Re: Prosauropods

["Jaime A. Headden" <qilongia@yahoo.com>]

George Olshevsky (Dinogeorge@aol.com) wrote:

<Still don't see how sauropods could reevolve a two-phalanx fifth pedal
digit, which is vestigial (one phalanx) or lost (zero phalanges) in all
known prosauropods. Unless of course the fifth digit is lost repeatedly
and independently in each separate prosauropod lineage in this cladogram,
and also in ornithischians and in theropods.>

  The fifth toe does not appear to be present in *Thecodontosaurus* and
only one phalanx is present in most plateosaurs and *Anchisaurus*, as well
as *Herrerasaurus* and, presumably, *Eoraptor*. This citing of a
re-evolution is, I think, not a good choice of reason to inhibit the
numerous other characters. Indeed, it does nothing to contradict my
phylogeny, in which all "Prosauropods" are still basal to Sauropoda within
Sauropodomorpha; even *Saturnalia* is so sauropodomorphan that it is
unlikely to ever permit the restriction of any current "prosauropod" from
Sauropodomorpha.

<More interesting is to consider the possibility that sauropods are basal
to all other dinosaurs, that prosauropods and ornithischians are sister
groups, and that theropods are a sister group to the
sauropod+ornithischian group. This is what the feet say, anyway.>

  Not exactly, and interesting only in trying to use the feet, which are
shown to develop into the sauropod condition from a cursorial, gracile
one, to support this schism. Sauropods are hardly basal to other
dinosaurs, and the numerous saurischian apomorphies and basal dinosaur
studies, as gone over extensively way back when Kinman was challenging the
phylogenies, show the exact reversal. You would have to selectively ignore
about twenty+ characters providing the present topology of

--+--ornithischians
  `--+--+--plateosaurs
     |  `--sauropods
     `--theropods

  in order to reach such a conclusion. As noted embryonically, birds
retain the ability, though coupled with an inhibitor and no longer
expressive, to form chondrostomes for the phalanges of the other two
digits of the hand, one medial and one lateral. It is not then far-fetched
to presume the ability to paedomorphically retain the phalanges late into
the evolutionary lineage. That *Herrerasaurus* still possesses a broad,
subcursorial pes, being very prosauropod-like in its design, but with a
theropodan ankle and hip, it would seem that this condition is basal for
saurischians in specific, and perhaps dinosaurs in general. Only in some
of the studier "prosauropods" do I note the presence of a facet on the
first phalanx of the fifth digit (pdV-1) to support a second, and this is
in *Plateosaurus* and *Ammosaurus*, both plateosaurs. *Anchisaurus* has a
conical, tapered distal first phalanx, and *Thecodontosaurus* had at least
one phalanx but I do not think it has been recovered (the metatarsal is
somewhat indicative of this, I beleive). This was a non-functional digit
with regards to locomotion, and may have formed the basis of a "spur" or
some such ... wild speculation with the last bit.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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