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Re: Long-necked stegosaur (the reason for long necks?)



--- On Sun, 3/8/09, Jonas Weselake-George <paleo@ncf.ca> wrote:

> Sauropods had relatively high ground loading and limited
> traction. Any
> terrain too soft, too unstable, too hard, too uneven, could
> be problematic.

Why so? Elephants apparently like swamps, and have been observed at high 
altitudes...

In soft ground, the common idea that great size is a disadvantage is not 
supported by the physics or empirical evidence (once the threshold of surface 
yield stress has been exceeded). The 'drag' on a limb is proportional to 
surface area, and the absolute power available to move the limb is proportional 
to mass, therefore increasing size increases the power to 'drag' ration. Also 
most swamps have a 'bottom', and the longer your legs are, the more likely you 
can reach it. 

Elephants do very well in swamps, are used for logging in mountains and rain 
forests where other animals aren't suitable, and certainly quadrupeds are more 
stable than bipeds, pound for pound. Therefore soft ground would represent a 
(large) theropod-free haven to a sauropod, certain rather novel theories about 
tread frequency and evolutionary over-compensation notwithstanding.

Don't know about the hard stuff; what you say seems logical.

> This has impacts beyond simply keeping Sauropods out of
> some environments: In essentially any habitat type, there
> would have remained patches of food which would be
> inaccessible. The question is only the percentage of food
> that would be inaccessible in a given habitat.
> 
> The obvious solution is to grow a longer neck. You can
> reach to consume
> foliage growing in ponds, 

Or, you could hang out in a cool, safe pond, and nibble around the edges...

The evo-model re elicitation of long necks remains exactly the the same of 
course, so on your basic point we agree.

> you can consume ferns dotting a
> rocky outcrop, you
> can pass in between >500 ton trees to feed on the
> undergrowth behind them -
> if you have the right neck.

Good point about the trees. What a minute! 500 tons? That is huge. Did you know 
that the largest tree trunk preserved in the fossil record (~4.5m diameter, 35m 
bp, iirc*) is significantly smaller than the General Sherman and many other 
living trees?

*correction and/or ref GREATLY appreciated.

> So, having a longer neck, especially if there is
> competition over food
> gathering, is the only feasible solution for herbivores
> that weight more
> than ten tons. 

Elephants don't have long necks, but trunks are functionally similar relative 
to resource acquisition...

> I'm a little mystified why this
> isn't more widely advanced
> (in fact I haven't seen it anywhere in the literature).
> There are a lot of
> interesting hypotheses regarding sauropod neck length (I
> counted ten last
> time I checked), but this may be the simplest and most
> obvious one.
> 
> Anyway, the basic point is that we tend to over-idealise
> the environments
> that could have existed. A lot of adaptations make a lot
> more sense if one
> considers how animals could exist in contemporary terrains.
> 
> Even something as simple considering width (a
> characteristic all too often overlooked) can lead to
> interesting conclusions: One can guess that Tenotosaurus is
> laterally flattened (and elongated) for moving through
> woodlands/scrub, whereas, Sauropelta has adaptations suited
> to comparatively open, exposed environments.
> 
> It would be interesting to find out what the rear half of
> Miragia and
> whether there are any Kentrosaurus like spines that might
> have effected
> mobility in some terrain types. If the animal can rear up
> on its hind legs,
> that might also have an impact: Even a few percent more
> calories from being
> able to reach the lower branches of trees may be
> significant at evolutionary
> time scales. If you throw in some exotic environmental
> factors (eg.
> involving drought or fires in some types of floral
> communities)...
> 
> S!
> 
> -Jonas Weselake-George
> 
> P.S. Regarding neck calculations in a given environment its
> important to
> recognise the role of 'edge' effects (as used in
> conservation ecology
> theories) in heterogeneous terrain. This is import for
> calculating the
> amount of accessible food increased by neck length for any
> given corridor.
> It may also be important for considering the long term
> impact of Sauropods
> on plant community structures.
> 
> P.S.S.
> I have doubts about the energetic argument that has been
> advanced for
> Sauropods using the neck to sweep through vegetation
> without having to walk.
> But it is interesting to note that the energetic cost may
> have been greater
> for toppling dozens of trees (instead of using the neck to
> penetrate deep
> into a stand of trees). 

Good point. Ah, but I bet those lurking theropods liked that situation too. 
Chomp! Still, turning down cheap food isn't wise, even if it doesn't always 
turn out well...

Don