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RE: evolution of theropod wrist motion




James Norton wrote:

>If the BCF folks are correct, and theropod dinosaurs are the
>secondarily 
>flightless descendents of early feathered flyers,

These days, you don't need to be a disciple of BCF (sensu Olshevsky) to 
believe that certain "traditional" theropod dinosaurs are secondarily 
flightless avians.  There is a view that a subset of theropods
(dromaeosaurids, for example) may share a common ancestor with
pygostylian birds, with both descended from a flighted
_Archaeopteryx_-like ancestor.

>wouldn't there have been enough evolutionary time between the earliest 
> >theropods and Deinonychus, for example, for theropods to have
regained 
> >the wrist mobility one would expect in a ground-based predator?

Assuming (for the moment) that _Deinonychus_ did evolved from a powered 
flyer, and it is (therefore) secondarily flightless, there may not have
been any selective pressure to regain wrist mobility.  Flying theropods
may have been accustomed to using their hands and feet to dispatch prey,
for fear of fouling the wing feathers.  This behavior was inherited by
putatively secondarily flightless theropods like _Deinonychus_.  The
hands did retain their use in "clamping" prey (as decribed in my last
post).

Under the more traditional scenario, the bird-like features of
dromaeosaurids and other maniraptorans were originally evolved for
catching and holding prey: long arms, long hands, semilunate carpal,
furcula, ossified sternals.  These features were later exapted into the
flight apparatus of birds - not the other way round, as BCF posits.

> Why would the limited wrist mobility persist?

The semilunate carpal not only linked the motion of the hand to that of
the rest of the forelimb during the "predatory stroke", it might also
have reinforced the predator's grip when it was holding the prey.  

Limited mobility is not always a disadvantage.  For comparison, look at
another pulley-like structure which has served its owners well: the
trochlear astragalus of artiodactyl mammals.  The added stability and
altered range of motion is a good adaptation for terrestrial running.

The semilunate carpal, though, would have been a downright nuisance for
certain modes of behavior - especially involve pronation/supination.
Ornithomimosaurs lost their swivel-wrist.  These theropods were almost
certainly not predators.  The manus of ornithomimosaurs was once thought
to be used as a hook-like device (such as for grabbing branches).  
(Warning: Rampant speculation coming up!)  With the jaws of
ornithomimosaurs now thought to be used for water-feeding, the hands may
have been used for scrabbling in the sediment.  A swivel-wrist would
have been useless for this purpose.

(Warning: More rampant speculation ahead!) The therizinosaurids, which
were probably herbivores, did retain the semilunate carpal (albeit as
two elements - secondarily separated?)  However, if therizinosaurids did
feed on trees, then they might have reached up or ahead for branches.
Long arms and hands would be very useful.  


Tracy Ford wrote:

>And its better to catch things with two hands than one :) 

>I'm sure that's what the little league coach would say (I've never been
>in little league, but that's what my soft ball team coach said). 

And in cricket too!  :-)



Tim