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RE: WHAT are segnosaurs?



Tracy Ford (dino.hunter@home.com) wrote:

<What if Prosauropods where primitive theropods (as a few of
them were thought of by Huene and I think it wasn't all do to
the missinterpeted rauischid teeth) and not related to sauropods
at all? (Which I think anyway).>

  I think "Prosauropoda" as classically used is a polyphyletic
assemblage. The problem has been using plesiomorphies to tie
many "prosauropods" into a group that diverged from sauropods
essentially. The basis is not that big, and in fact some basal
"prosauropods" such as thecodontosaurs and anchisaurs (=
ammosaurs) appear to have features that remove them from the
lineage after the split, but occured before it. A "Plateosauria"
is more probable, linking plateosaurids and melanorosaurids
together, but that too many be paraphyletic. What does seem
interesting is the few plesiomorphies that have been used to
keep Melanorosauridae out of the running for sauropod ancestors,
assuming they were "too derived to be sauropod ancestors" as I
quote from whom I cannot recall now. The fact is, either
plateosaurids or melanorosaurids/blikanasaurids have more
features in common with sauropods than anchisaurs or
thecodontosaurs do.

  Two new taxa, *Isanosaurus* and *Yimenosaurus* have not been
added to any strict analysis that I know of except to say that
the first is a very basal sauropods (it had some plesiomorphies
reminiscent of "prosauropods") and that the second [described as
a prosauropod] has a skull that could be the very key to
sauropod evolution, being both sauropodan and prosauropodan in
its features. This makes prosauropods likely paraphyletic, but
I'd like to have some times trying this out :).

  Meanwhile, with some basal "prosauropods" outside of the
{*Plateosaurus* + *Saltasaurus*} clade, Sauropodomorpha get's a
little more interesting: the forms become increasingly more
"predatory" or "carnivorous," with longer, thinner and perhaps
recurved teeth, fully bipedal with smaller guts, larger heads,
longer lower legs, that the most basal sauropodomorphan was
probably quite "theropod" looking, as they get older or more
primitive. Now, Huene's original hypothesis was based on having
rauisuchid or theropod (?) skull material and teeth associated
with "prosauropod" postcrania, and possible other theropod
material, with *Aliwalia* being a notorious example of a
"predatory prosaruopod". When one looks at herrerasaurids and
thecodontosaurids together one sees that these two forms are
quite similar and that there is not that much variation that is
taking place between the two compared to, say, the
diplodocimorph/camarasauromorph split. What _has_ taken place,
apparently, is that one lineage has become omnivorous, trending
towards herbivory, and the other has stayed carnivorous. I would
suggest that "prosauropods" excluding the huge ones like
melanorosaurids, were still omnivores.

  Thus if we see basal prosauropods looking theropodan, there's
probably a good reason for this, due to the basal derivation
from predacous archosaurs like lagosuchids, etc. I haven't read
a whole lot of Huene's work, and wish I could, but he may have
very well stumbled onto this himself, but based on limited data,
may not have been able to elaborate so that his colleagues would
have gained from the insight. Huene has had, so far as I can
recall, one of the best track-records in paleontology.

  Appologies to Mickey for usurping his dialogue :)

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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