Re: New alvarezsaurid

["Matthew Troutman" <m_troutman@hotmail.com>]

>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 carina is not seen in basal birds such as Archaeopteryx and 
Confuciusornis so it is not seen in the common ancestor of allosaurs and 
birds. 

><< 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.

 The development of volant adaptations is essential for flight. If an 
ancestral bird did not have modified pectoral musclature, a posteriorly 
directed coracoid, or assymetric flight feathers, flight would be 
impossible. So flight adaptations must have evolved for reasons other 
than flapping flight.

>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...

   Yes, it wasn't the point I was trying to make. I was merely trying to 
make the point that volant adaptations could evolve for reasons other 
than flight.

>> Paedomorphic changes are always 
> seen with the loss of flight. >>

... 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.
>

   Look at the paleognathus lithornids. They are present in the 
Paleocene and their descendents ( ratites ) have been around for tens of 
millions of years and they have not lost any important apomorphies. New 
World vultures ( Vulturidae ) have been around for tens of millions of 
years and they have not lost any of their stork ancestor's 
characteristics. 

MattTroutman

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