The sudden appearance of shelly fossils between about 544 to 542 Ma is the most astonishing feature of biological evolution, especially as representatives of every modern animal phylum (and some which have vanished) appear at that time. A means to explain this short-lived blossoming has eluded palaeontologists. Part of the problem is that the record of the immediately preceding Neoproterozoic Era cannot resolve whether the phyla sprang up at the same time as they developed hard parts, or had been evolving as flaccid forms for much longer. Another aspect is the difficulty in accounting for the sudden adoption of calcium carbonate and phosphate hard parts. It seems inescapable that the issue of hard parts, which is really what the “Explosion” is all about, cannot be separated from the chemistry of seawater at the time.
A new insight into what was going on was presented at the October GSA meeting in Denver by John Grotzinger and colleagues at MIT, who have been examining drill cores through the Precambrian-Cambrian boundary beneath the south Omani oil fields. The Late Neoproterozoic basin in which the deposits began to form was a semi-enclosed basin, dominated by stromatolitic carbonates. Seawater in it contained excess calcium and carbonate ions. Periodically, the basin was cut off and evaporites began to form; it became hypersaline. In the cyclical sequence the very earliest carbonate-shelled organisms (Cloudinia and Namacalathus) left fossil remains. However, in cycles of earliest Cambrian age they simply disappear, not merely in Oman but world wide. Moreover, rocks from which they are missing show abnormally light d13C, generally interpreted as a result of mass extinction. The demise of two organisms, albeit the only ones that could have left any record, may not seem very dramatic. But Grotzinger and colleagues suggest that a sudden extinction could mark a critical period in evolution that both reduced the population of all organisms and sterilised ecological niches for future adaptive radiation. Interesting, but still not explaining why hard parts were adopted to become so very necessary in subsequent animal evolution.
Source: Kerr, R.A. A trigger for the Cambrian Explosion? Science, v. 298, p. 1547.