This period lasted about 53 million years and marked a dramatic burst of evolutionary changes in life on Earth, known as the "Cambrian Explosion. 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 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 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.
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This period lasted about 53 million years and marked a dramatic burst of evolutionary changes in life on Earth, known as the "Cambrian Explosion. 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 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 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. 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. A fossilized Spartobranchus tenuis from the Burgess shale in Canada. The animal contains features of modern acorn worms and modern tube worms called pterobranches.
Image credit: JB Caron At this point, no life yet existed on land; all life was aquatic. These multicellular organisms were the first to show evidence of a bilateral body plan.
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. 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.
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. For example, Halkieria were slug-shaped animals with shell caps at either end.
Some specimens have been found in curled up defensive postures like modern pill bugs. The fearsome meter-long super-predator Anomalocaris. 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.
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.
Prog Biophys Mol Biol. Epub Mar Cause of Cambrian Explosion - Terrestrial or Cosmic? Electronic address: ejsteele cyo. Comment in Prog Biophys Mol Biol.
See Article History Cambrian explosion, the unparalleled emergence of organisms between million and approximately million years ago at the beginning of the Cambrian Period. The event was characterized by the appearance of many of the major phyla between 20 and 35 that make up modern animal life. Many other phyla also evolved during this time, the great majority of which became extinct during the following 50 to million years. Ironically, many of the most successful modern phyla including the chordates , which encompass all vertebrates are rare elements in Cambrian assemblages; phyla that include the arthropods and sponges contained the most numerically dominant taxa taxonomic groups during the Cambrian, and those were the taxa that became extinct. The first life-forms were small and simple. Later forms were more complicated and diverse.
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