Yes, of course, you're absolutely right that some partial background, even if it's only surface on which the animal to step on will greatly benefit the artwork. Maybe someday I'll get around to add it. It's just that for my project, which is eventually putting all these tyrannosaurids into poster showing their phylogeny, I don't need any kind of background (even if there is one, it will get digitally erased eventually) and thus I'm just saving time by not drawing it. But your point is absolutely correct, and I agree with it!