Re: Dryptosaurus species specifics [long]

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

Mickey Mortimer (Mickey_Mortimer111@msn.com) wrote:

<Most of what you wrote is irrelevent.>

  This is a matter of opinion.

<However, I wasn't arguing synonymy be rejected because of the low number
of characters distinguishing specimens, but rather because of the
(admittedly subjective) low importance of the single character.>

  This is a subjective issue that I do not think matters. That a character
would appear to distinguish the two, and your argument it is immaterial,
the buirden is upon you to show how, and then support the lack of
likelihood the two are distinct.

  Note the holotype of Tyrannosaurus has a narrower
> ilial
> peduncle on the ischium than either AMNH 5027 or RTMP 81.61 (Carpenter,
> 1990).  But virtually no one recommends these be placed in separate
> species.
> That this type of variation is known to be intraspecific in a related
> taxon
> strongly suggests it is in Bahariasaurus as well.

<As an aside, it's inappropiate to claim Heckert and others erect
ornithischian tooth taxa "without comparative positioning of the element
in a series" when they do try to place each tooth in a certain area of the
jaw. Basal ornithischian teeth are rather homodont anyway, usually grading
in a predictable pattern from front to back.>

  I can only hope that this opinion of homodonty applies to just Mickey.
Those who work on ornithischians, for instance, especially basal forms,
recognize that there is no such thing as a strict homodonty among
ornithischians without selecting a few select clades. For instance, in
ceratopsids, and iguanodontids, the crowns _are_ relatively homodont, as
well as in hadorsauroids. However, for nearly the entirety of the
remainder of Ornithischia, this is not true, but that teeth vary in
morphology mesial to distal in the jaw, and between jaws, and a crown can
be identified as to left or right. Not always, but most of the time. But
this is not really true the more basal you get, where teeth not only get
more similar among taxa, but further heterodont and vary between upper and
lower jaws. But there's a point, as in general theropods, that identifying
a maxillary and dentary crown cannot be done, as in *Lesothosaurus*, in
which these crowns are identical except for location of the wear facets;
but these teeth usually lack assymetry or wear facets, and the
identifications of such crowns and their diagnoses can only be relegated
to crown shape based on assumptions without jaw placement information.
Thus, the teeth can belong to similar taxa in which heterodonty was
rampant. That the authors of said Late Triassic teeth ignore the caveat of
heterodonty is surprising, given their apparently familiarity with the
conditions in basal ornithischians and so-called "hypsilophodontians."

   I, personally, see that an unplaced element cannot be significantly
placed without comparative data on the morphology of the material it was
related to. Caniniform teeth can be found in ornithischians in the
maxilla, dentary, and premaxilla, and to assume that finding a canifiform
and serrated conical crown and calling it a dentary tooth is preblematic.
Caniniform teeth are found in pachycephalosaurians, heterodontosaurids,
ceratopsians, and *Echinodon;* to which forms should the conical teeth of
*Paronychodon,* often identified as pachy caniniforms, be identified if
they are, indeed, ornithischians? What theropod has the type's morphology
to support referral on ziphodont crowns with lingual ridges to the genus
hypodigm? That such teeth appear in varied taxa and conical teeth with
additional ridges can appear in non-ornithischians in which the crowns are
normally ziphodont (semi-heterodonty is known in tyrannosaurids and
"ceratosaurs" _sensu lato_) reduces the quality of such diagnoses on their
face; such caution in naming taxa on just teeth, or referring teeth to
other tooth-taxa is questionable. Find a jaw, and this becomes easier to
do; but without that jaw, it is a lot harder to do anything diagnostic or
comparative with that tooth, based on the above reasons.

  The same is true for assuming an isolated centrum belongs to a
particular portion of a caudal series, rather than another. Such an issue
has been raised in the variable morphology of titanosaurian caudal
vertebrae within a single specimen's series (unpublished material). If one
wishes to argue this is restricted to titanosaurs, then the issue of the
variability is just as arguable for any other taxon if it can appear in
one, much less the variability of the shape of the centrum facets in
sauropod tails within a series. I can apply the relative convexity of the
cranial facet in *Spinosaurus* vertebrae as part fo a series which, as in
*Baryonyx*, from increasing to decreasing convexity, as it appears to be
true for all other tetanurans; if this is done, the length of the neural
spine and its orientation are wildly varied in vertebrae for which the
centra and neural arches are assuredly associated or fused (e.g., two
consecutive vertebrae have neural spines almost at two orders of magnitude
in length from one another, the anterior of the two shorter by almost half
and oriented caudally, and the second oriented vertically, where more
caudal vertebrae are cranially to caudally oriented -- the tallest neural
arch appears to be a caudal dorsal spine, whereas centra associated or
fused to neural spines of similar near-flattened centra show remarkably
divergent and inconsistent spine shape and length).

