Re: Largest Dinosaur?

[Mike Taylor <mike@tecc.co.uk>]

> Date: Wed, 12 Sep 2001 10:12:25 +1000
> From: "David Elliott" <dalelemu@hotmail.com>
> 
>> So given some abstract biomechanical reasoning that says no animal
>> can grow heavier than 140 tons, and some bones that suggest an
>> animal damn well _did_ grow heavier, I know which one I will
>> believe :-)
>  
> But isn't there a question of wether or not the bones do suggest
> this?

Yes!  Absolutely.

> I don't know much about this area beyond R McNeil Alexander's
> "Dynamics of Dinosaurs and Other Extinct Giants" (at least ten years
> old i think), but assuming the method (i.e. measure the volume of a
> model, assume an average density based on living animals, and
> calculate the mass from that) described in that book was used for
> the weights given in the earlier post, there is the obivous problem
> of what density to assume.

Yes.  There are _at least_ three different methods of estimating
dinosaur masses, and for obvious none of them is reliable.  They are
(1) make a scale model, measure its volume, scale up and multiply by
density; (2) measure cross-sectional area of load-bearing bones and
interpolate on a graph of area-vs-mass for known animals; and (3)
Greg Paul's favourite method (see the recent SciAm book) where he
basically draws the animal, cuts the picture into slices and estimate
its mass from, er, the area.  Or something.

(1) and (3) are both obviously succeptible to the interpretation of
the model-maker/artist.  The "classic" mass of 80 tons for
_Brachiosaurus_ comes from Colbert's 1962 application of method (1) to
a model which Matt Wedel describes as ``morbidly obese.  It looked
like a freakin' Macy's parade balloon.  The limb muscles go well
beyond the boundaries of the girdles, a sure sign that someone was
just throwing clay on an armature without doing a musculoskeletal
reconstruction first.''  :-) R. McNeil Alexander similarly used the
BMNH plastic _Brachiosaurus_ to arrive at an estimate of 46 tons, and
Greg Paul has done something similar with his own bones-and-muscle
reconstruction and reached an estimate of "only" 32 tons.

Method (2) has its own obvious problem -- real animals don't stay on
or even very close to the "best line" through the points you get by
plotting limb-bone cross-sectional area vs. mass.  For example, on the
basis of such a "best line", you'd expect elephants to have thicker
bones than they actually have, and hippos to have thinner bones.  So
that method is probably not good for much better than a
within-a-factor-of-two estimate.  For what it's worth, Anderson et
al. used this method to estimate a _Brachiosaurus_ mass similar to
Greg Paul's 32 tons.

There may be other mass-estimating methods, I don't know.  Anyone?

> In "Dynamics" the author references estimates of the average density
> of crocodiles, because they are close relatives of dinosaurs. But
> crocodiles are obviously nowhere near giant sauropods in size, and i
> would assume that - if there is a restriction on mass - giant
> sauropods would presumably have adaptations lowering their average
> density - adaptations for which there would probably be no living
> analogue.  So i'm guessing that using any living animal to estimate
> mass would be inherently flawed if the sauropods did approach the
> weight limit. (i just had an image of low density sauropods floating
> away in a breeze...)
>  
> What are sauropod bones like on the inside?

They vary enormously.  Limb bones are very solid (no marrow cavity),
but the vertebrae, particularly of larger species, tend to be
extensively pneumatic.  To cite Matt Wedel again, his guess at
_Argentinosaurus_ density (and so, I guess, most large sauropods) is
70-75% -- but that's based on the assumption of a much more extensive
air-sac system than most people seem prepared to believe in.

> Having said that, how strong are the calculations that give the
> upper mass limit? Do they make any assumptions about the structure
> of the legs?

No idea, I would love to see the 140-ton paper.  Anyone have a
reference?  (Or better still, a link to a PDF!)

 _/|_    _______________________________________________________________
/o ) \/  Mike Taylor   <mike@miketaylor.org.uk>   www.miketaylor.org.uk
)_v__/\  "Do not be overcome by evil, but overcome evil with good" --
         Romans 12:21

        ... shocked, like everyone else, by yesterday's disaster.