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Re: Dinosaur Web Pages' Re-Opening



As you can see, I've had this one sitting in my file of e-mails to answer for
several months now.

In a message dated 97-03-10 12:53:22 EST, znc14@ttacs1.ttu.edu (Jonathan R.
Wagner) writes (responding to me):

<< >(1) Saurischia doesn't exist, or if it does, it is congruent to
Dinosauria
         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).>>

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_.
 
<< >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.
         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.>>

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.
 
<< >[...] important characters uniting the two groups into the clade
 >Phytodinosauria [i]nclude twin sternal plates,
         This is likely to be plesiomorphic.>>

Perhaps. And perhaps not.
 
<< >dermal armor

         Plesiomorphic for dinosauria? I do not recall evidence for off-axial
 dermal armor in any primitive dinosaur (and there is evidence for axial
 dermal armor in all three groups). I believe "_Lagosuchus_" has scutes. Is
 there any evidence tat early "phyotdinosaurs" had a greater degree of dermal
 armor?>>

You might be right about this. Of course, it depends on what counts as dermal
armor. Dermal armor is found in a number of phytodinosaurian groups but
not--unless you count the small dorsal ossicles of _Ceratosaurus_--in
Theropoda, as a rule.
 
<< >a functional fifth pedal digit
         Plesiomorphic for Ornithosuchia, and possibly more inclusive clades.
 I realize you (George) cannot accept reversals in the locomotor apparatus,
 but such an objection cannot be supported in the presence of clear
 phylogenetic analyses to the contrary. I do not recall there being any basal
 Ornithiscian with a functional fifth digit...>>

The analyses are hardly "clear." I have yet to see any cladistic analysis
that demonstrates beyond reasonable doubt that any terrestrial vertebrate has
>regained< a functional digit that was previously lost. Digital loss is a
one-way street: down. Indeed, the occasional cladistic analyses that purport
to demonstrate this phenomenon should be regarded with great skepticism. If
this >has< happened, it would be a "remarkable occurrence, requiring
remarkable proof." Not just some cladogram.
 
<< >leaf-shaped herbivorous dentition.
         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.>>

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.
 
<< >(3) Heterodontosaurids are not ornithopods [...]
 >and there is [not] an obturator process on the ischium. 
         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.>>

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.
 
<< >(4) Segnosaurs (= therizinosaurs) are not theropods.
         Therizinosaurs (= segnosaurs).
 
 >theropods, the semilunate carpal evidently formed by fusion of the proximal
 >carpals (radiale and intermedium), whereas in segnosaurs it formed by the
         Hasn't this particular homology been questioned?  I believe that
 Hinchcliffe's (?) work on avian ontogeny has some things to say about
this.>>

I'm going by Ostrom's description of the semilunate carpal in _Deinonychus_.
 
<< >carpal structure differs greatly from that of theropods and cannot be
derived
         "Always with you what cannot be done...">>

Like I said before, a remarkable occurrence requiring remarkable proof.
 
<< >from theirs. The forelimb and manus of segnosaurs are highly specialized,
so
 >the fact that it the manus is tridactyl should not be used as a character
 >uniting segnosaurs with theropods.
         This is patently ludicrous! Ornithomimids has a far more specialized
 manus, and no one argue the homology of their manus! Again you eliminate
 homology a priori of a phylogenetic analyses.>>

Sorry, the ornithomimid manus is nothing like as specialized as a segnosaur
manus. "Far more specialized" is going off the deep end.
 
<< >The ascending process on the astragalus differs in detail from that found
 in >theropods and probably developed independently.
         So your argument then becomes "any feature which is significantly
 derived from the ancestral condition may no longer be considered
 homologous"?  Let's let the data speak for themselves: therizinosaurs and
 other theropods have large astragalar as.p.s.>>

They certainly do, and in my phylogeny they developed them independently. I
never said, "Any feature significantly derived from the ancestral condition
may no longer be considered homologous." That's nonsense.
 
<<>The feet
         The feet show clear signs of being secondarily quadradactyl (Russell
 and Dong 1993).>>

Nonsense. The signs, if any, are not clear at all.
 
