Re: cause of Gigantism in sauropods

[David Marjanovic <david.marjanovic@gmx.at>]

>  "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).

But is Cope's Rule valid?

That's a bit difficult to test. I tried to test it for dinosaurs in my 
Master-equivalent thesis. Result: "yes, but" -- it is only valid in the 
Late Cretaceous and only for some clades. Sauropodomorpha and Sauropoda 
are _not_ among the clades for which Cope's "Rule" holds.

I haven't published this, because I wasn't able to use the probably best 
method, it hadn't yet been tested which method actually is best, and I 
need to expand the dataset. The paper which tests the methods has come 
out now, but I don't currently have time to expand the dataset, rerun 
the analyses, and write a paper.

What I find for sauropodomorphs in general and for sauropods in 
particular is passive expansion of the size range, followed by 
extinction of the small members. This is different from a driven trend.

>  Now, I have been nursing my own pet hypothesis for a score of years,
>  a hypothesis that argues that large size _was_ driven;

Whether it was driven can be told from just looking at the data. It does 
not need to be deduced from theoretical considerations.

Of course "the data" consist of measurements, stratigraphic positions, 
and a phylogeny. Assembling all that takes a while. And "looking"... see 
above for methods, except I didn't mention playing around with the 
branch lengths.

>  dinosaurs above a size that inhibited concealment

What about the sea turtle method: lay the eggs en masse when not 
watched, bury the eggs, then abscond? That makes concealing the eggs -- 
or, rather, enough eggs -- possible.

>  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.

Running is impeded by later stages of gravidity. That's a disadvantage 
of placentals not shared by marsupials and egg-layers.

>  early in the paper predation is given credit for being the potential
>  major selective value of large size, i.e., it drives selection.

There are always selection pressures for larger as well as for smaller size.

>  therapods [...] therapods

Ouch.