Re: Late night thoughts: Pathetica and Interspersal

[Tim Williams <twilliams_alpha@hotmail.com>]

Don Ohmes wrote:

> You will have further noticed that in this thread he makes claims about 
> maximum sauropod size that yield a logical conclusion; that directional 
> selection for size never occurred in the largest land animals ever to have 
> lived. So much for evolution, apparently. He says they were always all the 
> same size anyway, or possibly just appeared at full size and then decreased 
> in size,

No, David didn't say that at all.  What he said was that after a dramatic 
increase in size that occurred early in their evolution (from 
_Anchisaurus_-sized to _Isanosaurus_-sized and beyond), sauropods had 
essentially hit their maximum size by the Middle Jurassic.  In other words, 
the biggest sauropods of the mid-Jurassic were as big as the biggest 
sauropods at any point thereafter (except for maybe _Amphicoelias 
fragillimus_ of the Late Jurassic).

Further, it doesn't matter how you define "big".  Within each of the major 
sauropod lineages (diplodocoids, brachiosaurids, titanosaurs) average size 
appears to have peaked, and then gone downhill.  For example, although the 
Titanosauria produced some extremely large taxa in the later Cretaceous 
(e.g., _Argentinosaurus_, _Puertasaurus_) the average is dragged down by the 
appearance of a slew of much smaller titanosaurs (e.g., saltasaurids).  This 
is what Carrano found.  You may argue that certain sauropods got bigger as a 
defense mechanism, which might well be true.  But this argument of 
"directional selection" cannot be upheld as a general overarching theme for 
sauropod evolution.

> and that both patterns falsify predation as a factor in sauropod size, 
> which they do not.

If your hypothesis is that sauropods were driven to large body size by 
theropods (and/or vice versa), you must demonstrate a correlation.  Such a 
hypothesis makes a prediction that there is a correlation between increased 
body size in theropods and sauropods.  Thus, it can be tested.  If such a 
correlation is demonstrated (quantitatively) to exist, then we have room to 
speculate on causation.

I hope this doesn't inflame the debate any further.

Cheers

Tim

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