Re: bipedal

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

At 09:05 AM 10/31/96 -0500, Paul Sparks wrote:

>Stevet sez that most mammals have stronger rear bodies than front.
>Unfortunately that is not true, (ref some recent PBS thing) It may look that
>way until you dress one of them out and see where all the meat (muscle) is.
>An extreme case is the mountain goat where there is damn near no meat aft of
>its belly button. On the other hand look at a croc standing on the bottom
>with its head out of the water. It looks just like a theropod. Surprised me
>when I saw it. We primates differ because we liked to do terrible things in
>trees. comments?

Stop me if you've heard this one before... :-)

Mammals are therapsid synapsids.  As has been long pointed out, therapsids
in general, from Moschops and dicynodonts and Cynognathus up through the
Mammalia, have more powerful forequaters than hindquaters (or, as Nick
Hotton says, they have "front-wheel drive").  The major thrust muscles for
the hindlimb attach to the hips, so that the tail is not as important in
locomotion as in diapsid reptiles (see below).  This is the reason most
therapsid tails are fairly short or, if long (as in cats, for example) are
not particularly thick or deep.  This is also the reason humans have buttocks...

(Also, many primates DO have more powerfully built forequartes than
hindquarters: baboons, gibbons, gorillas, and the occasional Mr. (or Ms.)
Universe).

Crocs and dinosaurs (and lizards and tuataras, etc.) are diapsid reptiles.
Diapsids are characterized by hindlimbs longer than the forelimbs.  Almost
all diapsids are capable of going momentarily bipedal at high speed runs
(some, such as the frilled lizard, can do so at relatively low speeds).
Their main thrust muscles for the hindlimb attach from the femur and pelvis
to the tail.  Thus, most diapsids retain a long, powerful tail.  This is
also the reason why dinosaurs did not have buttocks, and why the presence of
such on Russell's infamous "dinosauroid" was considered so unlikely.

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 the Holy Grail"
---------- subtitle --[Monty Python ik den Holy Grailen]

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