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Thank you for your reply!

In any event, it's difficult to see how a new body form, if such shows up, is going to start developing enough descendants of various types to get recognized as a whole new phylum. The precursors would probably get gobbled up before the process developed very far.

That counter-argument is why I mentioned the periods of mass extinctions when there was plenty of opportunity.

At any rate, according to the NASA guy, a sweeping hormonal change in a population could cause an entire new body plan in sufficient numbers to emerge and survive.

If it happened back then, then what would prevent a sweeping hormonal change in the population of humans which would cause enough with 3 eyes, 4 arms and antenna to emerge and survive?

People today would not call "that" a new body plan per se but they might if it happened to worms. Nevertheless, I believe it makes the point that something must be preventing sweeping changes to regulatory control genes after the Cambrian. And because of the eyes developing concurrently in both vertebrates and invertebrates, I suggest it is the control genes themselves resisting change.