The question is raised , how would we go about quantifying the amorphous concept of "diversity"? Is it an attribute of evolving species in selected environments that is conserved? I posit that it is conserved and though it is true that a great deal of animal forms came into existence in a relatively short space of 25 million years, these forms came into being at a time when the genetic information for all animals consisted of many more shorter segments of nucleotides. It seems intuitive that these many number of shorter segments would allow for the emergence of ready variations in species. As animals evolved and early forms competed, those which acquired the most beneficial traits for survival in the early environment of development (the seas) were more likely to survive and radiate. So the ability to radiate into new environments is directly facilitated by the species having the genetic complement necessary to survive in those new environments. So as time went on the descendant species combined survival traits of their ancestors or of parallel species that for one reason or another failed to survive, being so possessed of survival genes these species in essence harbored the diversity that formerly resided in distinct species in fewer descendant species. The diversity is conserved but in so being conserved, the diversity is difficult to extract from subsequent rock strata analysis of "apparent" complexity in living forms that developed in the hundreds of millions of years since the Cambrian "acceleration". Where I don't use the term in quotes to describe evolution increasing in rate but rather acceleration describes the increased ability for animal forms to survive in their nascent environments as through selective processes they evolve traits that are conducive to survival in the environment.
The engine of this acceleration may be for example locked in a radically different view of reproduction among early Cambrian species than we are aware of now. We know that many species exist in "ring" relationships, animals like fish, birds and frogs offer clear examples of divergent and convergent evolution of species along a geographic path. The existence of ring species with differential abilities to mate with members of populations that are geographically segregated, if run backward provides a perfect explanation for what might have existed among ALL species of early simple animal forms in the Cambrian era seas. What if these early forms were similar in their reproductive apparatus more than they were in their physical apparatus? Namely, what if what we "see" as obviously different creatures in the ancient rock strata , are creatures which actually could share genetic information through mating either through intention or by accident. We know that the evolution of internal methods of gestation comes almost 150 million years later, in a population of organisms with similar genetic mating apparatus it should not be surprising at all that a riot of physical forms could arise, as only with the emergence of dominant strategies for survival in these physical forms could less robust forms be weeded out, as it were through selection and continued genetic isolation.
As genetic complement increased in the diversifying species the likelihood of viable cross breeding between species with different physical appearance would go down until we have what exists today. Less apparent diversity but highly specialized and robust species thus representing the early diversity within existing species. I am going to give these ideas some more thought to determine some tests to the hypotheses.