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Re: Late night thoughts: Pathetica and Interspersal

Currently there's so much traffic that I sometimes receive replies before the originals. So I'll only answer this one post now. I don't have oodles of time anyway...

----- Original Message ----- From: "Tim Williams" <twilliams_alpha@hotmail.com>
Sent: Wednesday, June 20, 2007 7:32 AM

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.

No, not that either. I said that after a drastic increase in the size _range_, into both directions, with -- in sum -- _no trend_, the smallest ones (like *Anchisaurus*) died out, and the bigger ones kept diversifying, though neither the minimum nor the maximum size changed much from the MJ onwards.

Again: there is no trend in sauropod size over the Mesozoic, and there is no trend in sauropodomorph size over the Mesozoic, according to my unpublished results.

Within each of the major sauropod lineages (diplodocoids,
brachiosaurids, titanosaurs) average size
appears to have peaked, and then gone downhill.

No, not even. There is no trend. :-)

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.

Yes. I only found a statistically not significant decrease, perhaps because I used fewer immature sauropods as data points, so I found no statistically significant trend, only diversification and extinction.

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.

That's because directional selection would result in a trend, and there is no trend, unless my analysis is flawed.

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.

The method I used takes the phylogeny into account and compares descendants to their reconstructed ancestors. If repeated size increases and extinctions had happened, I'd have found that as a trend of size increase. I didn't.

Hone et al. did not reconstruct the sizes of ancestors and just compared younger relatives to older relatives. They, too, didn't find a trend.