An article in Nature attempts to account for the Cambrian explosion in part as follows:
Formation of the ‘Great Unconformity’as a trigger for the Cambrian explosion
From the Abstract:
. Here we use new stratigraphic and geochemical data to show that early Palaeozoic marine sediments deposited approximately 540–480?Myr ago record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust.
The resultant globally occurring stratigraphic surface, which in most regions separates continental crystalline basement rock from much younger Cambrian shallow marine sedimentary deposits, is known as the Great Unconformity.
Thus, the formation of the Great Unconformity may have been an environmental trigger for the evolution of biomineralization and the ‘Cambrian explosion’ of ecologic and taxonomic diversity following the Neoproterozoic emergence of animals.
The last two explain-aways to whistle across this desk were: deposits of sediment enabled life forms to build shells and a natural selection war occurred around eyes. As to the latter, well, perhaps so, but the suddenly emerging Cambrian eyes were very advanced, even by today’s standards. Wonder what the next explain-away will be after this.
They are still looking for something easy and it isn’t there.
See also: New Scientist on the subject.