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ARTHROLOGY AND FLIGHT CONTROL





<<With the aid of gusts, getting off the ground is dead easy.>>

Rayner (1991) proposed a ground-up model with a glider that used the 
wind to glide off the ground.  He still preferred the arboreal gliding 
idea because of the easier adaptive steps, but he came down on the side 
of cursorial origins of avian flight because he could see no way that 
the ancestors of _Archaeopteryx_ were arboreal.  (It should be noted 
that some are working on this idea, and are coming down on the side of 
arboreal _Archaeopteryx_ and its ancestors).  Rayner calculated that an 
air speed of 7-9 m/s would be appropriate for take-off.  Rayner seemed 
to think that the protobird could just run and jump into the wind.  I 
agree that this model might make the origin of flight of the ground-up 
possible, but I think that it fails in other regards.  

The biggest fault of this hypothesis is that it would require a great 
amount of flight control.  Flight requires a great deal of control from 
the neuromuscular system and within the wrist.  Jenkins et al. (1988) 
showed through use cineroradigraphs that the wrist goes through several 
motions through the flight stroke.  These motions are generalized 
through all birds (except for hummingbirds to an extent) and that can 
easily tell one that these motions are very important sustaining control 
and equilibrium in flight.  However, no one in 1988 had studied the 
motions of the wrist (arthrology; the study of joint movements); the 
studies on the wrist started in 1992 with Rick Vasquez's study.  Vasquez 
used anatids to show that the carpals in the wrist go through several 
motions through the flight stroke.  For example, during the downstroke 
the manus (carpometacarpus in proper avian anatomy) is kept from 
hyperpronating by the cuneiform (ulnarae).  The typical avian ulnarae is 
heart-shaped (contrary to my past claims, this feature is probably not 
present in _Archaeopteryx_) and the V-shaped groove articulates with the 
carpometacarpus while the ulnar facet (Facies articularis ulnaris) 
articulates with the ulna.  Resistance causes the carpometacarpus to 
press against the cuneiform, which in turn drives against the articular 
ridge of the ulna and stops any dislocation.  During gliding the radiale 
(scapholunar) locks the manus in place.  Etc.  

Anyway, it seems that _Archaeopteryx_ and its theropodian outgroups lack 
the unique features of the avian wrist which allow these special 
kinematics to take place.  Vasquez said it best:

"These are important implications here for both the flight capabilities 
of the fossil _Archaeopteryx_ and for the evolution of powered flight.  
_Archaeopteryx_ is well known from six specimens and in none of these 
does the wrist exhibit the high degree of specialization found in modern 
birds.  For example, in the Berlin and Eichstatt specimens, in which the 
details of the wrist are best preserved, the carpometacarpus lacks any 
indication of a ventral ridge and the ulna does not exhibit a pronounced 
articular ridge.  Moreover, both the cuneiform and scapholunar are 
relatively small and the articular facets that characterize these carpal 
elements in the modern avian wrist appear to be missing (Wellnhofer, 
'74; Ostrom, '76).  It is difficult to imagine that such a "primitive" 
wrist would have been able to keep the manus from hyperpronating during 
the downstroke.  Equally unlikely would be its ability to rotate the 
manus systematically from the plane of the body, and then back again, 
during the upstroke-downstroke wing cycle.  In short, the wrist 
structure of _Archaeopteryx_ strongly suggests that it was incapable of 
executing the same kinematics displayed by modern birds during flapping 
flight." (Vasquez 1992; 266).  

This is not to say that _Archaeopteryx_ was not capable of powered 
flight, the wrist kinematics in modern birds allow slow flight and 
better control during all phases of flight.  There have been many 
critics of Vasquez; Feduccia (1996) tried to explain that the fossils of 
_Archaeopteryx_ lack any cartilageous imprints and cartilage makes up a 
big part of the avian wrist, so the wrist of _Archaeopteryx_ can look 
like anything.  Regardless, the two carpal elements (cuneiform and 
scapholunar) are too small and medially placed for this to make up too 
much of a difference.  

The point that is trying to be made here is that the wrists of 
_Archaeopteryx_ and its theropodian outgroups show that the kinematics 
present in modern avians were not present.  That these kinematics were 
not present suggests that early birds and their ancestors were incapable 
of flying very well in windy conditions.  Gliders today can maneuver 
through wind, but with great difficulty.  This would argue against the 
"windy" hypothesis because, though it is possible to maneuver through 
wind no matter what kind of flight, it would still be hard to do so.

Matt Troutman  

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