Período Cambriano: Fatos e Informações
O Período Cambriano é o primeiro período de tempo geológico da Era Paleozóica (o “tempo da vida antiga”). Este período durou cerca de 53 milhões de anos e marcou uma explosão dramática de mudanças evolutivas na vida na Terra, conhecida como a "Explosão Cambriana". Entre os animais que evoluíram durante esse período, estavam os cordados - animais com cordas nervosas dorsais; braquiópodes encorpados, que se assemelhavam a amêijoas; e artrópodes - ancestrais de aranhas, insetos e crustáceos.
Though there is some scientific debate about what fossil strata should
mark the beginning of the period, the International Geological Congress
places the lower boundary of the period at 543 million years ago with
the first appearance in the fossil record of worms that made horizontal
burrows. The end of the Cambrian Period is marked by evidence in the
fossil record of a mass extinction event about 490 million years ago.
The Cambrian Period was followed by the Ordovician Period.
The period gets its name from Cambria, the Roman name for Wales, where
Adam Sedgwick, one of the pioneers of geology, studied rock strata. Charles Darwin was one of his students. (Sedgwick, however, never accepted Darwin's theory of evolution and natural selection.)
Climate of the Cambrian Period
In the early Cambrian, Earth was generally cold but was gradually
warming as the glaciers of the late Proterozoic Eon receded. Tectonic
evidence suggests that the single supercontinent Rodinia broke apart and
by the early to mid-Cambrian there were two continents. Gondwana, near
the South Pole, was a supercontinent that later formed much of the land
area of modern Africa, Australia, South America, Antarctica and parts of
Asia. Laurentia, nearer the equator, was composed of landmasses that
currently make up much of North America and part of Europe. Increased
coastal area and flooding due to glacial retreat created more shallow
sea environments.
At this point, no life yet existed on land; all life was aquatic. Very
early in the Cambrian the sea floor was covered by a “mat” of microbial
life above a thick layer of oxygen-free mud. The first multicellular life forms
had evolved in the late Proterozoic to “graze” on the microbes. These
multicellular organisms were the first to show evidence of a bilateral
body plan. These near-microscopic “worms” began to burrow, mixing and
oxygenating the mud of the ocean floor. During this time, dissolved
oxygen was increasing in the water because of the presence of
cyanobacteria. The first animals to develop calcium carbonate
exoskeletons built coral reefs. [Image Gallery: Cambrian Creatures: Primitive Sea Life]
The middle of the Cambrian Period began with an extinction event. Many
of the reef-building organisms died out, as well as the most primitive
trilobites. One hypothesis suggests that this was due to a temporary
depletion of oxygen caused by an upwelling of cooler water from deep
ocean areas. This upwelling eventually resulted in a variety of marine
environments ranging from the deep ocean to the shallow coastal zones.
Scientists hypothesize that this increase in available ecological niches
set the stage for the abrupt radiation in life forms commonly called
the “Cambrian Explosion.”
Fossils of the Cambrian Period
Scientists find some of the best specimens for the “evolutionary
experiments” of the Cambrian Period in the fossil beds of the Sirius
Passet formation in Greenland; Chenjiang, China; and the Burgess Shale
of British Columbia. These formations are remarkable because the
conditions of fossilization led to impressions of both hard and soft
body parts and the most complete records of the varieties of organisms
alive in the Cambrian Period.
The Sirius Passet formation has fossils estimated to be from the early
Cambrian Period. Arthropods are the most abundant, although the groups
are not as diverse as those found in the later Burgess Shale formation.
The Sirius Passet has the first fossil indications of complex predator/prey relationships. For example, Halkieria
were slug-shaped animals with shell caps at either end. The rest of the
body was covered in smaller armor plates over a soft snail-like “foot.”
It is unclear whether they are more closely related to the annelids,
such as modern-day earthworms and leeches, or are a primitive mollusk.
Some specimens have been found in curled up defensive postures like
modern pill bugs. Predator/prey relationships provide intensive
selection pressures that lead to rapid speciation and evolutionary
change.
Burgess Shale fossils are from the late Cambrian. Diversity had increased dramatically. There are at least 12 species of trilobite
in the Burgess Shale; whereas in the Sirius Passet, there are only two.
It is clear that representatives of every animal phylum, excepting only
the Bryozoa, existed by this time.
The largest predator was Anomalocaris,
a free-swimming animal that undulated through the water by flexing its
lobed body. It had true compound eyes and two claw-tipped appendages in
front of its mouth. It was the largest most fearsome predator of the
Cambrian Period, but did not survive into the Ordovician.
The earliest
known chordate animal, the Pikaia, was about 1.5 inches (4 centimeters) long. Pikaia
had a nerve cord that was visible as a ridge starting behind its head
and extending almost to the tip of the body. The fine detail preserved
in the Burgess Shale clearly shows that Pikaia had the segmented muscle structure of later chordates and vertebrates. Haikouichythes, thought by some to be the earliest jawless fish, were also found in the Burgess Shale.
A mass extinction event closed the Cambrian Period. Early Ordovician sediments found in South America are of glacial origin. James F. Miller
of Southwest Missouri State University suggests that glaciers and a
colder climate may have been the cause of the mass extinction of the
fauna that evolved in the warm Cambrian oceans. Glacial ice would have
also locked up much of the free ocean water, reducing both the oxygen in
the water and the area available for shallow water species.
Related pages
Time periodsPrecambrian: Facts About the Beginning of Time
Paleozoic Era: Facts & Information
- Cambrian Period: Facts & Information
- Silurian Period Facts: Climate, Animals & Plants
- Devonian Period: Climate, Animals & Plants
- Permian Period: Climate, Animals & Plants
- Triassic Period Facts: Climate, Animals & Plants
- Jurassic Period Facts
- Cretaceous Period: Facts About Animals, Plants & Climate
Dinosaurs
- A Brief History of Dinosaurs
- Allosaurus: Facts About the 'Different Lizard'
- Ankylosaurus: Facts About the Armored Dinosaur
- Apatosaurus: Facts About the 'Deceptive Lizard'
- Archaeopteryx: Facts about the Transitional Fossil
- Brachiosaurus: Facts About the Giraffe-like Dinosaur
- Diplodocus: Facts About the Longest Dinosaur
- Giganotosaurus: Facts about the 'Giant Southern Lizard'
- Pterodactyl, Pteranodon & Other Flying 'Dinosaurs'
- Spinosaurus: The Largest Carnivorous Dinosaur
- Stegosaurus: Bony Plates & Tiny Brain
- Triceratops: Facts about the Three-horned Dinosaur
- Tyrannosaurus Rex: Facts about T. Rex, King of the Dinosaurs
- Velociraptor: Facts about the 'Speedy Thief'
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