Re: Megaraptor (was Re: Pygostyle)

[NJPharris@aol.com]

In a message dated 9/23/99 2:31:24 AM Pacific Daylight Time, 
h2068d4p@rz.hu-berlin.de writes:

> 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.

The foot of _Alvarezsaurus_ is rather like that of a dromaeosaur, with a 
shortened MT II, inwardly-diverging pedal digit II, and laterally bowed MT 
IV.  At least one recent analysis (can't remember whose, at the moment), 
placed Alvarezsauridae just outside of (_Archaeopteryx_ + modern birds), a 
position quite near that of the Dromaeosauridae.

So it is possible that the Alvarezsauridae arose from within or near the 
Dromaeosauridae, and that _Megaraptor_, with its dromaeosaur-like pedal claw 
and alvarezsaur-like forearm, is a basal alvarezsaur.

Still, of course, this is an awful lot to rest on one fragmentary specimen!

--Nick P.