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Re: Running with arms out



At 03:15 PM 5/9/99 +0100, John Jackson wrote:

>In Tom Holtz's reply to Larry Febo: Date: Fri, 7 May 1999 17:55:57 -0400
>(EDT):
>
>>Consider this scenario.  I'm not saying it is correct, but I am offering it
>as a possibility:
>I) Ancestral dinosaurian carnivores with moderate length arms, wrists
>without much "folding" ability in the manner of birds.
>II) Selection favors variants with a longer reach (longer arms, longer
>hands).
>III)  Selection favors variants among long-armed dinosaurs with the best
>ability to fold and tuck the arms.
>IV)  Having evolved the fold-and-tuck structures, variants with greatly
>elongated arms can develop and still use their forelimbs to sieze prey or to
>climb with, without the arms getting in the way while running.
>
>Although this is all a just-so story, it matches the pattern we see in
>theropod evolution (I being basal theropods and ceratosaurs, II being basal
>tetanurines, III being basal avetheropods (with small semilunate carpal
>block) such as carnosaurs, _Coelurus_, _Scipionyx_, tyrannosaurids, and
>maybe ancestral ornithomimosaurs, and IV being long-armed maniraptorans.
><
>
>I would agree with that as a possible scenario but with two reservations:
>
>Is a gradual lengthening of arms really there?  (This is a delicate point
>because in any theory there has to be a lengthening of the arms, but Larry's
>(and my, Larry!) position  advocates an explosive initial lengthening for
>flight purposes; however, there might also be a lengthening for
>tree-climbing purposes.  In this case, we might expect any lengthening to be
>associated more with the smaller specimens, as is I believe the trend.)
>
>Why did this new way of doing an old trick only appear after 100 mys?

Well, as for "gradual" versus "punctuated", we really can't say because of
the lack of good continuous fossil sequences.

However, the pattern is there: basal tetanurines have bigger hands as a
portion of the forearm than ceratosaurs, avetheropods have a semilunate
carpal block, coelurosaurs have a narrower hand (forgot that part),
maniraptorans have greatly elongated arms.

Now, in fact, all of these changes would have occurred in the first eighty
million years or less of the Age of Dinosaurs, not after 100 million years
had passed.  If _Cryolophosaurus_ is indeed a carnosaur, then we get to
stage III by the late Early Jurassic, forty million years or so after the
oldest known dinos.  If the Yunnan therizinosauroid is indeed a
therizinosauroid, then we get to stage IV within the first thirty or forty
million years; if not, we've definitely got to that stage by the Late Jurassic.

The early record is poor, but it's there.

>Also, in reply to a posting by Dan Pigdon, Tom says:
>
>Simply sticking your arms out remains unfeasible.  The paper demonstrated
>that the forelimbs of a running bird such as _Ax_ could contribute both lift
>and thrust (which is not the slightest bit worthy of space in "Nature"), and
>this might help it run faster (which it *didn't* demonstrate, but wouldn't
>be very surprising at all to most people and was not to my mind worthy of
>publication anywhere).

Well, true in a sense.  However, several people are on record as saying that
there would be no advantage to flpping while on the ground, but Burgers &
Chiappe suggested that in fact there was.  In that sense it was important.
I, too, would like to see a more elaborate version of the paper.

>However, the other aspect for which I don't have a generic name is: isn't it
>an astounding coincidence that such a detailed form would have been so
>amazingly perfect for flight?

Except it wasn't "perfect for flight", at least not at the beginning: there
is clearly an elaboration and refinement of the flight apparatus (in the
wing and in the tail) post-_Archaeopteryx_ (indeed, regardless of whether
dromaeos, oviraptorosaurs, or troodontids are within Aves or not): the wings
of Archie & _Confuciusornis_ lack many of the sophistications present in
enantiornithines and ornithurines.

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