I wonder how many cryptocarpic cacti there are? That's really a curious means of protecting the seeds. Are they all Mammillarias? Do they all have a common ancestor, I wonder? If other plants outside of cacti developed the same trait, then I'd think it would have been a response to environmental pressure. To me, it would indicate a long period of time, though it might be limited geographically. Cataclysmic events don't give life forms much of a chance to adapt.
