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

Re: Please remind me!



Sherry Michael wrote:

>Have you ever noticed that sometimes the more you learn the less you
>recall?
>
>Someone asked me why, "the lizard hipped dinosaurs are said to have
>birdlike hips".  You know, I understood this reasoning once, but after
>looking over a few texts, I have something incomplete. Does it have
>something to do with similar muscle attachments? Why did birds "revert"
>back to the bird hipped form? A change of distribution in posture due to
>no tails and wishbones?

Wishbones have nothing to do with it: some theropods with typical
saurischian pelves (allosaurids, tyrannosauruds, etc.) have good, honest,
true, homologuous wishbones (to distinguish from the probably-convergent
wishbones of some of the Triassic diapsids, e.g. Longisquama).

The tails may have everything to do with it: in dromaeosaurids and early
birds, the swinging back of the pubis happens along with a transformation of
the tail.  Whereas earlier theropods had a long muscle (the Caudofemoralis
longus, which started from the leg and stretched along much of the length of
the tail) as the primary thrust generator, dromaeosaurids and birds
rearranged the primary thrust to muscles on the hips and the base of the
tail.  At the same time, this freed up the length of the tail for use as a
dynamic stabilizer and (in later birds) allowed the shortening and fusion of
the tail bones.

Birds did not revert back to a bird-hipped form: tracing bird origins starts
with the primitive archosaur condition, to the condition in primitive
theropods and sauropodomorphs (the typical "saurischian" pelves), to the
swept back pubis in dromaeosaurids and birds.  The backwards pubes in
ornithischians and therizinosauroids almost certainly occured independantly
of birds.

This is very different from the probable origin of the backwards pubis in
ornithischians and therizinosauroids, in which the pubis probably swept
backwards to expand the gut size, allowing the bigger stomachs and
intestines for better digestion of plant matter.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"There are some who call me...  Tim."
-- Tim the Enchanter, "Monty Python and Quest for the Holy Grail"

"Tim?!?  They called me TIM?!?!"
-- me, on seeing the credits to "The Ultimate Guide to T. rex"  :-)