I really love how you nocked out the anatomy and biological details so thoroughly. are you able to produce stuff like this with enough ease? if so, i MIGHT have a job for you later. (it includes designing alot of well thought out aliens)
BONES!?! What is this earthly heresy?! EVEN A VERTEBRA AND RIBS!?!
Though I do have to say, I admire most of your designs (most of which I saw on your website). As we all know, four limbs is too earthly and cliched, as are jaws, yet you can blend the familiar (eyes and mouth on the head) with the alien (alien mouth types and non-earthlike head shapes, and the un-earthlike eyes which the untrained Human eye cannot even detect as "eyes"). For that, I must praise you.
The jaw bone that we see in today's vertebrates has evolved twice, with both instances being completely isolated from each other. Also, any land animal of a considerable size needs some form of infrastructure, whether it be an exoskeleton or an endoskeleton. as for four limbs, its well within the realm of possibilities. one leg would leave you're animal off balance, and more than eight can get overly complex with little benefit, so that leaves a 25% chance that four limbs will succeeded. Now stack that number, with the number of different species a planet can sustain.