<P> At the start of the Ediacaran period, much of the acritarch fauna, which had remained relatively unchanged for hundreds of millions of years, became extinct, to be replaced with a range of new, larger species, which would prove far more ephemeral . This radiation, the first in the fossil record, is followed soon after by an array of unfamiliar, large, fossils dubbed the Ediacara biota, which flourished for 40 million years until the start of the Cambrian . Most of this "Ediacara biota" were at least a few centimeters long, significantly larger than any earlier fossils . The organisms form three distinct assemblages, increasing in size and complexity as time progressed . </P> <P> Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud - filled bags, or quilted mattresses--one palæontologist proposed that the strangest organisms should be classified as a separate kingdom, Vendozoa . </P> <P> At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs (Kimberella), echinoderms (Arkarua); and arthropods (Spriggina, Parvancorina). Still, debate exists about the classification of these specimens, mainly because the diagnostic features that allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the ediacarans . However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal . These organisms are central to the debate about how abrupt the Cambrian explosion was . If some were early members of the animal phyla seen today, the "explosion" looks a lot less sudden than if all these organisms represent an unrelated "experiment", and were replaced by the animal kingdom fairly soon thereafter (40M years is "soon" by evolutionary and geological standards). </P> <P> Paul Knauth, a geologist at Arizona State University, maintains that photosynthesizing organisms such as algae, may have grown over a 750 - to 800 - million - year - old formation in Death Valley known as the Beck Spring Dolomite . In the early 1990s, samples from this 1,000 - foot thick layer of dolomite revealed that the region housed flourishing mats of photosynthesizing, unicellular life forms which antedated the Cambrian explosion . </P>

What was the cause of the cambrian explosion