> It could have served that purpose, but why did they evolved different design in neck-structure,
I don't doubt that their long necks and tails could serve various purposes, but adaptations aren't like intelligently designed tools (in which case the purpose of a particular tool is set beforehand), whatever use they could possibly have (assuming they're not being prevented from that use by the animals behaviour) would have some effect on the loss/retention/exageration of that feature.
And an SA:V ratio isn't something that the animal's behaviour would limit, that is, it's use lies in its very presence (i.e. we can argue over wether Triceratops used it's horns for fighting predators or for intra-specific confrontations, but an elephants ears help it cool off to some extent wether the elephant uses them to best advantage or not). So wether or not the necks/tails originally evolved for some other reason, or diverged to be useful in different ways to different animals, there still must be some bias in favour of any bau-plan (wow, i actually got to use that term in a sentence!) that reduced SA:V in large animals, whatever their metabolism?
I didn't mean to say that would be the only use, or the only reason they evolved, after all SA:V considerations aren't mutually exclusive with any of the alternative uses you mentioned. IIRC, the context of my reply was to imply that whatever other uses sails, ridges, and humps had, SA:V was probably a consideration as well.
I just always thought the shape might help to some significant extent. SA:V considerations are one of the very first things we learn about in biology, after all.
Maybe it's even one of the reasons sauropods included the largest land vertebrates we know of? We don't see ornithischians growing to the sizes of the bigger sauropods. They are all in roughly the same size groups that mammals have spanned, aren't they? (note: im not looking at any data as i type that sentence, feel free to correct me (as if i have to ask here!).)
(And if you look at outlines of Dicraeosaurs, their necks may be relatively short, but that is just "relatively").