How did the paleolithic man managed to get loads of magnesium?

I've been searching for magnesium-rich foods to take instead of supplements, and they appear to be in very short order: flaxseed, brazil nuts, spinach, a couple of very specific others, and pretty much the rest is all neolithic food.

How did the paleo man, with no possibility of consuming these very specific magnesium-rich foods, managed to get his high intake of this mineral - 700mg according to Primal Body, Primal Mind?

Thanks!