Re: Amargasaurus...

["Steve Brusatte" <dinoland@lycos.com>]

On Thu, 26 Apr 2001 21:21:48  
 T. Mike Keesey wrote:

>Large _Carcharodontosaurinae_ would have towered over it, though.
>(cf. the Luis Rey pic I posted a link to)

Yes, large carcharodontosaurs would have towered over it.  However, lets look 
at hadrosaurs for a moment.  Amargasaurus was about 12 meters long, and lived 
along with Giganotosaurus and this newly discovered critter.  There were 
several species of hadrosaurid dinosaurs, including Parasaurolophus and 
Lambeosaurus, that were the same general size (sometimes even smaller), and 
lived alongside Tyrannosaurus, a predator about the same size as Giganotosaurus 
(give or take a few centimeters).   However, no hadrosaur that I know of has 
any sort of protection.  Wouldn't they be just as vulnerable?  
>
>It still seems to me that they are large enough to ward off bites,
>especially if they had sharp keratinous sheaths.

Oh, yes they are large enough to ward off bites, and keratin would have only 
added to their defense purposes.  I have no aversion to accepting the fact that 
they may have been used for defense.  I do not believe, however, that they 
evolved for defense purposes.  I believe much of the same about ceratopsian 
horns.  I am a big believer in Sampson's work, stating that these horns and 
frills were only present in adults.  This signals (if you read the evidence the 
way I do), that these structures were used for sexual display purposes only.  
However, I am sure that some adults utilized them in other ways, such to ward 
off predators.  Many structures in modern animals have several functions.  Take 
the cloaca of reptiles, for example.  Did it first evolve for waste release or 
reproduction purposes?  I doubt it evolved for both in the first place, but 
over time it accompanied both uses.  

I am sure that Amargasaurus individuals used these spines for both defense at 
some point...but I think they evolved for display purposes and over time may 
have been either modified to accompany defense purposes or have taken up a 
defense function.  

>> the predator could simply bite the side or underside of the neck and
>> get right past them.
>
>It couldn't bite the side more than a nip unless it opened its mouth wide
>enough to encompass the spines, and then we're back to square one. Biting
>from the underside would work, but look at it this way: what's better,
>protection from the top and sides, or no protection at all? In an
>environment where there are such tall predatory theropods a sauropod with
>neck spines is going to have some kind of advantage over one without them.

Absolutely.  As I've been saying, these spines may have evolved for display 
purposes, and at some point a mutation made them either larger, stronger, or 
covered with hard keratin.  Evolution may have then supported the individuals 
with this mutation.  

>> If Amargasaurus was to truly evolve a protective mechanism for its
>> neck, I would think that a nice, thick set of armor would be much
>> better and much more protective.
>
>Too bad _Amargasaurus_ didn't think of that....
>Of course, this may be one reason why titanosaurs outlasted
>dicraeosaurids.
>
>> These spines seem very ostentatious...and rarely are "showy" objects
>> used for defense purposes (at least exclusively).
>
>That's possible, too. I can't imagine that they didn't have some role in
>protecting the neck, one of the most vulnerable parts of any
>sauropodomorph.

The neck is very vulnerable, especially with big ass predators like 
Giganotosaurus.  I just doubt that such an ostentatious structure would evolve 
for defense purposes only.  Evolution may even FROWN upon an individual with 
such an ostentatious structure used for defense only.  These spines, especially 
if covered with keratin, were likely very expensive biologically to maintain.  
I would think that, if provided with the right mutation (if..if..if), evolution 
would favor a simpler defense structure.  Often display structures ARE very 
biologically expensive.  Sex is very important in animal (yep), and I think 
that this fact is often overlooked in dinosaur paleontology.  However, there 
has been a "sexual revolution" during the last 15 years or so, and display and 
dimorphic features are now being described more than ever.  

Open to criticism!

Steve

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