Re: Megaraptor (was Re: Pygostyle)

[Oliver Rauhut <h2068d4p@rz.hu-berlin.de>]

On Wed, 22 Sep 1999, dbensen wrote:

> >>_Megaraptor_ the same as _Unenlagia_?  _Megaraptor_'s big megalosaur-like
> arms would look quite funny hanging from the shoulders of a dino-bird.  Do
> you mean _Unquillosaurus_?<<
> Speaking of Megaraptor, somebody knows more about it than I do.
> It isn't a deinonychosaur, is it?  I didn't think they lived in South America.
> 
> I asked this question a few years back to Jeff Poling, but nobody knew much
> about Megaraptor then.  Is Megaraptor a giant noasaur or what?

Megaraptor and Unenlagia are from the same Formation, but that's all there
is in terms of comparable facts about these animals. Megaraptor is based
on an ulna, a manual digit, a fragmentary metatarsal, and a large claw,
presumably the ungual of digit II of the pes. Unenlagia is based on some
dorsal vertebrae, a sacrum, a scapula, humerus, most of the pelvis, femur,
and tibia. Since there are no overlapping elements, nothing can be said
about possible synonymy. However, Megaraptor is considerably larger than
Unenlagia, and the latter does surely not represent a very young
individual, but a subadult at best. 
The systematic position of Megaraptor is problematic (as pointed out by
Novas): The ulna has a very well developed olecranon process, unlike any
other maniraptoran (with the exception of Alvarezsaurids), but the pedal
ungual has the same morphology as it is seen in the pedal ungual II in
dromaeosaurids, including a sharp ventral margin and an asymmetric
arrangement of the claw grooves. Thus, Megaraptor is either a basal
theropod or basal tetanuran with a convergent development of an enlarged
pedal ungual, or a dromaeosaur which reversed the reduction of the
olecranon process. A third possibility would be that the association of
the material is erronous, but that seems unlikely. I personally think that
the dromaeosaur explanation is more likely (since the olecranon process
is a muscle attachment: muscle attachments tend to be more strongly
developed in large animals; furthermore, we know that this character is
reversed in at least one other maniraptoran group (unless you accept
Martins 1998 and Serenos 1999 interpretation of alvarezsaurids as  a
sister group to ornithomimosaurs), but this is untested by cladistic
analysis. I fear that the solution to this problem can only come from more
material.