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Re: cause of Gigantism in sauropods



>>So my question is: how do the relative sizes of lions vs. elephants
>>compare to that of therapods vs. large sauropods? 

Depends on the particular theropod and sauropod, but let's say Morrison era...

The 'common' big theropod was Allosaurus fragilis. Here 
(http://palaeo-electronica.org/2009_3/186/index.html ) is a Palaeo Electronica 
article giving a range of 1350-1850 kg for the 'Big Al' specimen of Allosaurus 
fragilis. Now, that specimen was only subadult... so let's say 1.5-2 tonnes is 
a reasonable range for adult A. fragilis.  

And A. amplexus, or "Epanterias", was significantly bigger, as was 
Saurophaganax (Allosaurus?) maximus. I don't have good (=peer-reviewed) sources 
for masses of either of these... Mickey Mortimer's suggested 4.5 tons for 
amplexus, ~3 to 6 tons for Saurophaganax on the DML, but these were *old* posts 
(http://dml.cmnh.org/2001Jan/msg00445.html , 
http://dml.cmnh.org/2002Aug/msg00194.html , and 
http://dml.cmnh.org/2003Jul/msg00355.html )  so I'm not sure if this is his 
current opinion (if you're reading this, do those numbers still look right?)

Let's, again, be conservative and say 4 tons for our biggest allosaurid.

Matt Wedel's 2005 sauropod-pneumaticity-and-mass-estimates paper gave 12 tonnes 
for Diplodocus carnegii; that's about 6-8 times bigger than our A. fragilis, 
and maybe 3 times bigger than the giant allosaurids.

Giraffatitan was somewhere in the general ballpark of 30 tonnes, though 
estimates vary... so about 15-20 times bigger than A. fragilis ... something 
like 7.5 times bigger than the giant allosaurids.

The 2007 Supersaurus paper estimated it at 35-40 tonnes... so maybe 17 - 26 
times bigger than fragilis and 9 - 10 times bigger than the giant allosaurids.
---

Now, elephants and lions. Here the size gap is like 20x, even if we err large 
on the lions (200kg) and small on the elephants (4000kg)... a 25x gap or more 
might be more realistic, especially since lionesses do most of the hunting

So in the Morrison, the sauropod-theropod size gap seems smaller than the 
elephant-lion one. I see little reason to believe that Saurophaganax or A. 
maximus could not take down even Giraffatitan or Supersaurus.

On the other hand, if Amphicoelias fragillimus was really as big as has been 
estimated, it totally blows everything else out of the water, out-massing even 
the 4 ton giant allosaurid by up to 30 times. It seems a little out of place 
compared to the other Morrison sauropods, and I have to wonder if it was really 
that large (maybe it wasn't actually proportioned like Diplodocus?) We really 
need to find another specimen of the thing...

William Miller

----- Original Message -----
From: "John Bois" <mjohn.bois@gmail.com>
To: "DINOSAUR Mailing List" <DINOSAUR@usc.edu>
Sent: Sunday, February 6, 2011 9:30:34 PM GMT -06:00 US/Canada Central
Subject: cause of Gigantism in sauropods

Reading: Biology of Sauropod Dinosaurs, P. Martin Sander, et al.,
(2010) I was flummoxed by its logic. They offer a simple explanation
of Cope's rule: "...as life diversifies, there is always room for body
size to expand in one direction: to the top. As habitat is partitioned
and ecospace becomes crowded, one way out is evolution towards larger
body size" (pg. 8). They call this a "...simple (if not simplistic)
explanation..." and then go on to use it as total justification for
their thesis: "Given that Cope's Rule in its most general formulation
is valid...the question with regard to sauropod dinosaurs must be what
limited their body size...not what drove body size increase" (pg.20).
Then follows a description of the marvelous adaptations which
successfully removed those limits.

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.
This had to put a premium on defense, a factor that mammals with baby
inside do not have to worry about, i.e., they can simply run away. As
a hypothesis it can stand in line behind all the others, but it should
not be ruled out on the basis of the above arbitrary claim. And 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.

So my question is: how do the relative sizes of lions vs. elephants
compare to that of therapods vs. large sauropods? At least with the
taxa shown in the paper, it looks to be in favor of therapods. (I
understand that predator/prey relationships were diverse over the
years)