Re: dinosaur humps

["T. Mike Keesey" <tmk@dinosauricon.com>]

On Fri, 10 Mar 2000, Larry Febo wrote:

> OK. Forget about the elongated spines. What keyed me into the idea
> that ornithischians may have at one time been arboreal was the mention
> of the narrowness of the body from side to side. Seems that was also
> true of Megalancosaur.

Again, I think this is a derived condition of hadrosauroids.

> Of course then there are the ankylosaurs that certainly were not that way.

Quite the opposite -- the broadest of all dinosaurs.
Pachycephalosaurs were also quite broad.

> Then again, they are also highly derived.

Hence we should look to primitive forms, which are neither very broad nor
very narrow.
 
>  And then there`s the retroverted pubis,
(Technically a pubic process, isn't it?)
> that may have been an original
> adaptation for getting the center of gravity closer to tree trunks and
> branches for more stability.

Interesting. That's been proposed (and seems likely) for eumaniraptors.
For therizinosaurs and ornithischians, however, it's generally thought to
be for increasing the volume of the gut, since they were herbivorous.

> I believe I read this in Romer`s book on vert.
> evolution, although the idea was later discounted (no exact reason given as
> to why it was rejected,  it seems perfectly reasonable to me).And , of
> course, the pelvic girdle of prolacertilians seem much too primitive to make
> any comparison with that of ornithischians.
> 
> Anyway, you don`t think that early ornithischian ancestors could have been
> arboreal?

Scansorial early on perhaps. Ornithischians more primitive than
_Lesothosaurus_ (which already has the predentary bone and the pelvic
condition) are not known, after all. And all dinosaurs descend from
"lagosuchian" stock, which seem capable of scansoriality.

-- T. Michael Keesey .................................. <tmk@dinosauricon.com>
   My Worlds (including The Dinosauricon) ... <http://dinosauricon.com/keesey>
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