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Dino cladistics - long with requotations (was Re: Dinosaur Web...



Sorry about the requotations here, but they are kind of necessary in this
context.

At 05:32 AM 3/9/97 -0500, Dinogeorge wrote:
>
>Now let me air some of my beefs with the dinosaur cladogram itself--not your
>problem, since you follow conventional wisdom. Call this THE FIVE GREAT
>ERRORS OF DINOSAUR CLADISTICS.

Note that "disagreement with your results" does not equal "error".
"Discordance", perhaps, but not "error".

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

Under your analysis.  Under Benton's analyses (note that the 1990 paper is a
review, NOT an anlaysis), or Gauthier's 1986 classic, or Novas' work, etc.,
these characters support a monophyletic union of sauropodomorphs and
theropods outside of ornithischians.  We look forward to the addition of new
characters or new character polarities to test this hypothesis.

>Sauropodomorpha and Ornithischia share a more recent common ancestor than
>either group shares with Theropoda; important characters uniting the two
>groups into the clade Phytodinosauria include twin sternal plates, dermal
>armor, a functional fifth pedal digit (retained in a small number of
>ankylosaurians), and leaf-shaped herbivorous dentition.

Although some of these are potential synapomorphies, the retention of a
primitive feature in two groups is a symplesiomorphy, and does not support
(nor deny) a special relationship between the two groups.  The retention of
limbs in _Sphenodon_ and _Lacerta_ does not mean that they are more closely
related to each other than to _Coluber_.

>(2) Thyreophora doesn't exist. The dermal armor and a few superficial cranial
>similarities have led cladists astray into thinking that stegosaurs and
>ankylosaurs share a more recent common ancestor than either group shares with
>other ornithischians.

(And George, with his special pipeline to the Tree of Life, was luckily not
led astray...) :-)

>Dermal armor is plesiomorphic for all of Ornithischia
>(and indeed Phytodinosauria), and the pectoral, pelvic, limb, and vertebral
>anatomies of stegosaurs and ankylosaurs could scarcely differ more.
>Examination of pelvic and limb anatomy indicates that Stegosauria and
>Cerapoda share a more recent common ancestor within Ornithischia than either
>group shares with Ankylosauria. By the way, _Emausaurus_ is not a basal
>thyreophoran but a basal stegosaur remarkably similar to _Huayangosaurus_ and
>sharing about a dozen cranial characters with it.

I agree with you that some of the commonly accepted aspects of ornithischian
evolution have NOT been fully tested in public (it remains the only major
group of dinosaurs for which an explicit data matrix is not published).  The
characters you indicate should indeed be added to any future cladistic
analysis, to see if the Sereno model, or the Olsehvsky model, or some other
topology is supported.

>(3) Heterodontosaurids are not ornithopods (as pointed out by Albert Santa
>Luca in his description of _Heterodontosaurus_). At best they are basal to
>the clade Marginocephalia, united with it by the presence of a dentary fang,
>premaxillary-maxillary diastema, jugal with well-developed boss (which merges
>with and helps to enlarge the jugal in marginocephalians), and three fanglike
>premaxillary teeth.

As above, these charactes should be included in future analyses.  I actually
am quite fond of the idea of heterodontosaurids as basal marginocephalians,
but require convincing.

>In ornithopods (other than heterodontosaurids) the
>prepubic process is enormous and invariably extends forward of the
>preacetabular process of the ilium, and there is an obturator process on the
>ischium. Neither of these features appears in heterodontosaurids.

The lack of these derived features in heterodontosaurids does not in anyway
invalidate the concept of Heterodontosauridae + (Hypsilophodontia +
Iguanodontia)!  The features you cite are simply derived traits of the
euornithopods, and do indicate their special relationship with each other
outside of all other dinosaurs.

>(4) Segnosaurs (= therizinosaurs) are not theropods. There is not one
>theropod feature of segnosaurs that doesn't also appear in at least one other
>group of phytodinosaurs, except for the so-called "semilunate" carpal,
>tridactyl manus, and ascending process on the astragalus.

Ummm... Braincase?

>In maniraptoran
>theropods, the semilunate carpal evidently formed by fusion of the proximal
>carpals (radiale and intermedium), whereas in segnosaurs it formed by the
>fusion of distal carpals I and II, as seen in _Alxasaurus_, whose illustrated
>carpal structure differs greatly from that of theropods and cannot be derived
>from theirs.

As stated time and time again on the dinonet: identification of the
homologies of the elements in the semilunate carpal block of any dinosaur
(including birds) with those in more primitive amniotes is problematic.
Interpretations vary.  However, the topology of these elements remain the
same across the forms.

>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.

Actually, their forelimb is only "highly specialized" if you consider them
to be "phytodinosaurs".  Other than bulk, they are quite typical of
maniraptoriform coelurosaur forelimbs.

>The ascending process on the astragalus
>differs in detail from that found in theropods and probably developed
>independently.

There are differences in the a.a.p. of therizinosauroids and other theropods
(just as there are differences among the a.a.p. form in the various bird
subgroups), but that does not lead to the conclusion of the sentence above.
IF the most parsimonious distribution of derived character states among the
forms places therizinosauroids outside of Theropoda, THEN the simplest
explanation is that the differences between the theropod and
therizinosauroid a.a.p. is homoplastic; IF the distribution instead places
therizinosauroids within Theropoda, the differences are simply
autapomorphies of Therizinosauroidea.
>
>(5) _Mononykus_ is not a bird, nor is it an alvarezsaurid. It's a perfectly
>good arctometatarsalian or avimimiform theropod with a highly derived,
>birdlike forelimb, pelvic girdle, and skull.

The monophyly of Alvarezsauridae (= _Alvarezsaurus_ + (_Patagonykus_ +
_Mononykus_)) is established in the papers of the Gondwana Dinosaur volume
(finally got my copy!!).  I'll have more to say about that when I get some
time.  However, it must be admitted that the union of the whole group is
much weaker than the union of _Patagonykus_ and _Mononykus_, which clearly
do form their own taxon.

And, I hate to admit it, but the damn things are getting more and more birdy
all the time...  Yikes!

Hope this helps.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661