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

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 02:41 PM 2/24/99 -0000, John Jackson wrote:
>I had got the impression from somewhere that the Raho. was considered
>close(r) to droms.  I wonder how that happened.

Don't know: a lot of stuff gets out there (onto the Internet) which isn't
backed up by data...

>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.

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.  Sterna have been recovered for a few
tyrannosaurids, but not most specimens; they are recovered from a few
ornithomimosaurs, but not many.  They are recovered from one _Archaeopteryx_
(where it isn't proportionally much different in size than in other typical
coelurosaurs), but not the others.  It seems pretty clear that we aren't
seeing multiple evolutionary origins of sterna, but instead a spotty pattern
of ossification of this (otherwise non-ossified) structure and/or failure to
recognize and recover the element in the field.

Same thing with uncinates: spotty record among fossils, but present among
living birds and crocs.  May well be an archosaurian feature, which was
mostly cartilage in life, and only ossified in some groups of theropods.
The fact that not all dromaeosaurids and oviraptorids show these structures
suggest that there was variable ossification and preservation.

As for the pubic orientation: that seems to be the only strong feature which
is more "bird-like" in dromaeosaurids than _Archaeopteryx_ (or _Rahonavis_,
which has an _Archaeopteryx_-like pubis orientation).  Compare this to
additional cranial and postcranial features supporting an Archie-modern bird
relationship to the exclusion of dromaeosaurids.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661