[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Syntarsus [longish]



Tim Williams wrote:

>True, cranial crests of one shape or another pop up in various 
>theropod crests.  But _paired nasolacrimal crests_ are found in only 
>two (possibly three) species that I know of, and all are coelophysoids 
>- Dilophosaurus wetherilli, Syntarsus kayentakatae, and (rumour has 
>it) one of the Ghost Ranch theropods, possibly a Syntarsus species.

        Rumors rumors rumors, oh please please publish soon. (it's kind of a
song... I believe the name is "Russell and Charig, stop sittin' on them
Spinosaurs").
        Although the defence will stipulate to the apparent uniqueness of
the "coelophysoid" crest structure, recall that specifics of the lacrimal
crest structre (specifically the lacrimal foramen) were used to link
tyrannosaurs and allosaurs in the paraphyletic Carnosauria (sensu Gauthier).

>Paired nasolacrimal crests is quite an elaborate character, and one 
>that I don't believe evolved more than once in ceratosaur evolution.  
>Therefore, I think it's a useful synapomorphic character.

        Has anyone here seen the crests of Syntarsus kayentakae?  I have
seen a so-so cast, and they were so hard to pick up that the gentleman from
the museum who was showing them to me actually could not tell where they
were (and didn't believe me when I showed the fragmented edge of one to
him).  While we do have a good idea of what they might have been like, I
believe that most of our reconstruction of these crests is _based_ on the
crest structure of D. wetherilli.
        Pardon my tone, but who are are any of us to say how nature may
work?  We may say that this structure is so complex (can't say that I agree
on that, but anyway...) that it is unlikely to have evolved twice, but this
must be wieghed with other characters, to determine the most parsimonious
evolutionary hypothesis.  It is statements like the one made above which
guided, and still guide, may very intelligent people (from Romer to
Olshevsky) to premature conclusions about the phylogeny of a group because
they (the person, not the group) were unwilling to accept that a certain
structure *might* be homoplastic (homoplaisic?)(ie. a reversal, or a result
of parallel or convergent evolution).
        By me old Bio 202 training, not lost on me even as I plow through
the pleasures of Structural Geology, there's a little joy called preaption
(now, I hear, replaced with the concept of exapation, which now has me
royally confused).  Basically, the idea is that an organism can have the
potential to develop a character without actually expressing it, which said
potential can be passed to daughter species, which then may or may not
express that character( BOY does it sound hokey when it's written out like
that).
        In the example we are using here, the ur-coelophysoid may have had
everything it needed to develop crests (in fact, probably did), but may not
have actually developed them (in the same way that it had arms and perhaps
feathers, and had lots of what it needed to be a bird, but it didn't express
those features, even though a decendant of one of it's ancestor *did*
express them).  However, it's decendants, dilophosaurs and some
coelophysids, then expressed the trait (more appropriately, they both
followed the same evolutionary path from similar genetic makeups, and
reached the same conclusion).  This is a gross oversimplification, but you
can see the danger in picking one trait to define your phylogeny on.
        I still say that if you're gonna base your phylogeny on a "by hand"
analysis of characters, try not to bas it on characters that have a *direct*
effect on reproduction, feeding strategy, movement, or gross longbone ratios
(in that order).  You might ask, what does that leave?  Hey, that's why I
don't do it that way!  :)

>S. rhodesiensis does not belong to Coelophysis.  S. kayentakatae 
>deserves its own genus.  As for Liliensternus liliensterni, I'm not 
>too sure where it sits in coelophysoid evolution.

        Very likely close to Dilophosaurus.  As I recall, the
Liliensternus+Dilophosaurus clade is supported almost wholly on postcranial
features.  I think.
        If recall the current pravda, Syntarsus is seperated from
Coelophysis on the basis of the hole in te roof of it's skull, which S.
rhodesiensis and S. kayentakae share, and the GRTs do not.  So, as far as
that goes, you are correct, S. rhodesiensis does not belong in Coelophysis.
On the other hand, can we go around erecting genera on the basis of a sexual
display characteristic?
 
>Among theropods, paired cranial crests are known only for 
>coelophysoid species (two Dilophosaurus species, one Syntarsus 
>species), so I'd be surprised if it wasn't a synapomorphy for a 
>clade within the Ceratosauria.

        1.  As Charig and Milner have pointed out, if it's a "synapomorphy"
for a sinlge clade, not emphasising that it joins two clades, it is an
*autopomorphy*.  Of course, in any case, I think it's safe to say that it is
certainly a general trend in the Coelophysoidea, and possibly the
Ceratosauria (sensu Gauthier).
        2.  Anyway, the only way you're going to convince me of this is to
set up a data matrix (you can cop some characters off of Rowe in various
places, including _The Dinosauria_), add the feature "paired naso-lacrimal
crests, the data for this character will look something like
        Liliensternus, Elaphrosaurus, Sarcosaurus, Genusaurus: ?
        Dilophosaurus, S. kayentakae:                          1
        Ceratosaurus, Coelophysis (GRT), S. rhodesiensis:      0
        Do a little research, add some other characters (I think there may
be some in Holtz 1994), then run it.  If you use at least 40 characters and
get Dilophosaurus and S. kaentakae as a clade, I beleive that both myself
and Tim Rowe will probably have a tasty bite of hat.  But you will have
achieved a laudable scientific goal:  testing a theory.
        :)
        Wagner
+--------------------------------------------------------------------------+
|   ********************PLEASE NOTE NEW E-MAIL ADDRESS******************   |
| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
| Lubbock, TX 79409                               and if they don't clade, |
| znc14@ttacs1.ttu.edu                   Then they're no friends of mine." |
+--------------------------------------------------------------------------+