Double Cranial Crests

["Jaime A. Headden" <qilongia@yahoo.com>]

Randall Irmis (rbi@dana.ucc.nau.edu) wrote:

<In a paper, or on list?>

  On list. However, many ecologists and neontologists will tell
you display structures are so plastic as to be virtually useless
in regards to reconstructing phylogeny. They are prone to
convergences, etc.

<And they are all ceratosaurs, either coelophysoids or sister
taxa to the Coelophysoidea.  Plus, the formation and morphology
of the crest is almost identical. I have a hard time believing
that this is convergence.>

  It must be convergence. It is irrelevant that they are all
"ceratosaurs", because these taxa do not form a monophyletic
group diagnosed by the paired cranial crests.

  Two coelophysids, *Coeolophysis bauri* and *Syntarsus
rhodesiensis*, lack the cranial crests. This makes the
structures in *Syntarsus kayentakatae* and *Dilophosaurus*
convergent, without them being sistergroups to the exclusion of
*Coelophysis*, etc.. *Dilophosaurus* is most likely the sister
group to *Ceratosaurus* + *Abelisaurus*, and is thus the only
neoceratosaur with paired cranial crests. The feature in
"Ceratosauria" of raised edges of the nasals would most likely
be exapted into crests as a display feature, but that is about
it.

<And again, this taxon is not a ceratosaur, and has a different
crest form. Cranial ornamentation is not useful in phylogeny
when comparing distantly related taxa with ornamentation that is
morphologically different.>

  You are arguing my point, here:

  They are exactly analogous to the essential feature: they
arise from the exact positions on the skull as in other
paired-crest theropods, whereas *Ceratosaurus* has a midline
nasal ridge, with rounded lateral edges, abelisaurid skulls have
only the rounded lateral edges.

<However, when you have three taxa that are closely related with
identical ornamentation, that is completely different.>

  Not closely related enough.

  But given the disparity of taxa having the same plastic
deformation, and not having a monophyletic phylogeny exclusive
of non-crested forms, makes the feature phylogenetically
useless.

<And I follow Downs (2000) synonymization of S. rhodesiensis
with the genus Coelophysis.  Therefore, these "ridges" are
probably homologous.>

  Downs follows Paul, who suggested this as early as 1986, and
well-published in 1988, and 1994. Long before Downs.

  But as long as the Kayenta double-crested form possesses
autapomorphies of *Syntarsus* exclusive of *Coelophysis* or
*Dilophosaurus* (see Rowe, 1989, which I agree with) having
recoded Sereno, 1999 to deal with the influx of a great deal of
taxa, including *S. kayentakatae,* *Syntarsus* has two species
closer to one another than either is to *Coelophysis,* this is
problematic. It still does not solve the issue as
*Dilophosaurus* and *Syntarsus kayentakatae* lack any
synapomorphies that are not plesiomorphic to "Ceratosauria"
besides the form of the crests, and the postcrania of the
Kayenta form is still diagnostically coelophysid (coelophysine, even).

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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