"It hasn't been clear why," says Don Canfield of the University of Southern Denmark in Odense. But Canfield and his collaborator, James Farquhar of the University of Maryland in College Park, have a theory to explain it. It's a murky story with a moral: never underestimate the power of a scum-sucking, ocean-bottom worm."
"The planet has long been seen as a driver of evolution. But this, say Canfield and Farquhar, is an important example of how animals, too, can create a global geochemical signal that, in turn, twists evolution's corkscrew. "The Cambrian explosion needs to be viewed as not the consequence of geochemical perturbations, but the cause of them," says Nick Butterfield of the University of Cambridge, UK, who was not affiliated with the study."
Before these sea-floor animals began their steady churn, sulphate — arriving in seas in the run-off from rivers — would largely be turned into hydrogen sulphide by bacteria living in the ocean floor. The sulphide would then be converted to pyrite (FeS2), which, once buried, removes the sulphate from the system. Once bioturbation turned on, however, oxygen in the deep ocean could mix more freely with the sediments, allowing bacteria and other processes to recycle pyrite and turn it back to sulphate. This excess sulphate would have reached a saturation point, giving rise to the formation of gypsum deposits — a mineral that, along with sulphate levels, also happened to rise in the rock record around this time.
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According to this website: "The Cambrian Period occurred 543-490 million years ago.The Cambrian Period marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. It was once thought that the Cambrian rocks contained the first and oldest fossil animals, but these are now to be found in the earlier Vendian strata."
"Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa (Bryoza were "moss animals," and aquatic organisms, living for the most part in colonies of interconnected individuals. A few to many millions of these individuals may form one colony. Some bryozoans encrust rocky surfaces, shells, or algae. (source:http://www.ucmp.berkeley.edu/bryozoa/bryozoa.html