Re: Dino sterna (was Re: Rahonavis....a Bird?....a Dino?)

["John V Jackson" <jjackson@interalpha.co.uk>]

--Original Message-- From: Thomas R. Holtz, Jr. : Wednesday, February 24,
1999 08:48 PM



>At 02:41 PM 2/24/99 -0000, John Jackson wrote:
[snip]
>
>>In replying to Larry Febo:
>>>Okay, even given the premise that 'dromaeosaurids' ARE _Rahonavis_'s
>>>ancestors, why COULDN'T _Archaeopteryx_ also be a descendant?
>>
>>Apart from the very odd breastplate reversal that would be required (see
>>above), and the fact that this would make Archae the only flying bird that
>>had moved its pubis forward, it would require Archae to have lost a
feature
>>(uncinate processes) that have been conserved by 99.95% of all birds that
>>had them in their ancestry, volant or flightless.  Three "bet-your-shirt"
>>flight features - probably trap-door developments too.
>
>Except that two of these (sterna and uncinates) are really just questions
of
>ossification.

The process of ossification spreads through the cartilage progressively,
probably from various points, and the process caught different specimens at
different stages.  But the claim, 'all the Ax specimens died young', or 'Ax
had a less ossified sternum than droms' cannot be made because a comparison
of corresponding points in the structures shows that the sterna actually
were smaller than in droms.

Modern birds have ossifying uncinates.  So did the raptors and the ovis
though they were not always preserved.  Enants and arctos didn't.  No amount
of doubt over preservation can obscure that now.  We've got seven
Archaeopteryx specimens, some quite well on towards adulthood if not already
there, and the preservation was so good the feather imprints survived.  I
believe that evidence is adequate to conclude Ax didn't have ossifying
uncinates.


>
>If we used the breastplate argument, than spinosaurids and _Sinraptor_
(both
>of which have fused, keeled sterna) were closer to modern birds than
Archie,
>dromaeosaurids, oviraptorids, etc.

Nobody uses individual characters on their own.  Their significance can only
be understood in combination.