RE: Broader than Dinosauria [Additional Info]

[Tim Williams <TiJaWi@corn2.agron.iastate.edu>]

Ken Kinman wrote:

>Although the twist-thumb is apparently present in most theropods
>and prosauropods (any sauropods??), it is unfortunately known in only >one
ornithischian (Heterodontosaurus tucki).   It may be more >parsimonious to
view the twist-thumb as having arisen separately in >ornithischians and
saurischians, rather than multiple losses in >ornithischians 

If heterodontosaurids are the only ornithischians to exhibit a "twist-thumb"
(i.e. with the phalangeal articulations compelling the pollex to converge
with the other digits during flexion of the manus), then it was lost three
times in the Ornithischia: Thyreophora, Euornithopoda, and Marginocephalia.
(This is the case irrespective of whether heterodontosaurids are basal
ornithopods or basal marginocephalians.  I'm not aware of the details of
_Lesothosaurus_'s manus.)  Compare this to how many times teeth were lost
within the Theropoda.

Bakker and Galton's hypothesis regards the "twist-thumb" as a primitive
dinosaurian trait.  If not, it was evolved multiple times in the course of
dinosaurian evolution.  I can't see how this scenario could be more
parsimonious.


> P.S.  By the way, I believe it was one of Jaime's posts that mentions
>the lack of a twist-thumb in segnosaurs.  If they are the only theropods
>to lack the twist-thumb, it would be interesting to know why they were >the
only theropods to have lost it.

Herbivory?  If the hands were used to bring tree branches to within reach of
the mouth, then there was no need for a partly opposable pollex.

This may have been the rationale behind its loss in other herbivorous
dinosaurs.  Or it may have been due to browsing at ground-level
(thyreophorans?); or to increased quadrupedality (sauropods?); or more than
one of the above.  Perhaps the twist-thumb was an asset to bipedal omnivores
or carnivores; otherwise it was of limited utility, and lost.

All a lot of "just-so"-ing (sensu Tom Holtz) to be sure.  But I think
there's sufficient evidence to keep the idea of a single origin for the
"twist-thumb" alive.



Tim