Re: forelimbs

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 01:52 PM 6/23/99 -0100, B.Dol@skn.sc.philips.com wrote:
>I have a question. Last night I was thinking about theropods and I 
>wondered about the forelimbs. Very small indeed in some species. What 
>I would like to know is the following:
>Do these forelimbs have restrictions in movement?

Well, all forelimbs have some restrictions in movement.  However, it is
certainly true that some primitive (coelophysids) and specialized
(tyrannosaurids, _Carnotaurus_, etc.) taxa would have more limited mobility
and range than forms like allosaurids, maniraptorans, etc.

>Could they be 
>swayed out sideways or only forwards/backwards?

For most theropods, there would be more limited movement out to the side.
Maniraptorans (esp. troodontids and dromaeosaurids) have more laterally
oriented shoulder joints, so their arms could stick out more towards the side.

>If there were 
>restrictions then: did some species suffer (i.e smaller and/or 
>larger) more restrictions than others or did this decrease come with 
>time?

Yes, some had more limited motion ("suffer" is a loaded term: I don't think
that sharks "suffer" as predators even though they can't come on land) than
others.  In a reduced form, dinosaurs primitively would have a relatively
limited range of forelimb motions; in Tetanurae this range becomes greater
(secondarily reduced in a few forms), greater still in Maniraptora, and
greatest in Avialae (birds).

(Simultaneously, though, the degrees of freedom of motion in the wrist
decrease: primitive dinosaurs would have more directions of wrist motion
than tetanurines (esp. maniraptorans (esp. birds)).  The latter have fewer
directions they can more their wrist, but within that plane they can move it
through a big angle).

Hope this helps.

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