Re: What is a Dinosaur? and semilunate carpal

["Matthew Bonnan" <mbonnan@hotmail.com>]

twilliams_alpha@hotmail.com writes:
>
> << I wonder if this feature, like the increasingly inflexible wrist of
>  tetanurines, was designed to prevent the forelimb and hand from
"wobbling"
>  while gripping prey - in other words, providing a more stable and secure
>  grip while engaging struggling prey. >>

Dinogeorge responds: > No, it was "designed" to prevent wobbling while the 
animal was winging its
> way through the air. Wobbling wings don't work very well, and any
> anti-wobbling adaptations would be selected for in the evolution of
flight.
> The anti-wobbling features of the forelimb were later inherited by the
> flightless descendants of the winged ancestral tetanurans and exapted for
> other purposes.


The forearms of crocodilians and most lepidosaurs are rigidly constructed.  
The radius and ulna articulate proximally and distally like part and 
counterpart pieces.  The radius does not have a rounded radial head; the 
head of the radius is usually angular, and it is physically impossible to 
move the radius about the ulna as we can do in our own forearms.  If you 
take an alligator forelimb and hold it so the ventral surface faces 
anteriorly, you are essentially holding the forelimb of a predatory 
dinosaur: the radius and ulna are interlocked together and the radius lies 
anterior and lateral to the ulna proximally, and anterior to the ulna 
distally.  The manus is turned sideways so that the palms face inward in a 
prayer-like manner.

Therefore, the rigidity of the dinosaurian forearm, regardless of the BCF or 
birds after dinosaurs scenario, probably developed >prior< to the evolution 
of flight or even bipedalism.  Whereas rigidity became a means for achieving 
stable forelimbs in flight and/or predation later on, this rigidity was 
already present in the ancestors to dinosaurs.

Matt Bonnan
Assitant Professor
Dept. Biological Sciences
Western Illinois University
Macomb, IL 61455

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