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

["Colin McHenry" <cmchenry@westserv.net.au>]

Wow, four of my favourite questions in one post.  Good stuff.

----- Original Message ----- 
From: "Vladimír Socha" <Seismosaurus@seznam.cz>
To: <dinosaur@usc.edu>
Sent: Friday, May 30, 2003 6:59 PM
Subject: Spinosaurus questions and the presence of air-sacs in Dinosauria
(quite long)


> 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.

I've always had a problem with the fish specialisation story in spinosaurs
(not that I disagree with it - I just don't understand it).  Could someone
explain what it is based upon?  Gut contents, or a biomechanical
interpretation of the skull?  If the latter, is it based upon the festooning
of the jaw line, or tooth morphology, or....or ...dunno what else might be
involved.

The reason I ask is that I had always assumed the fish eating story in
Barynonyx was based on the jaw festooning as well as the gut content.  The
obvious comparison to make is with the festooned jaw in crocs....but my
problem with this is that, so far, in my understanding of croc skulls the
festooning is not a direct consquence of piscivory.  I'll explain this in a
seperate post sometime soon, I hope.  Right now, my model of croc jaw shape
works when I consider crocs, but the logic in the model, which does explain
crocs pretty well, breaks down
when I try to apply it to a fish eating spinosaur.  Help!

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.

Big assumption, in my opinion.

 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.

There is no way an animal that size would get too cold at night, not in the
climate it was in.  Look at the thermoregulatory models for big crocs that
Frank Seebacher has been working on.

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

Exactly

> Also (which is more important,IMHO) it was probably used as a cooling
> aparate preventing animal from overwarming in the super-warm climate.

That may be more likely

> 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?

Suggestive, isn't it?

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.

I think you've just explained the reason to yourself.  In the absense of a
good neotype, *Spinosaurus aegyptiacus* has valid taxonomic status, but just
about everything else you might ask about the animal lies more in the realm
of mythology than it does science.  Photographs and drawings of bombed
specimens are not exactly a sound basis for scientific investigations of
palaeobiology.  Although they certainly do fuel one's curiosity....

> 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?

How about the reason that works for camels - increase in thermal inertia so
the animal doesn't fry its brains during the day?  Excess heat load can then
be dumped at night.  By this logic, the animal is thermoregulating by
increasing mass and decreasing SA:vol ratio.  In its own merits, this
strategy could be just as viable in a hot climate as using a sail to dump
heat....A speculative suggestion would be that the hump strategy would work
better in continental environments, where diurnal variation is huge, with
very high maximum temperatures (requiring a large body mass to provide
sufficient thermal inertia) and low minimums (low enough to dump the excess
heat load).  On the other hand, a sail might be a better option in a coastal
environment, where diurnal fluctuations are less (so that a lower mass does
provide sufficient thermal inertia) and a reliable sea breeze, relatively
cool, can be used to keep body temperatures down (allowing high day time
activity levels than non-sailed rivals?).


I'll leave it to others more qualified than I to decide whether the
vertebral anatomy is more consistent with the sail or the hump...but it
seems that either one would give the animal options in a hot climate,
dependign on the excat details of its ecology.

Of course, if the hump is used as a food store, and the animal has to go for
long periods without food, than the animal may be have a hump and a sail at
different times.

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".

Sailers vs Humpers,eh?  The mind boggles.....

> 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.

Sounds pretty stupid, put like that, doesn't it?  Hey, maybe if it sounds
stupid, then it is......oh, never mind.

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
>

Some good questions.  I'd be very interested to see what others on the list
think of them.

Slante
Colin

"You can tell whether a man is clever by his answers. You can tell whether a
man is wise by his questions." - Mahfouz Naguib

BTW, check out John Conway's philosophy quotes page for a quote for every
occaision -
http://homepage.mac.com/john_conway/_philosophy/quotationsIII.html