Thanks for your reply.
I wouldn’t classify 40 million years as an explosion either, but I’m not an evolutionary biologist, so what do I know. Anyway, even a novice like me can speculate what mammals might have looked like 40 million years ago based on the fossil record and compared to extant mammal specimens we have today. From this I acknowledge that it is a sufficiently long time for species to undergo profound changes.
NosyNed writes:
I don't see what is particularly wrong with the "hard shell" theory in any case. There are traces of multi cellular life back to around 600 million years and one would not expect much to remain of worms after 600 million years would one? Hard parts had to come in at some point. Obviously (I hope it is), that would produce a quantum leap in preservation even if there was no real change in the diversity or number of living things around at the time.
Regarding the hard shell theory, there does seem to be much speculation among experts in the field, which is why I posed the question initially.
Consider the following:
"Beyond the latest Precambrian there occurred what has appropriately been called an explosion of life forms, many of which seem to be extraordinary experiments in animal design. For a long time it was supposed that the idea of a sudden rise of complex forms of life in the Cambrian Period (on the Proterozoic-Phanerozoic border) was in fact a fallacy created by the nature of the fossil record, and that it simply represented the time when the first shelled creatures began to appear. Since shells are hard objects, they are much more capable of being preserved than soft-bodied creatures.
However, from recent research it really does look as though the Earth presented these early organisms with a "clean sheet" upon which to develop all manner of designs."
(Dr. David Norman, Prehistoric Life: The Rise of the Vertebrates, pub. Boxtree limited, 1994, p. 32) Dr. Norman is Director of the Sedgwick Museum and lectures on paleontology and evolution at the University of Cambridge.
BTW, If there is another thread where the pre-Cambrian "hard shell" theory has been discused, please provide the link
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