<< >jaws
         The lower jaw of _Erlicosaurus_ is similar to that of _Harpymimus_,
 as P. Buckholz [sic? sorry Pete] has pointed out.>>

It's much more similar to the lower jaws of >any< prosauropod, as well as
many sauropods and even stegosaurs.
 
<< >teeth
         These teeth are possibly just as similar to those of _Mononykus_ and
 _Pelecanomimus_ as they are to your "phytodinosauria".>>

They are nearly identical (no pun intended) in conformation, relative size,
to the teeth of most prosauropods. Including the interdental plates.
 
<< >pelvis
         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.>>

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.
 
<<         CHARACTERS WHICH INDICATE THAT THERIZINOSAURS ARE CEOLUROSAURS:
         (Abridged)
         1.  Manus tridactyl>>

Convergent.

<<         2.  Manual proportions similar to other ceolurosaurian groups>>

Convergent. Too vague to be a real character.

<<         3.  Lip on manual ungual II (Archaey, Oviraptorsauria)>>

So does this make segnosaurs archaeopterygids or oviraptorosaurs? Convergent.

<<         4.  Enlarged preacetabular portion of Ilium (Neotheropoda)>>

Also noted in all Ornithischia; probably a synapomorphy of segnosaurs and
ornithischians, convergent with theropods.

<<         5.  Elongate ascending process of astragalus>>

Just been through this one. Convergent.

<<         6.  Leaf-shaped teeth (Ornithomimidae, Mononykus)>>

So--are segnosaur ornithomimimds or mononykosaurs? Also noted in
ornithischians and sauropodomorphs.

<<         7.  Preacetabular blade of ilium dorsovetrally elongate (ovir,
microv)>>

Segnosaur apomorphy.

<<         8.      "       "       "       with hooked anterior margin
(ornith, ov)>>

Again, does this make them ornithomimosaurs or oviraptorosaurs? Convergent.

<<         9.  Antero-dorsal expansion of lip of acetabulum (ov, microv)>>

Ditto.

<<         10. Obturator process triangular (ceolurosauria)>>

Segnosaur apomorphy. The obturator process of segnosaurs is in a different
location on the ischium. Ornithopods have an obturator process; does this
make coelurosaurs ornithopods or vice versa?

<<         11. Pubic boot enlarged.>>

Too vague. Enlarged which way? Anteriorly, posteriorly, both?

<<         12. ?Ventral migration of obturator process (questionable)>>

I'll say. See above.

<<         13. Strap-shaped scapula (Theropoda)>>

Too vague. Depending on what you consider "strap-shaped," sauropods,
prosauropods, and ornithischians also have strap-shaped scapulae.

<<         14. Tridactyl pes (reversed in Therizinosauroidea)>>

Bah.

<<         15. Semilunate carpal with transverse trochlea backs dig I and II
         etc etc etc...>>

Convergent; see above. The carpus of _Alxasaurus_ is much different from the
derived carpus of maniraptorans. Or manuraptors, as Charig & Milner have it.

What you have here is a list of characters found randomly among theropods and
other dinosaurs that could easily have been acquired by convergence during
the long interval of segnosaur evolution for which there is as yet no fossil
record. There's >nothing< like a robust character suite here--just a few
lumps and bumps
 
<< >and limbs of segnosaurs all derive much more readily from
prosauropod-like
 >sauropodomorphs than from theropods
         And snakes derive more readily from eels, but that doesn't make it
 right.>>

Aw come on. Take almost any part of the segnosaur skeleton, from skull to
tail, and compare it with the corresponding region in prosauropods. The
similarities are too many to be lightly dismissed.
 
<< >(5) _Mononykus_ is not a bird, nor is it an alvarezsaurid. It's a
perfectly
         In the absence of a phylogenetic hypothesis to the contrary, we are
 left with this conclusion. Care to clade a better one?>>

Since I don't accept cladistic analysis as anything more than simply one more
way of constructing phylogenies, I don't see what good this would do.
 
<< >good arctometatarsalian or avimimiform theropod with a highly derived,
         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. >>

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.