I'll make some wild guesses. I don't have any supporting references, but since bones have the highest concentration of Mg in the body and since animals love to chew large bones, my guess is that the highest concentration of Mg in the bones (or perhaps the highest concentration of edible Mg) is near the surface where it can be gnawed off. Thus, bone chewing would be not just for pleasure, nor just for jaw exercise, but also for Mg nutrition. Also, canids are known to bury bones, where they ferment and become more easily chewed, so that perhaps more of the Mg can be obtained and absorbed. Of course, bones would have been more Mg-rich eons ago when soils were richer in Mg.
There are of course also other sources of Mg, such as green plants, seafoods, sea salts and anadromous fish like salmon, all of which has been mentioned before in the forum. Plus, sacred healing lakes/ponds/springs tended to be rich in Mg and/or sulfur (think epsom salts). Andadromous fish tend to be sacred also, especially the salmon.
In Irish pagan tradition, the salmon was a magical wise being that when consumed could bestow those who ate it with wisdom and the gift of prophecy (foresight). Thousands of years later, scientists rediscovered the brain-boosting powers of wild salmon.
I don't think it's just coincidence that if one reads about the old ways, old myths, etc., that everything seems to fall into place (or maybe I've got it completely wrong
). Not everything old is superstition, foolish barbarity or made-up nonsense. Some of it is effective practices based on time-tested experience. One can also learn useful things by studying wild animals, which the ancients understood and many scientists that observe wild animals have found out.
The modern notion that some seem to have of chewing little or nothing and juicing, blenderizing, grinding or swallowing lots of foods forever, based on bizarre theories, makes the teeth into useless ornaments and makes no sense to me. I can see maybe doing such things when one is seriously ill or underweight or has very poor digestion, but not for the longer term, unless perhaps one is toothless, and even then there are dentures.
"One must attend in medical practice not primarily to plausible theories but to experience combined with reason." - Hippocrates, Precepts