Re: Rahonavis....a Bird?....a Dino?

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

At 12:34 PM 2/24/99 -0000, John Jackson wrote:
>What is the preferred orthodox BAMM view on the relationships between
>Rahonavis/Archaeopteryx/droms?:

Well, since _Rahonavis_  has only been included in a few analyses so far,
there aren't really any "orthodox" views on it.

>1)  Neither of the two flying forms is descended from the other, so either
>flight must have evolved twice or they must have descended from a flying
>common ancestor.  The latter would make the similarity between Rahonavis and
>droms difficult to reconcile with BAMM.

No it wouldn't, not by a long shot!  It WOULD strongly support
dromaeosaurids being the sister group to a _Raho._ + (Archie + later birds)
or Archie + (_Raho._ + later birds) clade, and it would certainly support a
sickle claw being ancestral to the bird condition if the former option were
true, but it is fully compatible with the 'standard model'.

Note that _Rahonavis_ does have features it shares with Archie and later
birds (the proximodorsal process of the ischium, among others) that
dromaeosaurids lack, and other features (enlarged neural canals) which are
absent in dromaeosaurs and other non-avians but which cannot be examined in
_Archaeopteryx_.

>2)  Rahonavis is descended from Archaeopteryx;  this would make the
>similarity between Rahonavis and droms difficult to reconcile with BAMM.

Other than the sickle claw, _Rahonavis_ does not possess any particularly
dromaeosaurid-like features that _Archaeopteryx_ lacks.  And, as above,
_Raho._, Archie, and later birds all share features not found in
dromaeosaurids or other theropod groups.  _Rahonavis_' position isn't
certain, but that uncertainty seems to concern whether it is outside Archie
plus later birds, closer to Archie than to later birds, or closer to later
birds than to Archie.

>3)  Archaeopteryx is descended from Rahonavis.  This must be the BAMM
>favourite.

Based on...?

Heck, if you think people are still on an "ancestor hunt", join the latter
third of the 20th Century!

In any case, Forster et al. did find some trees where _Rahonavis_ did form a
clade with _Archaeopteryx_ to the exclusion of ornithurine birds, but that
doesn't imply an ancestor-descendant relationship.

My own study places _Rahonavis_ outside _Archaeopteryx_ + later birds, but
(as noted previously) the large amount of missing data make this uncertain.
Either way, these hypotheses do not suggest ancestor-descendant
relationships, but rather the relative sequence of branching events.

>As it happens, the crucial wing/shoulder/and maybe even uncinate
>parts that would probably have shown Rahonavis to be a more advanced flier
>than Archaeopteryx were not found.

Could well be, especially if the forearms found with _Rahonavis_ are indeed
part of this animal (rather than _Vorona_ or another bird).

>But if Rahonavis were shown to be a more
>advanced flier than Archaeopteryx, say if a better Rahonavis skeleton were
>found, what would be the BAMM view?

Presumably, based on the character evidence you suggest, Archie + (_Raho._ +
later birds).

>In fact, is there anything in what we
>have of Rahonavis that suggests it is a more advanced flier?

The elongate forearm (radius and ulna) and the "quill nodes" are more
similar to modern birds than to _Archaeopteryx_, so it might have been a
better flier than _Archaeopteryx_.  However, it lacks a pygostyle and the
relatively derived pelves of even _Confuciusornis_, and so lacked these
flight specializations.

>Does anyone
>consider Rahonavis to have been flightless (and without a flying ancestor)?

Martin, Feduccia, Ruben et al. suggest that the forearms of _Rahonavis_
actually belong to _Vorona_, and that _Rahonavis_ is just a little bitty
dromaeosaurid.  (Yeah, one that shares derived features of the non-forearm
skeleton with _Archaeopteryx_ and later birds that are not found in
_Velociraptor_, _Deinonychus_, _Saurornitholestes_, etc...).  Of course, the
SAME people say that there are no dinosaurs small enough to habitually climb
trees.  One or the other, guys, but not both at once.

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