Spinosaurus questions and the presence of air-sacs in Dinosauria (quite long)

[Vladimír Socha <Seismosaurus@seznam.cz>]

Good day to all,
I'd like to make clear some points regarding _Spinosaurus
aegyptiacus_:
1.) Why would a specialist fish-eater like Spinosaurus grow into so
monstrous size? Large tyrannosaurids and carcharodontosaurins also
grew big (but not so big, as it seems) but there was a reason:
hunting large and/or dangerous prey. If Spinosaurus' diet was
composed of just "Lepidotes-like" fishes (although large and heavy),
it would be unnecessarily large. One would think it was able to
attack also large (dinosaurian) prey. OTOH, its dorsal "sail" would
be vulnerable during such action. So my question is: How was it
probably with its diet?

2.) I have one problem with Spinosaurus sail (leave alone the debate
over whether it was a sail or a hump). I suppose all theropods from
_Eoraptor lunensis_ to _Passer domesticus_ were/are endothermic
creatures. Including Spinosaurus. It was living in a very warm
climate. So what was the main role of this morphological structure?
Clearly not the same as in poikilotherm pelycosaurs like Dimetrodon
or Edaphosaurus. OK, the dinosaur used it in thermoregulation, say,
in the morning to warm up more quickly. But was it really necessary?
It was very large, why have it not gained gigantothermy when adult?
Also (which is more important,IMHO) it was probably used as a cooling
aparate preventing animal from overwarming in the super-warm climate.
Sice the dorsal sail evolved also in basal hadrosauriform
_Ouranosaurus nigeriensis_ living in about the same time and
geographical area of central northern Gondwana, was the climate
somehow specific in requiring this morphology? In theropods, the
dorsal sail is present in Spinosaurids (Spinosaurus, Altispinax,
Suchomimus) and Allosaurids (Saurophaganax and Acrocanthosaurus). Is
this the complete distribution of d.s. in theropods? And, leave alone
phylogeny, what is the morphological/physiological relativnes of this
trait in those groups? And why is it not present in other groups of
large theropods living in the similar climate conditions
(Ceratosaurian Abelisauroids, Spinosauroid Megalosaurids, various
Coelurosaurians).  

3.) Given the facts that are obvious (the size of now destroyed _S.
aegyptiacus_ holotype remains and also MNHN SAM remains), why are all
studies refer to _G. carolinii_ as the largest theropod ever to have
lived? It's estimated to be "just" 12.5 m long and some 8 tons heavy,
while Spinosaurus holotype was clearly larger by both measures,
though unfortunately, this is not testable nowadays.

4.) As for the sail v. hump debate. I favor the "sail version". It
was useful for a different kinds of thermoregulation (i.e. either
positive or negative) and could also have served for intimidation
and/or display (e.g. by changing its colour) while the hump would
seem to be kinda useless. What would it be good for? Statokinetic
organ for keeping balance? A water reserve store? No, this would
rather make dinosaur more heavy and would lower its "keeping-balance"
ability. And for what reason water reserve, if it was fishing, i.e.
living in the close vicinity to a water supply? Just some thoughts, I
hope I won't be ostracized from those who favor the "hump theory".

5.) As for the air-sacs, which is a trait claimed to be present in
all members of Dinosauria. If I understand it correctly, it's based
on cladistic rule saying that this trait was present in the most
recent common ancestor of all Dinosauria (Ornithodira for that
matter), i.e. Triceratops and Passer plus all of its descendants, so
that all dinosaurs possesed it. But what function it played in less
advanced dinosaurs and slow thyreophorans? Was it reduced or even
secondarily lost? It has an usage only for an agile, fast moving
animals with a high rate of metabolism enabling them more
sofisticated and efficient breathing. In sauropods, it gained
secondarily other function by relieving the mass of their vertebras.
But in middle sized, slow moving stegosaurs and ankylosaurs? OK, who
would deny them the right to breath more efficiently, but still...;o)
Thanks, Vlad               

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