shallow passive margins along the edges

of the rifted fragments. It is thought that as rifting separated

continental fragments from Rodinia, they moved across warm

oceans, and new life-forms developed on these shallow passive

margins and inland seas. When these “continental icebergs”

carrying new life collided with the supercontinent of Gondwana,

the new life-forms could rapidly expand and diversify,

then compete with the next organism brought in by the next

continent. This process happened over and over again, with

the formation and breakup of the two Late Proterozoic-

Cambrian supercontinents of Rodinia and Gondwana.

North America began rifting away from Gondwana as it

was forming, with the rifting becoming successful enough to

generate rift-type volcanism by 570 million years ago, and an

ocean named Iapetus by 500 million years ago. Iapetus saw

some convergent activity in the Cambrian, but it was not until

the Middle Ordovician that this ocean experienced major contractional

events. North and south China had begun rifting off

of Gondwana in the Cambrian, as did Kazakhstan, Siberia,

and Baltica. The margins of these continental fragments subsided

and accommodated the deposition of thick, carbonate

passive margin sequences that heralded the rapid development

of life, some of which are now petroleum provinces.

A Middle Cambrian sequence of fine-grained turbidites

near Calgary, Alberta, Canada, has yielded a truly remarkable

group of extremely well-preserved fauna. The Burgess

Shale was discovered by Charles D. Walcott in 1909 and is

thought to preserve organisms deposited in a lagoon that was

buried suddenly in anaerobic muds, resulting in the organisms

being so well preserved that even the soft parts show

fine detail. Fossils from the Burgess Shale and related rocks

have revealed much of what we know about the early lifeforms

in the Cambrian, and it represents the earliest known

fauna. The Burgess Shale has yielded some of the best preserved

jellyfish, worms, sponges, brachiopods, trilobites,

arthropods, mollusks, and first invertebrate chordates.

One of the important steps for the rapid expansion of

life-forms in the Cambrian was the rapid radiation of

acritarchs, which are small spores of planktonic algae such as

green algae or dinoflagellates. The acritarchs served as the

primary source of food and the base of the food chain for the

higher animals that developed. Acritarchs appeared first during

the Late Proterozoic, but 75 percent of all their taxa

became extinct in the Late Proterozoic glaciation. The Late

Proterozoic-Cambrian transition also saw the first appearance

of trace fossils of soft-bodied organisms, showing that

they developed and rapidly diversified in this time period.

Traces of worm paths are most common, where they

searched for food or burrowed into soft sediments.

Shelly fossils first appeared slightly later in the Cambrian,

during the Tommotian, a 15-million-year-long stage added to

the base of the Cambrian time scale in the 1970s. Most of the

early shelly fossils were small (1–2 mm) conical shells, tubes,

plates, or spicules made of calcite or calcium phosphate. They

represent different phyla, including mollusks, brachiopods,

armored worms, sponges, and archeocyathid reefs. The next

major phase of the Cambrian radiation saw calcite shells

added to trilobites, enabling their widespread preservation.

Trilobites rapidly became abundant, forming about 95 percent

of all preserved Cambrian fossils. However, since trilobites

form only 10 percent of the Burgess Shale, it is thought that

this number is biased in favor of the hard-shelled trilobites

over other soft-bodied organisms. Trilobites experienced five

major extinctions in the Cambrian, each one followed by an

adaptive radiation and expansion of new species into vacant

ecological niches. Other arthropods to appear in the Cambrian

include crustaceans (lobsters, crabs, shrimp, ostracods, and

barnacles) and chelicerates (scorpions, spiders, mites, ticks,

and horseshoe crabs).

Mollusks also appeared for the first time at the beginning

of the Cambrian, resulting in the first clam (pelecypod)

by the end of the Cambrian. Snails (gastropods) also

emerged, including those with multiple gas-filled chambers,

which includes the cephalopods. Echinoderms with hard

skeletons first appeared in the Early Cambrian, and these

include starfish (asteroids), brittle stars (ophiuroids), sea

urchins (echinoids), and sea cucumbers (holothuroids).

See also GONDWANA; SUPERCONTINENT CYCLE; VENDIAN.

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