Donnie skeletal anatomy! Click for better quality
Alright so softshell turtles right. They funky. It’s a lot more obvious that their shells are part of their ribs, as you can actually see the faint outline of their fused vertebrae and ribs on their shells. The reason their shells are so soft is because the edges and much of the lower half of their shell is actually all just cartilage and skin. Their plastron are weird, because the bone plates are largely separate and pointy, held together by harder cartilage the same way human ribs are attached around the sternum.
Donnie was weird to figure out on account of the cartilage lower half of his softshell, but I tried to compromise with his lower back vertebrae being unfused to still allow for the higher mobility required for his waist. Another fun fact about softshell is that they don’t actually have scutes or scales, but because of the look of his plastron and consistency with his brothers in the show, I’m just going to pretend that Draxum mixed in some DNA to let him grow scales for extra protection even if he has less coverage than his brothers. He can “breathe” water by absorbing oxygen through the skin of his throat, not actually by inhaling it. While most turtles can do this to a point, soft shells can stay quite active while doing so.
Personally I like to think of Donnie’s softshell as both a weakness and a strength depending on the situation! While more vulnerable, his skin can heal faster than scutes, and if he sustains blunt damage to his shell he can more easily disperse the force of it because of the give to his cartilage, rather than have the force crack his shell and cause a much more traumatic injury. Kind of like how seatbelts have some give to them to ease into a stop instead of giving someone whiplash.
Just some personal head canons! Feel free to disagree or expand upon :)
