Some quickish muscle drawings, as people have been requesting wyvern anatomy and I myself have to figure out what wyverns in Aerie will look like. They'll have to exist alongside the larger hexapods without competing for the same niche. And will most likely be less intelligent and more colonial than their six-limbed cousins. A good comparison between the wyverns and the hexapods in Aerie would be that between wolves and coyotes. Farmers don't like either of them, but one is smaller, more troublesome, and much more numerous.
There are multiple ways to structure a wyvern. Whether they're bird, bat or pterosaur based.
Both of these are very bird like, with keeled sternums, coracoids and furcula.
The first wyvern walks like a bat or a pterosaur would, and can gallop quite quickly when using its wings as forearms. It's thoracic vertebrae is only partially fused, leaving the spine semi- flexible. I imagine this guy as gliding and climbing a lot. This dragon is what an early wyvern could look like when transitioning from a hexapod to a four limbed biped, depending on the phylogeny.
The bottom wyvern is essentially a modified heron, with a birds skeleton. This guy is smaller and can take off much more quickly. He's a bit more like a medieval wyvern with a snakish neck and tail, and a short bulbous midsection.