[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Giant flightless birds



----- Original Message -----
From: Thomas R. Holtz, Jr. <th81@umail.umd.edu>
To: Larry Febo <larryf@capital.net>
Cc: <dinosaur@usc.edu>
Sent: Tuesday, May 11, 1999 10:31 AM
Subject: Re: Giant flightless birds

(snip)

>
> >Archaeopteryx had three
> >free digits (even pterosaurs had three free digits), but they don`t seem
to
> >be opposable.
>
> Take a look again: digit I is just about as opposable as in _Deinonychus_,
> _Troodon_, etc.  Remember that dinosaur digit opposability is achieved by
> rotation of the metacarpal-phalangeal joint, rather than at the
> carpal-metacarpal joint (as in, for example, humans).  Also, dinosaur
> "opposability" is not a precision grip between the fingers (as in humans),
> but simply bringing digit I in towards the palm at a different angle than
> the others.
>

(Hmm, I thought I had once read something about Archies digits being
opposable.)

(more snip)

> Oh, but tyrannosaurids DO have about the same opposability as in other
> tetanurines: digit I does come in at an angle to the rest of the hand.
The
> main difference with tyrannosaurid hands is that they are proportionately
so
> small and they have lost the third finger.
>

Dr Holtz, I learn something new every time I have a discussion with you! I
never thought of T_Rex as having opposable digits. I`ve never seen it
pictured as having what looked like opposable claws. But to what degree?
Could it have grasped something one "handed"? I`d still like to make the
arguement that it couldn`t have been that efficient at grasping, else why
would it have degenerated? More likely it`s lack of grasping ability lead to
it`s forearms becoming somewhat vestigial structures. (I specify "somewhat"
vestigial because perhaps they were used to manipulate its eggs in the
nest??)

Are there any three digit (manus) theropods that also have very short arms?


> >Again, it seems to make more
> >sense that a terrestrial animal would always be in need of good grasping
> >ability of its forelimbs for ther procurement of prey.
>
> Tell that to a dog, a hyena, or other jaw-based hunter!!  These animals do
> not use the forelimb to aquire prey: at best they use them to stabilize
the
> victim while the jaws do their work.  Interestingly, these predators also
> have very powerful jaws and neck muscles, claws which are not well curved
> (compared to cats), and very long and slender legs: these same features
are
> also seen in tyrannosaurids.
>

I should have specified "bipedal" cursorial animals....and I know you can
say that T_Rex, Mononychus, and many flightless birds do fine, but would a
terrestrial animal have any enviornmental pressures to lose functional
grasping arms. I mean why would they evolve in that direction? I can give
what I believe is a good reason, that being, all of the above were
secondarily flightless, and were "stuck" with the genetic baggage they
inherited from their flying ancestors. I cannot think of any reason a
Primarily flightless biped would have to loose its long forearm reach and
ability to grasp objects.



>>The only reason for
>>loss of such ability, (that I can visualize) would be that the condition
was
>>inherited from an Avian ancestor that itself had lost the third digit.


> And yet I can see another.  What we must now do is find some way of
choosing
> between these models other than our own particular preferences.
>

I`ve tried to make a logical arguement here based on how " natural
selection" might have operated. I guess more solid proof will someday have
to come from more complete fossil evidence. (Finding a fossil bird closer to
Tetanurines than Maniraptors would be nice!)

> Thomas R. Holtz, Jr.
> Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
> Dept. of Geology              Email:tholtz@geol.umd.edu
> University of Maryland        Phone:301-405-4084
> College Park, MD  20742       Fax:  301-314-9661
>