Amargasaurus With a Hump?

[amargasaurus cazaui]

Or....those fish are preserved in the swamps because they died there after being stranded by receding floodwaters, not because they habitually lived there. The same could be said for the dinosaurs themselves. Big animals and dense swamps are not the best combination. If a big animal is going to hang out in such a habitat it needs to be either very maneuverable (crocs) so it can go around the vegetation, or very powerful (elephants) so it can move the vegetation out of its way. Carrying a giant sail on your back is counterproductive in either scenario.

I really think you're going down a blind alley here. Also, as you yourself pointed out, these three animals have very different anatomies of their elongated vertebral spines; there's no particular reason to think that they're all doing the same things with them.

Fair enough and back to square one...but still there it is...in the same general living area, a sauropod, an ouranosaurus and a spinosaur, all with heavily modified spines......is there a parallel suggestable in the dinosaur kingdom anywhere else, where multiple lifestyles and kinds of animals all share extreme modifications to the same area of their bodies? If the answer is no, then it is a valid question and does beg an answer....what was happening in this particular area to cause this modification in the animals? Were the predators somehow more numerous? Was there an ancestral bias to spined animals? Could the modifications somehow relate to climate, or warmth? Were the modifications a form of highly derived sexual display?

[Newt]

I'm just spitballing here, but there are several lines of evidence that lead me to believe what we're seeing here is mainly sexual display structures. For convenience I will refer to elongated vertebral spines as "crests" below, which is not meant to imply anything about soft tissue.

1. Distribution through the dinosaur phylogeny. Freaky crestbacks are widely scattered and isolated in the dino tree. It seems to me that (non-sexual) adaptive features would be distributed more normally, with entire lineages showing varying degrees of crestedness. Instead crests seem to spring up fully-formed here and there, which smacks of runaway sexual selection to me.

2. Lack of correlated features. If crests are associated with some peculiar lifestyle, why do the animals not have lots of other weird features? Without their crests, Ouranosaurus, Amargasaurus, Concavenator, Acrocanthosaurus, etc. are very similar to their close, crest-less relatives.

Spinosaurids are the exceptions to the first two points, which leads me to think it's more likely they were using their crests for something other than impressing one another.

3. Modern analogues. Where do we see tall vertebral crests on modern animals? Fish and aquatic amphibians have dorsal fins, but they are not good structural analogues for dino sails, and most dinos were thoroughly terrestrial. That leaves the wide diversity of lizards and mammals with vertebral crests. Of course, these are mainly made of integumentary structures in these animals - but hair, feathers, and scales don't....wait for it...scale. If you're really big and want an impressive crest, you'll need a bony support structure. And what do those lizards and mammals use their vertebral crests for? Display.

As an extra bonus, if we assume that these are display structures, their phylogenetic isolation becomes less dramatic. We know of integumentary crests in sauropods, hadrosaurs and your psittacosaurid friends, and considering how few good soft-tissue specimens we have of other taxa, it seems quite conceivable these features were widespread in the dino tree. We also know of bony head crests in many dinosaurs. It's hard to see what other than display most of these structures could have been used for. If bony vertebral crests are extensions or exaggerations of widespread display structures, they become merely extreme examples of existing features rather than unique features in themselves.