Thank you both for your thoughts, which I agree with.
My understanding of evolution is evolving and I learn more every day. It is a subject that I, living in Australia as I do, have not needed to address. So it is a huge learning curve for me.
I can see that that period of the Cambrian age would be known as a "radiation". Tell me if my understanding is incorrect, but I expect this was the outcome of the development of complex cells and hence the formation of animal life (0.6 GYA). I did not know that the subsequent mass extinctions would be known as a "radiation". Have I got that wrong?
No additional phyla have been formed since the Cambrian radiation and each subsequent cataclysmic saw massive removal of species, with each event survived by species that possessed the appropriate mutations.
Doug