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Re: Monolophosaurus



        I am soooo happy! Non-coelurosaur theropods are back with a
vengeance! And you all thought everything had already been said. Oh, Nick
Pharris, wherefore art thou?

Tim Williams wrote:
>As I understand the term, the "Carnosauria" is a paraphyletic assemblage of
>non-coelurosaurian tetanurines (megalosaurids, allosauroids, maybe
>spinosauroids).
        I don't know if the *original* idea was that it should be
paraphyletic, but it turned out that way. Gauthier coded it as an OTU in his
big (1986) work, but that was probably more because his stated purpose was
apparently not to explore ingroup relationships among non-avian theropods so
much as to explore bird relationships (Holtz? posting to this list). It was
treated as monphyletic (largely as _Allosaurus_ and tyrannosaurs, with some
hangers-on and taxa incertae sedis) in The Dinosauria, because the concept
of coelurosaurian tyrannosaurs was not in favor at the time. Things were
pretty much status quo until Holtz's 1994 paper, after which people have
been talking about Carnosauria being paraphyletic.
        Old school taxonomy creates typological classes of animals. when
those collections turn out not to fit some criterion of authenticity (e.g.
monophyly), the name is abandoned (presumably due to a faulty "taxon
concept"), or the taxon is rearranged. Most people have assumed that since
the taxon as composed did not consistute a clade, the name should be
abandoned. Also, I imagine Greg Paul's _PDW_ may have had something to do
with this.
        However, in the spirit of phylogenetic taxonomy, some workers have
chosen to honor Gauthier's use of Carnosauria as the sister group to
Coelurosauria (to ease definitional problems?). They have chosen to apply
the definition Carnosauria == { _Allosaurus_ > _Neornithes_ } (or something
similar). This Carnosauria will never be paraphyletic.

>Others regard the Carnosauria as a valid clade - the monophyletic sister-group
>to the Coelurosauria
        This would again be the defined Carnosauria. Yes, it is
monophyletic, but it is likely that it does not contain the taxa you listed
above. I don't think anyone who talks about a monophlyetic Carnosauria
thinks it includes tyrannosaurs, except maybe Norell and Novacek.

>(and the basal clade of the Avetheropoda)
        There is no one "basal clade of Avetheropoda". "Basal" is relative
to what "direction" you take in your view. If I am looking at carnosaur
evolution, Coelurosauria is the most basal clade of Avetheropoda.

>I believe the original description of 
>_Monololophosaurus_ regarded it as a "megalosaur-grade" theropod - 
>essentially a basal tetanurine or basal "carnosaur". [...] have moved 
>_Monolophosaurus_ further up the tree and put it somewhere among the 
>Allosauroidea.
        Actually, AFAICT, "megalosaur-grade" contains almost *NO*
phylogenetic information whatsoever. _Yangchuanosaurus_ is of a "megalosaur
grade" to some workers (e.g. Paul, Hammer and Hickerson), yet it has been
considered an "advanced allosauroid (sinraptorid)" by pretty much everyone
recently. The criteria for "megalosaur gradedness" seem to be
symplesiomorphies (as is implied by the word "grade". Symplesiomorphy won't
tell you much about where a taxon lies on the cladogram, just look at our
"megalosaur-grade" sinraptorid. I guess there was an assumption that
"megalosaurs" were non-avetheropodan tetanurans, but I think if you read the
original description of _Monolophosaurus_ you will find that the describers
are clear in not attributing phylogenetic significance to the grade assignment.

