>A giraffe grew a long neck to reach the tops of trees for food.
I had the great good fortune and fun of sharing a biology conference gig with embryological specialist Dr Jack Cohen in New Zealand last week (who, BTW, constructed the aliens in various hard sf novels, e.g. LEGACY OF HEOROT by Nivem, Pournelle and Barnes). We went to the zoo and looked at the giraffes. Did you know that these lovely beasts walk and run by lifting both legs on one side, then on the other? Jack pointed out that unlike most animals giraffes don't shove themselves along with powerful hind-quarters, they *pull* themselves forward with those high, immense muscles at their shoulders. To make a faster giraffe, selection needs to conserve and encourage more and more extra height at the front end. So how does the poor thing drink, with its head stuck up there in the clouds? Well, it could have a trunk, like a 'phant, or kneel down in a rather vulnerable fashion. Or have a *really really* long neck... As a handy spandrel, it can then also reach those tasty leaves in the higher branches.
This cuts both ways, of course, but I'm inclined to think that this is the likely vector through the morphological landscape, rather than the long neck constraining the gait and its enabling musculature.