The giant theropod Spinosaurus gets all the attention for being properly and dramatically enhanced with a huge bony sail on its back, but it’s far from the only dinosaur with them. Most intriguingly it’s far from the only dinosaur from the Cretaceous of North Africa with one: both the sauropod Rebbachisaurus and the iguanodontid Ouranosaurus also had sails.
The term ‘sail’ is often used to cover a multitude of different, if superficially similar, structures seen on the backs of various tetrapods (think of Dimetrodon) and doesn’t really have any strict definition beyond the fact that the neural spines are typically elongated. Among the dinosaurs one could argue for sails in thinks like the dicraeosaurids, Becklespinax, Acrocanthosaurus and perhaps even Diplodocus (though in this case due to the extra spines that appear on top of the back, rather than elongate vertebrae). It’s also largely a function of interpretation and balance – plenty of ornithopods have tall neural spines, but it would be odd to say they all have sails, so it has to be something quite special to classify when talking about them and thus Ouranosaurus steps up.
I tend to avoid covering ornithischians on here simply because I really don’t know that much about them and sadly don’t have the time to do the necessary reading, and so again this is going to be detail-lite I’m afraid. Ouranosaurus is a very typical iguanodontid, with all the classic features that one would expect of a member of this group (and indeed is a very close relative of Iguanodon, so really no surprises there). It is pretty well know as although only two specimens have been recorded (well, according to ‘The Dinosauria’) one is near complete giving us a very good idea of what the animal looked like as exemplified in the mount here, (inevitably from Fukui as indeed is the little life restoration at the top).
This was a big animal (at least by modern standards and even among the ornithopods) being about seven metres long and weighing a few tons, and critically, or at least, interestingly living in a pretty hot and possibly arid environment. This brings us back to the sail itself and the rather inevitable hypotheses that have been put forward for its function: as a heat controller, as a display structure or to help support a fat store. Almost equally inevitably none of these really stand up to scrutiny very well and what little evidence has really been put forward for any of them is really quite equivocal (e.g. if heat control was so essential they why did other, bigger dinosaurs not produce them living in equally hot environments, and why is there no evidence of especially high numbers of blood vessels to transfer said heat to the sail).
Right, I’ll cut myself off there before I descend into a highly uninteresting discussion of sail evolution. Doubtless a few people would find it interesting and I’ll probably come back to it sooner or later, but I really don’t have the time right now. I’m back off into the field this week to hunt for more dinosaurs (hooray) and with a very special paper coming out soon (and indeed quite possibly while I am away frustratingly).
Dave Hone's Archosaur Musings 
I’m interested in the sail more as a symptom of what must have been a common environmental pressure on the animals that lived there. I almost wonder if some kind of mimicry was being attempted. You see the same dynamic between Dimetrodon and Edaphosaurus.
Its not mimicry because Ouranosaurus and Spinosaurus arent contemporous. Ouranosaurus and Suchomimus are however, but the sails aren’t as similar.
Well there may be some overlap with odd speciemns, but yes in general Ouranosaurus overlaps more with the temporal / geographic range of the less-exaggeratedly-spined Suchomimus and not Spinosaurus. If this was a mimicry thing then even at long range the very different head shape of the spinosaurs versus an iguanodontid would be a give-away and if it’s environmnetal then why didn’t other big ornithopods of theropods (and there’s lots of these in these formations – Carcharodontosaurus, Deltadromeus and others) and the sauropods (some of which doe have tall and elaborate neural spines) also develop them? A common environmental pressure perhaps, but one that didn’t bother various close relatives and persisted in both time and space, and of course other unrelated taxa in other times and places also produce very similar structures with no apparent necessity for such an environment, or live in dry conditions without them. If you are going to posit such a reason you have to have a practical explanation for why it might occur. I think it’s simply a coincidence here that more than one lineage has them, unless I see a convincing case being brought forward for one of the alternatives.
Where does this idea come from that Rebbachisaurus (or any sauropod) had a sail? I’ve heard it before, so I am not picking on you in particular, Dave, but I just don’t know where the evidence is. As you can see at http://2.bp.blogspot.com/_m6sVhskT_Fs/RrDA4C7PbXI/AAAAAAAAAqw/0bXe_UtFLJg/s1600-h/PICT2578.jpg the neural spine of the one well-known Rebbachisaurus dorsal is broken off, so its height can’t be known, and in any case the dorsolaterally projecting transverse processes seem to preclude anything in the way of a midline sail. And the dorsals of better known rebacchisaurids such as Nigersaurus are not at all high — see http://svpow.files.wordpress.com/2008/06/nigersaurus-anterior-view.jpg and http://www.flickr.com/photos/taburin/2443131820/sizes/l/ for examples. If you want tall neural spines in sauropods, you’re better off looking at dicraeosaurids such as Amargasaurus: see http://svpow.wordpress.com/2008/07/03/my-neglected-paddle-shaped-dorsal-neural-spines-by-amargasaurus/ and http://picasaweb.google.com/lh/photo/prHCkOx-27KVuvwCYxrXqQ
I may not be able to give you an especially satisfactory answer Mike as I did the reading for this post a couple of weeks back. I did have to hunt down a reference for this and now and not sure where I got it from. I had long been told of a sail-backed sauropod in North Africa and went hunting. In Tom Holtz’s book he makes reference to the neural spines of Rebb. being ‘especially long’ and I may have taken my cue from that (not that I’m blaming Tom here, I’m trying to find out where i got my own information from). I knew it wasn’t much of one, and as I say above, what you call a sial is rather a matter of opinion, most people would porbably say the baryonychines had a bit of one, but their spines are shorter than those of pretty much any igunaodontid or hadrosaur and most people would say they *didn’t* have one. I wasn’t suggesting Rebb. had a big sail, more that it had enlarged spines that could be called one (though reading back over my post that’s certainly not clear given that I only really mention it in passing).