Re: New alvarezsaurid

[Dinogeorge <Dinogeorge@aol.com>]

In a message dated 98-03-28 22:58:11 EST, m_troutman@hotmail.com writes
(concerning the keeled sternum):

<< It could have evolved in the allosauroid ancestor ( or even in 
 allosaurs themselves ) for many reasons: climbing, prey handling, 
 fighting, etc. 
 
 As for alvarezsaurs, it could have been a holdover from volant 
 ancestors, or could have been lost and regained for the same reasons as 
 allosaurs. >>

The reason the keeled sternum evolved is unknowable and, actually, irrelvant
to the discussion. The only thing we know is that some allosaurids had it.
This makes it likely that the keeled sternum was already present in the common
ancestor of allosaurids and birds. In birds, it has become adapted for the
attachment of giant flight muscles; in the common ancestor of birds and
allosaurids, perhaps it was as well. Who knows?
 
<< The presense of volant adapations is not direct evidence of volant 
 ancestors.>>

The only time this would be logically true is at the exact instant of the
evolution of the first volant member of a lineage from an immediate non-volant
ancestor. It is also trivially true, in the sense that the presence of
adaptations cannot be >direct< evidence of anything except the presence of
those adaptations. Neither of these alternatives is particularly relevant to
the discussion.

<< Take Longisquama, it has a furcula, but I think that it is 
 clear that it is not a flightless descendent of volant ancestors. >>

We have only one specimen of _Longisquama_ that shows skeletal anatomy, and it
is only about 35% complete. What we have suggests it had some kind of a volant
lifestyle >itself< (let alone its ancestors). So--you're right: It's >not< a
flightless descendant of volant ancestors. It's a >volant< descendant of
volant ancestors! But somehow I don't think this is the point you were trying
to make...

<< Basically in allosaurs and other theorpods with volant adaptations, you 
 see no paedomorphic characteristics. Paedomorphic changes are always 
 seen with the loss of flight. >>

Perhaps this is true for birds of a modern aspect, but at our present state of
knowledge you simply cannot make this kind of generalization over all time and
over all theropod lineages. Second, there is certainly a wide gulf of time
(several tens of millions of years at least) between the divergence of the
allosaurid lineage from its volant ancestral forms and the appearance of giant
allosaurids in the Late Jurassic. Plenty of time for all kinds of other
adaptations and autapomorphies to appear (and disappear) in that lineage.
We're lucky we can still see the keeled sternum in a few members.