Indeed, there are certain changes in body plans whose description as a new phyla or not seems to be rather subjective. A breed of humans with three eyes, an exoskeleton, antenna and wings like a bumble bee might be difficult to classify - but I'd probably call it a new body plan.
But the point I was raising earlier about the “no new phyla after the Cambrian Explosion” goes to a different aspect – what appears to be a resistance to certain mutations in master control genes.
In the Gehrig example, the lowest life forms have no eyes and the eye gene with many sites has exponential possible combinations of amino acids. And yet the same combination of amino acids are selected for vision in all animals - vertebrates and invertebrates - across phyla. The gene experiments between the mouse and fly make the point rather vividly.
In the NASA article, the author suggests that microevolution cannot account for the fossil record. He discusses alternative models for macroevolution (genetic drift, etc.) - but strongly suggests that environmentally linked hormonal induced changes in the control genes can create the observed effect of quickly emerging and successful phyla.
When I take the Gehrig view in combination with the NASA article, it makes sense to me that the phenomenon of “no new body plans” could be easily explained by a stability – or immutability – of control genes setting in after the Cambrian. Without such an immutability – considering the intervening mass extinctions - one is left contemplating why no new body plans. The interest by the NASA scientist (and others) indicates this is serious inquiry.
If the NASA scientist is correct, then the environmental conditions which caused sweeping hormonal changes during the Cambrian explosion have not since recurred. There ought to be some evidence of such a one-time event either in the cosmos or the geologic record.
But with or without such evidence, highly immutable control genes not only help to explain what is observed in the fossil record and in the laboratory – but also indicate that evolution is not directionless. Direction in evolution goes against the notion of random mutations in evolution theory. Without evidence of natural causation, direction (immutability of control genes) also can be argued by intelligent design supporters.
The two phyla with durable skeletons that do not appear in Lower Cambrian time, the Chordata and Bryozoa, may have been represented then by soft-bodied lineages, for they have body plans that do not require durable skeletons. Indeed, the chordates appear in the Middle Cambrian, while durable chordate skeletons are not known until the Late Cambrian (Repetski, 1978).I really, really, really don't like old dates on assertions that there are no fossils for this or that. It's almost self-discrediting on the face of it, even before I bother to go check.
I don't really need to go back and look in this case, either. We've had FR threads on findings of "deep roots and tiny prototypes" for Cambrian body plans.
I'm feeling lazy, so I'll wait for the challenge before I go get it.