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THERIZINOSAURS, SYNAPOMORPHIES, AND PLESIOMORPHIES



George Olshevsky wrote (quoting Jon Wagner (quoting George Olshevsky)) [I
have inserted the initials of the author at the beginning of what they said
to make it easier]:

GO:(1) Saurischia doesn't exist, or if it does, it is congruent to
Dinosauria
JW:         Under the terms of phylogenetic taxonomy, by your phylogeny, the
two
 would be not be synonymous. Dinosauria is the stem based clade of the common
 ancestor of _Triceratops_ and Neornithes AAOID, Saurischia would be
 synonymous with Theropoda (all dinosaurs more closely related to Neornithes
 than to _Triceratops_(?), or Sauropodomorpha).

GO:Details, details. Dinosauria has so many definitions that you can take
your
pick. If Dinosauria is defined as the common ancestor of _Iguanodon_ and
_Megalosaurus_ plus all its descendants (best definition I've seen, if I do
say so myself), then Saurischia is congruent to it. If you define Dinosauria
as the stem group you describe, then Saurischia would by my phylogeny
certainly NOT be synonymous with Theropoda, since it includes the
phytodinosaur _Triceratops_.

PB:  Wagner is correct here.  Dinosauria is {Neornithes + _Triceratops_},
Saurischia is {Neornithes > _Triceratops_}, so if sauropods are found to be
clade closer to ornithischians than to theropods, Saurischia becomes
congruent with Theropoda {Neornithes > _Diplodocus}, and might become a
senior synonym (I am doing this from memory and I can't remember when they S
and Th were defined cladistically).
 
GO:All the characters that are listed by, say, Benton in _The
 Dinosauria_ that supposedly unite Theropoda and Sauropodomorpha into a
clade Saurischia are either plesiomorphies, homoplasies, or doubtful.
JW:         See Gauthier 1986
         To paraphrase nearly every current systematicist, the only way one
 can determine that a character state is plesiomorphic or homoplastic is by
 phylogenetic analysis.  Very few scientists will be interested in the a
 priori assumption of homoplaisy.

GO:If one can determine homoplasy within a phylogeny only by performing more
cladistic analysis, then cladistic analysis is tautological and thus not
scientific. If there is no way to check whether a cladogram models reality,
cladistics is unscientific and is simply an exercise is making pretty
patterns out of character matrices.
 
PB:I think it is really quite impossible to know the internal structure of
the Dinosauria until we get some hands and heads and feet (that haven't been
weird-ified).  For example, it is claimed that the 'Saurischian hand'
supposedly unites theropods and sauropods, but how can you know if it is a
true synapomorphy absent in the ancestoral dinosaur, or if it is simply
plesiomorphic for the Dinosauria as a whole, and the hand of ornithischina is
the one that is truely derived.  At present you can't.  The only way you
would be able to would be to compare it to a close dinosaurian outgroup like
say _Marasuchus_ (handless), _Lagerpeton_ (handless) or _Sclermochlus_
(handless).  Heck, the closest non-dinos you CAN compare hands with are
pterosaurs, and that's hardly helpful.  Ditto with heads.  _Santanadactylus_
is hardly useful as an outgroup when trying to decipher skull characters in
theropods, 
sauropods and ornithischians....

As for now, I'd consider Dinosauria an unresolved polytomy consisting of
theropods, sauropods, prosauropods and ornithischians.

GO:leaf-shaped herbivorous dentition.
JW:         As noted by Gauthier (1986), this morphology is a common adaption
of
 reptilian tooth pattern, as it occurs in lacertillians as well as several
 archosaur groups.  While it is a potential synapomorphy, the diagnostic
 utility of this feature is doubtful.>>

GO:At the level of Phytodinosauria, it's very close to being a synapomorphy.
It's certainly a derived state (primitive would be some kind of insectivorous
dentition). I know of no archosaurs with leaf-shaped dentition that are
anywhere close to Dinosauria. Aetosaurs are the only others, and they seem to
have acquired this as an apomorphy.