>Under this scheme, the Allosauroidea is the sister-group to the Coelurosauria.
        We have a couple of confusing points here, largely having to do with
there being two competing schemes of phylogenetic taxonomy here, those being
Sereno's way and everyone else's.
        Sereno does not seem interested in Carnosauria, and freely treats
Allosauroidea as if it were the sister group of Coelurosauria. He has not,
to my knowledge, given a definition of Allosauroidea, he's just labelled the
node (IMHO, no priority for this sort of thing. Blind nodelabelling is kind
of annoying). Others use Allosauroidea == { Allosauridae + Sinraptoridae },
or, reduced to the components, == { _Allosaurus_ + _Sinraptor_ }, with
Carnosauria as the stem conjugate to Coelurosauria (see above). This
definition is (I believe) derived from non-PT usage, and seems to be a good
one. Heck, Sereno may even be using this definition, but I can't tell. Since
people outside the discipline tend to look to Sereno, you see his usage in
books like the new edition of Benton's text ( also possibly because the
abandonment of Carnosauria makes more sense to people who follow older
taxonomic reasoning).
        Also, there seem to be two uses of the term "sister group", which I
will call operational and ideal. The operational use is the something like
"the most closely related taxon WITHIN AN ANALYSIS." The "ideal" use would
be something like "all taxa sharing a more recent common ancestor with each
other than with [the sister group of the sister group you are talking
about]." You will note that the second usage sounds alot like a stem-based
taxon definition. The operational definition is important in that such
sister-group relationships have a lot to do with the structure and
interpretaion of cladograms. Indeed, cladistics has been called "the search
for the sister group" [ref. available on request] in this context. However,
someone [Padian abnd May?] has pointed out that a *true* sister group, the
evolutionary sister clade to a clade, the "other half" of a speciation
event, can only be a stem-based taxon. So, if Allosauroidea is a node-based
taxon, is it the sister group to Coelurosauria? Maybe, using the first
usage, but only so long as there is nothing downclade of it within Avetheropoda.

>The thing is, the line between derived "carnosaurs" and primitive 
>"coelurosaurs" is a little blurred. Look at the carnosaur-like 
>features of _Ornitholestes_ 
        The question becomes, which of these features are symplesiomorphies?
Also, I think you'll find that your "derived" carnosaurs are the ones which
are retained in the new (and improved) Carnosauria. I guess this makes the
situation worse, huh?

>Future cladistic analyses could push other "carnosaurs" - like _Baryonyx_, 
        Huh? Carnosaur?

>_Proceratosaurus_
        Carnosaur?
 
>_Gasosaurus_ - into the Coelurosauria.  
        Has already been suggested once on the list by Holtz. We'll have to
wait for his next paper.

Jaime A. Headden wrote:
>  To clarify, Carnosauria is all taxa closer to *Allosaurus* than to
>*Ornithomimus*.
        Where did you get this? That's certainly an odd definition, and not
the one I am familar with...

>  Actually, megalosaur-grade tetanuries can be considered basal to
>Avetheropoda, making an un-named node, and may include such wonderful
>taxa as *Baryonyx* and *Spinosaurus* and the indominatable
>*Torvosaurus*.
        I find this unlikely. Tom Holtz has recently shared his (a priori)
opinion that "meggie" may be a carnosaur. Looking over the illustrations I
have has shown that this is indeed possible. Some of the evidence used to
link spinosaurs and "torvosaurs" (e.g. enlarged claw on manual digit I) has
a wider distribution and may not hold up. Spinosaurs are weird ducks, and
may or may not belong where they have been placed. Overall, I think support
for the topology you provide may be much weaker than you think.

>  In my view, Carnosauria has three grades: sinraptors, carcharodontosaurs,
>and allosaurs,
        These are CLADES, not grades. I haven't seen any papers which
serously suggest that any of these are paraphyletic (well, ok, maybe I've
seen one suggesting carchars are paraphyletic).

>all of which have so many similarities they might be included into one
>suprageneric taxon.
        Indeed, they are already included within several, e.g. Tetrapoda. ;)
        In all seriousness, given current tree topologies, these taxa
consitute (by definition, until the intrinsic potential for logical
inconsistancy is discovered) the Allosauroidea:
        Allosauridae == { _Allosaurus_ + _Acrocanthosaurus_ }
        Allosauroidea == { Allosauridae + Sinraptoridae }

>Concensus seems to show Bary as a megalosaur-grade theropod.
        Does consensus show that this *means* anything? :)

>  Elongate jaws, [...] corresponding enlargement of the teeth, nasal/roof of
>the skull ornamentation
        These hardly sound unique to "Mono to Bary and Torvo" as you
suggest, nor have you established if these are truly derived characters.

>dentary with great distal enlargement in front of the shallowest part of the
>bone
        Huh?

>ischium as long as the pubis or nearly so,
        Plesiomorphic.

>elongate mid-caudal neural spines,
        Interesting...

>short hind limbs, 
        Got morphometrics? I know these guys had longer legs than I do. :)

        Wagner        

----------------------------------------------------------------------------
    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
    "See that lump on his jaw? ...it's called 'lumpy jaw'!" - Steve Irwin