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

["Mickey Mortimer" <Mickey_Mortimer111@msn.com>]

Vladimír Socha wrote-

> 1.) Why would a specialist fish-eater like Spinosaurus grow into so
> monstrous size?

Who says it didn't eat dinosaurs too?  Baryonyx has Iguanodon bones in its
stomach.

> OTOH, its dorsal "sail" would
> be vulnerable during such action. So my question is: How was it
> probably with its diet?

Lots of predators have parts that would seem vulnerable, but which don't
prevent them from attacking large prey.  A lion's tail seems pretty easy to
bite off or crush, but they still hunt large artiodactyls.

> I suppose all theropods from
> _Eoraptor lunensis_ to _Passer domesticus_ were/are endothermic
> creatures.

Bad assumption, given the large variety of metabolisms such a clade was
likely to exhibit.

> So what was the main role of this morphological structure?

Possibly multiple uses- display, thermoregulation, ligament/muscle
attachment....

> It was very large, why have it not gained gigantothermy when adult?

Because the hump/sail still functioned for other purposes?  And perhaps
gigantothermy is unfeasible for large theropods.

> 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?

No, because Carcharodontosaurus and Lurdusaurus lacked humps/sails.

> 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?

The dorsal neural spines of Suchomimus are quite a bit lower than
Spinosaurus, or even Becklespinax (not Altispinax, the tooth; unless Rauhut
2000 is right).  Saurophaganax lacks high neural spines.  Other relatively
tall-spined taxa include sinraptorids and Ceratosaurus.

> And why is it not present in other groups of
> large theropods living in the similar climate conditions
> (Ceratosaurian Abelisauroids, Spinosauroid Megalosaurids, various
> Coelurosaurians).

Surely one animal can be more efficient than another.  It clearly wasn't
necessary for Spinosaurus et al. to be humped/sailed to survive, just
useful.

> 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.

Because people like the more complete remains of Giganotosaurus, which get
more press coverage than Stromer's Spinosaurus holotype.  And because no one
seemed to try to accurately determine Spinosaurus' length before I did.

> 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?

Any hump would be a muscular hump, like in bison, not a fatty (water
reserve) hump, like in camels.  Humps also increase the surface area to mass
ratio (for thermoregulation) and could have been brightly colored.  The
center of balance in theropods seems to be at about the same place the
hump/sail was highest, so that it wouldn't interfere with balance.

> 5.) As for the air-sacs, which is a trait claimed to be present in
> all members of Dinosauria.

Only Thecodontosaurus (?), theropods and sauropods preserve osteological
evidence for air sacs.  If ornithischians and other basal taxa had them,
they didn't pneumatize the bones.  They could have been lost, reduced, or
maybe those taxa never had them.  More work needs to be done looking for
evidence for air sacs in ornithischians, basal dinosaurs, and other
archosaurs.

Mickey Mortimer