@Markw84 are these areas of differential bone and scute growth explanations from your own observation, or from published works that come to mind as you review @Anyfoot 's post? Maybe both? Could you please offer the sources from your readings?
I am able to make a slow motion video in my minds eye of the bone filling-in/overtaking the shell's marrow from your explanation. It bears out precisely for a weird type of pyramiding seen by many in developing Pyxis planicauda. In them, as they grow, the side of the coastal scutes nearest the middle of the carapace can pyramid while the rest of the shell does not, giving them a somewhat two keeled look. Thanks much for the visually rich narrative. Exactly the sentences that I made bold. It's an explanation that fits the observation with P. planicauda.
Will,
The dearth of research about tortoises is daunting! There are no published papers I know of that explain this. Look at pyramiding. There is no place in the world where anyone has ever published anything on the physiology of pyramiding. It is often simply referred to as a mystery no one knows. My theory is the only one I have ever seen or heard of.
So it is the study of many, even marginally related subjects coupled with my own observations that leads me here. General physiology, anatomy and biology. Veterinary medicine. Orthopedics and orthodontics. Embryonic development - and there are some papers on tortoises/turtles there. Anything I can find on keratin growth. Meteorology, passive heating, electromagnetic radiation, and quantum mechanics. Construction. Etc, etc. etc. All of these and more have been areas of quite intensive study that has help piece things together on the pyramiding side. Mix that with decades of intensely watching tortoises grow and a mindset that HAS TO KNOW not just how things work but why!
General physiology in how bones are formed and grow is a lot of what I believe you are asking. Couple that with chelonian anatomy, and how keratin grows. Then lots of observation and theories seeing which ones prove false and don't fit all situations I run into, and which one stands.