RE: Revising Hou et al, 96 (very very long)

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

Rutger Jansma wrote

> but in my opinion,
> Sinornithosaurus could be to basal to be properly placed in any of the 
> groups. It has similarties to Archaeoperyx that must be taken into 
> account (which is in your opinion close to birds, as well as the 
> raptor, but more closely to the former than to the latter) such as the 
> very long fore limbs, but also many similarities to the 
> Deinonychosaurs, such as the tail. 

I might be missing something, but the similarities shared by
_Sinornithosaurus_ and _Archaeopteryx_ are likely to be primitive
eumaniraptoran traits.  These include the long forelimbs and broad pinnate
feathers.  The stiffened tail (elongated prezygopophyses & chevrons) is
probably a derived dromaeosaurid character, just as the fused pygostyle is a
derived avialan character.

> That this animal lacked the semi-lunate carpal, but when the two clans 
> diverged, they each evolved on of their own, which are all based on the 
> same type of construction. 

The absence of a semilunate carpal (SLC) - or similar structurs - in certain
maniraptoriforms, maniraptorans or eumaniraptorans may be due to secondary
loss.  The SLC was useful tucking the long hands against the forearm, and
integral to the rapid deployment of the forelimb in the predatory stroke.
However, the enlarged SLC (as seen in eumaniraptorans, oviraptorosaurs,
therizinosaurs and certain other critters) had the effect of largely
limiting the motion of the forelimb to a laterodorsal arc.  This aspect was
incorporated into the flight stroke of birds.  

For other theropod lineages, the SLC may have been a disadvantage, promoting
the degeneration of this structure.  Taxa such as _Protarchaeopteryx_ might
have reverted to a more primitive carpal configuration to give the wrist
more overall flexibility - especially torsional and dorso-palmar movement.  

Some basal maniraptoriforms show a SLC-like structure (probably incipient);
but any trace of this was lost in the ornithomimosaurs, which show
loosely-articulating discoidal carpal bones.  If ornithomimosaurs did dabble
in the water like ducks, then it might have been useful to have a hand
capable of a wide range of movements - such as for stirring and scooping up
sediment.


Tim


------------------------------------------------------------ 

Timothy J. Williams 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 3163