Again, I don't know here, so don't want to sound like I am stating fact. However, it seems to me, that the vertebrals are not "open" as they are the backbone of the tortoise. Certainly more pliable in a hatchling, but I would think fully formed and intact along the length. The costals, on the other hand, are modified ribs and go through a process of ossification to fill in to create a complete bony box.Ive been thinking the exact same today regarding your 2nd paragraph.
What I'm struggling to understand is this.....
If you had 2 torts from the same clutch, grew one slow and one fast and both filled over the same period of time then the one that grew faster would more likely show pyramiding because the new keratin growth could be pushing beyond the filling, this causing the downward bone growth. Think what I'm saying is, is there a point where we can grow them faster than the filling, are we playing catch up when they grow too fast, hence the variation form none to very minor pyramiding within the same clutch and conditions. Are we on the brink with keratin growth to filling.
I need to think about this more, it's opened a lot of new thoughts.
Do we have a picture of the vertebrals when not filled?
I have a 3 month old burred in the garden, will that show us?
So the way the spine grows from the beginning will set a direct to the vertebral bone plate as it grows. Since the seams are in a different location than the scute edges, and the areolae of the scute is fixed to its respective place on the vertebral bone, the bone itself is being deflected into a "tipped" plane.