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Re: cause of Gigantism in sauropods



Indeed, the long necks -- and likely rearing, at least in diplodocids and some 
others (I believe Sereno wrote that *Jobaria* could) -- makes reaching higher a 
sensible cause for large size; the only way left to reach higher was to get 
bigger!

The long necks definitely predate the large size in sauropodomorphs; maybe the 
very uneven weight distribution in some (I've read that Diplodocus had like 85% 
of its weight on the hind legs, though I don't know the original source for 
that information) was an adaptation to continue being able to rear as they grew 
very large? That's presumably testable by looking at how the weight 
distribution / position of the CG changes going from small, early 
'prosauropod'-type sauropodomorphs to the big late sauropods -- has anyone done 
that? If so, it might be telling.

And it can't be something as simple as 'the Mesozoic climate/ecosystem was 
better for big stuff' (though it could have been a factor), because our whales 
get bigger than even the biggest sauropods. Also, I'm not sure it's a Mesozoic, 
or a dinosaur, thing. The biggest land mammals -- indricotheres, 
*Deinotherium*, and the largest mammoths -- seem to have been comparable in 
mass to the largest non-sauropod dinosaurs -- probably the big hadrosaurs (*The 
Dinosauria* says that 'The largest hadrosaurids -- e.g. Shantungosaurus 
giganteus -- may have weighed as much as 16,000 kg') Really, it seems that all 
the big nonsauropod land animals -- big theropods like *Spinosaurus*, big 
hadrosaurs like *Shantungosaurus*, big proboscideans like *Deinotherium* and 
the biggest *Mammuthus*, big rhinos like *Paraceratherium*/*Indricotherium* -- 
top out somewhere in the 10-20 tonne range... while the sauropods blow on by to 
70-80 tonnes at least (*Argentinosaurus*) and maybe more. So even 
conservatively, sauropods outweigh their biggest competitors on land 4x. 
....Wow.

There seems to be something weird going on with sauropods -- probably important 
-- but what was it???

William Miller
----- Original Message -----
From: "Augusto Haro" <augustoharo@gmail.com>
To: dannj@alphalink.com.au
Cc: dinosaur@usc.edu
Sent: Monday, February 7, 2011 12:01:47 AM GMT -06:00 US/Canada Central
Subject: Re: cause of Gigantism in sauropods

2011/2/7 Dann Pigdon <dannj@alphalink.com.au>:
> On Mon, Feb 7th, 2011 at 2:30 PM, John Bois <mjohn.bois@gmail.com> wrote:
>
>> ... I would have thought it would rate a mention as an important difference
>> between extant mega herbivores and those of the Mesozoic. Indeed,
>> comparisons are frequently made between the two groups: early in the
>> paper predation is given credit for being the potential major
>> selective value of large size, i.e., it drives selection. This
>> apparent contradiction with the paper's thesis is resolved by saying
>> that sauropods, like elephants, outsized their predators, and that is
>> was limits on predator size (possibly due to bipedal body plan) that
>> made this no longer a driver of selection.
>
> The problem with growing large to outgrow *your own* predators is that you 
> first have to survive
> long enough for that to happen. This would have been especially difficult for 
> sauropods who started
> off so much more relatively smaller than their adult form, and who didn't 
> have the benefit of super-
> rich milk (a la whales) to grow fast on.

It seems, as far as I read, that histology suggests fast growth rates
for large dinosaurs, especially sauropods. Fast bone growth rates for
hadrosaurs have actually been seen as a way to avoid predation. I read
a paper (I think from Erickson) where it is claimed that Tyrannosaurus
grew faster than modern elephants.

>> Now, I have been nursing my own pet hypothesis for a score of years, a
>> hypothesis that argues that large size _was_ driven; driven by the
>> fact that dinosaurs above a size that inhibited concealment, had
>> to--if nest attendance was in operation--defend or abandon the nest.

What about exploiting a previously unexploited resource? (tree tops,
for example). Generally speaking, larger animals can, also, bite
harder, and are thus perhaps able to eat thougher stuff, although this
is not likely the case in sauropods. Large size can also expand the
preferred range of food in the case of predators, for tigers can hunt
from locusts to gaur, and I suppose the range is much lower for
smaller cats (may there be something similar in herbivores, accounting
for greater possible forces and thus an increase in the potential
range of size and thoughness of the exploited resource?).