Oddly enough, this only started working out when I decided to go small:
I was trying to figure out the Blacktooth’s silhouette, and couldn’t make it fit on the page. I had decided that I’d established (at least in part) that it lives underground and had also given it something of a low-slung carriage, but fitting the long neck and the (implied) long body was giving me pause, until I turned the book around and just went portrait instead of landscape:
My first impulse was to do something that crawled, but I felt that the upright posture was really important, if nothing else because it gave the dragon the opportunity to show off its sharklike head shape.
So I sort of compromised. Its belly definitely drags the ground, but it holds its head up. I also went and shortened the teeth, so the idea of it actually biting something seems a little more possible without its head flipping back on itself. I think I want to make the limns a little more flipperlike or something, almost like a mole’s: