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).