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Re: THERIZINOSAURS, SYNAPOMORPHIES, AND PLESIOMORPHIES



Sorry to take up so much bandwidth here, but I have to keep repeating the
quotes if my comments are to make sense. I'll preface them with REPLY:

In a message dated 97-09-04 15:17:31 EDT, Tetanurae writes:

<< 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).>>

REPLY: I seem to have misread Wagner's response here. Your notation makes it
clearer. Doesn't change the import of what I say, merely the disposition of
the sauropodomorphs.
  
<< 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.>>

REPLY: Sauropods, prosauropods, and ornithischians are more closely related
to one another than any of them is to theropods. I don't think the polytomy
is unresolved. I do, however, agree that we need to know more about the hands
of primitive dinosaurs.
 
<< 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.>>

REPLY: BULLATOSAURS?? Like _Troodon_?? Clearly?? Not a chance. There's some
convergence between segnosaur and bullatosaur teeth, but only because both
kinds are relatively small. Like the convergence between Archaeopteryx teeth
and croc teeth.
 
<< 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.>>

REPLY: Stegosaurs much less so than the other groups, of course. As far as
_Harpymimus_ goes, check your pictures: in Dinosauria, p. 234 fig. 8.6A is a
MEDIAL view of the dentary of _Harpymimus_, so compare it with, say p. 411
fig. 18.2B, the MEDIAL view of the dentary of _Segnosaurus_. Where's the
prearticular loop around the mandibular fenestra (present in S. and
_Erlikosaurus_, not in H.)? In  H., the teeth extend nearly to the end of the
dentary, which has an upwardly directed anterior tip (somewhat different in
this respect from most ornithomimids); in S. there is an edentulous gap
between the tip of the dentary and the first dentary tooth, and there is no
upwardly directed anterior tip. H. has no coronoid eminence to speak of, S.
has a reasonably well-defined one. It's possible that these drawings were
done from incomplete specimens, but I think the mandibles are way too
different even allowing for this.
 
<< 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.>>

REPLY: The pelvis of _Pisanosaurus_ is known only from a matrix impression,
so we have little idea what it was really like. But what looks like an
impresion of a propubic pelvis is much more likely to be that of a pelvis in
which there is a fairly well developed prepubic process. This is how
Bonaparte interpreted it in his original description. Everything else known
about _Pisanosaurus_ suggests this.

<< 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. >>

REPLY: Well, whichever part of _Avimimus_ is represented by the leg and tail,
that's the part that's related to _Mononykus_. I'm already including
_Mononykus_ and _Parvicursor_ in the same clade.