In the simplest sense, the Cambrian Explosion was a relatively sudden proliferation of animal types. The number of new species involved in this event is unknown, but it was several thousand at most and perhaps far less than this. The extraordinary aspect of this event was that the new species were spread among many new body plans. As we have said, each body plan defines a higher taxonomic category, such as a phylum or class. The Cambrian Explosion thus involved a large number of higher taxa, each of which was composed of just a few species. But are we simply seeing the advent of effective fossilization, rather than a real diversification "event"?
We recognize the Cambrian Explosion as such for a simple reason: We see a large number of fossils suddenly appear in the fossil record. But are we seeing a genuine flowering of new forms, or do the fossils merely mark the first appearance of skeletons in groups that had already been long established?
In other words, is the Cambrian Explosion merely an artifact of a very imperfect fossil record? Skeletons make fossilization possible; it may be that the actual diversification of body plans that appears to mark the Cambrian Explosion actually took place long before but is invisible to us because it took place among small animals without skeletons, which left no fossils.
This latter view can be considered the null hypothesis. What if there really wasn't a Cambrian Explosion at all? It may be that the various animal phyla accumulated in gradual fashion over the last billion years of the Pre-cambrian, evolving one from another, but doing so without leaving any identifiable fossil record. It was thus only the evolution of large size, and of skeletons allowing the preservation of fossils, that accounts for the "Cambrian Explosion."
Whether the Cambrian event included the diversification of body plans or consisted simply of the first evolution, by these various body plans, of skeletons and large size is a moot point. Something stimulated the evolution of many large animals with skeletons in a brief period of geological time. Furthermore, M. McMenamin and R. McMenamin, in their 1990 book The Emergence of Animals, emphasize that mineralized skeletons—and especially shells—profoundly influenced the evolution of new body plans. A wide variety of forms use their shells not only for protection but also as an integral part of feeding. Brachiopods and bivalves (both invertebrates with two shells) use the shell as an integral part of a filter-feeding process. It is hard to see how the basic body plan of each of these groups could have formed before shells did.
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