PB:Though it is possible, that the leaf shaped teeth are homologous, it is
equally possible that they are convergent.  Oddly enough though, the teeth of
prosauropods and ornithischians resemble eachother far more than either one
resembles those of therizinosaurs, which are clearly those of bullatosaurs.
 
GO:(3) Heterodontosaurids are not ornithopods [...]
and there is [not] an obturator process on the ischium. 
JW:         It is possible that many of common conceptions concerning the
 diagnostic utility of this element are in error.  I would argue that the
 *absence* of the obturator process is by no means significant, excep tint he
 context of one being present ancestrally, as it appears to have dissappeared
 several times over dinosaurian history, and may have even been redeveloped
 in some lineages.
         See Britt 1992 and Novas 1997.>>

GO:In Ornithischia, the obturator process developed well along in the group's
evolution. Along with having a prepubic process that extends farther forward
than the anterior process of the ilium, it is a synapomorphy of Ornithopoda.
 
PB:I agree with George that Heterodontosaurids (including _Echinodon_) are
probably not ornithopods, but instead basal marginocephalians by way of the
large jugal horns (which are quite different from those on zephyrosaurs),
three fang premax teeth and some other characters of their vertebral column
and pelvis.  Oddly enough, almost every single person who has ever worked on
Heterodontosaurs up close and personal insists that they are in no way shape
or form ornithopods, but somehow related to ceratopians or pachycephalosaurs
or both.

GO:jaws
JW:         The lower jaw of _Erlicosaurus_ is similar to that of
_Harpymimus_,
 as P. Buckholz [sic? sorry Pete] has pointed out.>>

GO:It's much more similar to the lower jaws of >any< prosauropod, as well as
many sauropods and even stegosaurs.
 
PB: Uh...  are you sure you're looking at the right picture George?  Try
_Harpymimus_, lingual view, nearly identical except for the tooth count, much
more so than in prosauropods or (are you serious?) stegosaurs.

GO:pelvis
JW:         Retroverted pubes are present in some maniraptors, and anyway
are,
 in this case, possibly related to herbivorous habits and are thus not to be
 excluded from the possibility of convergence.

GO:So are segnosaurs dromaeosaurids or birds? Those are the only theropods
that
display retroverted pubes. >Of course< retroverted pubes in segnosaurs are
convergent with theropod retroverted pubes; but they could well be homologous
with retroverted pubes in Ornithischia.

PB:  Are you sure?  It is quite clear then, that if the pubes are homologous
to those in ornithischians, then Therizinosaurs are ornithischains, and not
only that, ornithischains more derived than _Pisanosaurus_ which has a
*propubic* pelvis.  Is that what you're saying George?  If it isn't, then the
retroverted pubes are just as likely to be derived for theropod pubes
convergent to ornithischians.
 
GO:good arctometatarsalian or avimimiform theropod with a highly derived,
JW:         Avimimiform?  Do you have access to material on this taxon that
the
 rest of us do not?  I am interested to know what autopomorphies _Avimimus_
 and _Mononykus_ might share, which the latter taxon does not share with
 birds or arctomets. 
GO:Very slender, distally tapering fibula, closely appressed to the tibia,
for
one. _Mononykus_ can't be a bird because, like _Avimimus_, it doesn't have an
avian metatarsus; it has an arctometatarsalian metatarsus. And a well
developed tail with elongate chevrons. So the slender fibula becomes a
potential synapomorphy of _Mononykus_ and _Avimimus_. There may be others,
particularly among the femoral trochanters, but I haven't finished looking at
the literature.

PB: Ya, what is an avimimiforme?  It is an idea of mine though that
_Avimimus_ is a chimera, the head and some other stuff belonging to a new
oviraptorosaur (as hinted at by Perle to Novacek in Dinosaurs of the Flaming
Cliffs), and feet and legs belonging to some parvicursorine bird that died
kinda nearby.  If anyone has any literature that would explain the taphonomy
of _Avimimus_, I'd love it.

Peter Buchholz
Tetanurae@aol.com

This one day, I didn't eat for four days.