Final answer:
The rise in diversity of animal phyla during the Cambrian explosion is attributed to factors like environmental changes, ecological interactions, and genetic developments like the evolution of Hox genes.
Step-by-step explanation:
Which is not a reason for the rise in the diversity of animal phyla during the Cambrian is a question that taps into theories about the Cambrian explosion. Some theories suggest that environmental changes like rising atmospheric oxygen levels and increases in oceanic calcium concentrations were factors. Additionally, the availability of a vast continental shelf with numerous shallow lagoons provided the habitat diversity necessary for the emergence of varied animal forms.
Another theory posits ecological interactions such as changes in the food web and predator-prey dynamics as drivers. Furthermore, the evolution of Hox genes could have led to increased morphological complexity, spurring on diversification. However, the rise in diversity cannot be attributed to a single factor and may rather be a combination of various interrelated causes.