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At 01:16 AM 9/27/96 -0400, George O. wrote:
>And I have yet to see a well reasoned, detailed argument for the monophyly of
>"Saurischia." Benton (in _The Dinosauria_) lists 10 putative synapomorphies
>uniting Sauropodomorpha with Theropoda, but on closer inspection these
>characters are so minor that they could easily have arisen several times
The question then becomes, which is the most parsimonious
distribution of characters?
I do aggree that , with the exception of some of the criteria listed
in Sereno et al.'s Herresaurus and Eoraptor papers, these seem to all be
symplesiomorphies of the Dinosauria. Some of the Sereno stuff (sub-narial
foramen, I beleive) do sound like actual synapomorphies.
>For a cladogram uniting sauropodomorphs with ornithischians, see
Once again, bless Olshevsky for his refs!
>Cooper calls the clade Cohort Ornithischiformes and notes these dental
>characters as synapomorphies:
>Laterally compressed, leaf-shaped teeth bearing marginal denticles to the
>cutting edge; dentition heterodont with non-recessed, marginal cheek teeth.
This describes Eoraptor.
>Let me note that the teeth of the earliest prosauropods and ornithischians
>are virtually indistinguishable and isolated specimens are frequently
>mistaken for each other.
I recall that Segnosaur teeth are also like this. It seems that
there was one really good way to make dinosaurian vegetarian teeth that kept
coming up (I know, it's not the only shape, btw). This is also why I think
Sauropoda phylogenies which dependant on tooth shape are not good.
Fortunately, these are being replaced by cladistic studies (see Dr. Holtz's
earlier posting). IMHO, the leaf-shape, or a predeliction to it, was
genetically in the genome already, and was surpressed or expressed depending
on the needs of the clade.
This is one character, and not enough to build a clade upon.
>To these I would add: ilium with well-developed presacral anterior process
>embracing caudal dorsal vertebrae and/or ribs (uniting anchisaurid
>Prosauropoda, Segnosauria, and Ornithischia)
Sounds like it's related to herbivorous habits to me...
>and pendent 4th trochanter (uniting anchisaurid Prosauropoda and Ornithischia;
Hmmm... comments from the group?
>lost in known Segnosauria).
Or never there, or lost when the other theropods lost it...
>Bakker gives one further character, the "double breast bone,"
>or sternum comprising two unfused elements.
And he indicates that this is primitive for the dinosauria, if I am
not mistaken. Also, aren't there some theropods (Allosaurus?) which express
>Bakker evidently feels the relationship between ornithischians and sauropods
>(phytodinosaurs) is too obvious to require writing a paper on it.
Without going into this too much, let's just say that tat sounds
like poor science to me.
>Indeed, if sauropodomorphs are placed at the base of "Saurischia"
>between Ornithischia and Theropoda, then (most parsimoniously) the
>common dinosaurian ancestor was herbivorous and theropods were
CHARACTER POLARITY! Basal archosauromorphs, basal archosaurs and
pterosaurs are all carnivorous animals, if these are taken as the successive
outgroups of the Dinosauria (note I use three, more than the minimum of
two), we see
data set looks like this:
This produces two trees with Ornith as outgroup of Saurop+Therop,
one in which they theropods are plesiomorphically carniverous, and one in
which they are secondarily carniverous. Hardly conclusive. Besides,
assuming character irreversability, as you seem wont to do,
ornithiscian/sauropodomorph herbivory *has* to be convergent.
It seems to be a pretty well established phenomenon that major new
radiations of herbivores radiate from carnivor/insectivore stock, because
the latter are more generalized and quicker to fill niches recently vacated.
Or somesuch. Anyway, it's a reasonable generalization.
I'm not saying that I think Phytodinosauria is necessarily
polyphyletic, I'd just really like to see some sort of evidence that doesn't
involve plesiomorphic characters and evidence taken from the Segnosaurs. I
ahve something to share on this, but I'll write it up seperately...
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