http://www.physorg.com/news106370906.html
It makes more sense that a cartilaginous appendage would be found on an early coastal life form before things started drying up, let’s say, requiring that same life form to adapt to longer bouts with gravity subsequently stressing their limbs of cartilage.
It is recommended that people exercise their limbs when aging to improve their muscle mass on the bones which in turn contributes to calcium strengthening the bones that the muscle exercised is on. My apologies for talking in what seems like circles here, but one can appreciate the known linkage here. Use them, loose them, or modify them in this case through exercise.
By stressing the muscles that are attached to the bones in a positive gravity, while sloshing around in the early coastal muck, that early creature was certainly affecting its own change through the natural selection process. One can only presume at this time that there was something in the diet to be had near the coastline or that the coastal environment was more conducive to that creature’s existence than remaining in the water fully buoyant as a cetacean life form or shark is today. Food intake would supply energy and chemical components to affect such a change.
It seems more reasonable to suggest that a life form is able to change a small portion of it self, given the energy, coding, and resources necessary to affect physical change, than make a dramatic alteration or transformation all at once. I suspect that there have been many alterations in the early environments of ancestral creatures of varying degrees which have resulted in diversified present day life forms observed.