<They only vary in one character Sereno was able to find, which you have
yet to defend the importance of.>

  As may be clear yet, I am not trying to defend the distinctness of two
taxa; rather, the distinction of identifying taxa based on the so-called
applicability of distinct specimens based on similarity. The referral of
Egyptian material Stromer referred to *Bahariasaurus* (but not comprising
the type) to *Deltadromeus* does not preclude the applicability of the
type Egyptian material to the Moroccan taxon. Rather, the argument I've
been hearing is that the TYPE *Bahariasaurus* includes the referred
material, and thus Sereno et al., 1996, are applying that *Bahariasaurus*
is a nomen dubium based on so-called Egyptian *Deltadromeus* material.
This has avoided that the diagnostic features of *Deltadromeus* that
include the Egyptian material must therefore mean that the Egyptian and
Moroccan taxa are synonymous. But this is subjective. Similarity can lead
to lumping only as far as one can bend a line another uses to distinguish
two taxa. Dissimilarity on any score can do the same thing. Aside from the
tooth issue, the problem I have with saying *Deltadromeus* is
*Bahariasaurus* is to make assumptions that the two taxa cannot be
distinct; but this is not supportable on the given material. The latter is
larger with differing morphology, which Sereno et al. selected from the
hypodigm _similar_ material to their taxon as referrable. This was based
on gross similarity, but not explicit character material. The same
argument can be applied between (as it was in the 80's and early 90's)
that *Velociraptor* included  *Deinonychus* and *Saurornitholestes,* but
we can see where that argument has gone, as well. Perhaps a better
appreciation of the subjectivity of phylogenetic work will lead to better
analyses on some people's parts, but others seem to be aware of this
already, and their work appears to reflect this very well.

<You yourself stated the Baharija material was referrable to the same
taxon as Deltadromeus yesterday, and both Sereno and I agree.>

  I suggest what I wrote be read again; the typer was not referred, but
was demoted as non-diagnostic. A few elements were referred, but did not
comprise any portion of the type of *Bahariasaurus*. My statement on
likely synonymy aside, this was, again, subjective, and based on an
objective review (the goal of this post) I cannot refer any quality of
synonymy when the data would not suggest there was a clear case OF
synonymy, based on any lack of an alternate hypothesis. That the type for
*Baharaijasaurus* includes material of a sufficient size to be distinct
from *Deltadromeus* with a different set of elements known, makes the
referral VERY subjective apart from hypodigmatic referral (and even that
may be questionable, as the spacial and stratigraphical distance could
imply a speciation variable that some authors provide sufficient to
diagnose species [e.g., *Euronychodon* vs *Paronychodon*]).

<So since we all think Deltadromeus lived in the Baharija Formation
alongside Bahariasaurus, there is no stratigraphic reason to consider them
distinct.  There's no reason to consider Bahariasaurus a nomen dubium
either, unless you can present another taxon (besides Deltadromeus) it
can't be distinguished from.>

  Since the type of *Bahariasaurus* decides the basis of a referral to a
nomina valida or nomina dubia status, then one would have to show the
material was diagnostic of a species, and that *Deltadromeus* shared all
these features or a sufficient value of them. Rather, this is only
predicated on a referred ischium (Sereno et al., 1996).

<No, the holotype of Carcharodontosaurus is a partial maxilla, teeth,
nasal, parietal, cervical vertebrae, caudal vertebrae, manual ungual,
ilium, pubis, femur and fibula.>

  I do wonder what paper Mickey is reading. The type was distinguished as
*Megalosaurus saharicus* by Deperét & Savornin in 1928 as an unumbered
tooth in the collection of the MHNH; the referral of the partial skeleton
from Baharija in 1931 by Stromer was specifically designated as a referral
with distinction to the type tooth as bearing distinct features of the
crowns (however, this is clearly improper, as the referred maxilla have
perfectly bilaterally symetrical crowns, while the type crown shows a
distinct recurvature with concave distal margin, as in the Moroccan skull
(Sereno et al., 1996).

<I agree consistancy is important in accessing the importance of
differences, and that such information is lacking in Bahariasaurus. 
However, I would argue that this is reason to treat them as a single taxon
for the time being, just as people treat A. fragilis and C. saharicus as
single taxa because the intraspecific differences noted are not
distributed in a consistant manner (as far as we know).>

  With the presence of two skulls in the latter versus some 30 odd skulls
in the former, this "consistent manner" is not evidenced for the African
taxon. Especially as the second skull shows features the first skull does
not, and shares with the type crowns; crowns with similar symmetry as well
as asymmetry have been found from Morocco, Tunisia, Algeria, and Egypt,
implying either a consistent distribution of LK to UK
carcharodontosaurids, or a VERY long-lived genus (some odd 40+ my).

<If we followed your advice, all bahariasaur material except the holotypes
of the two taxa would be unassignable (including the Egyptian material you
think is synonymous with Deltadromeus).>

  As I said, I do not think they are synonymous. Just that I do not
contradict nor think myself familiar enough to contradict or affirm their
assignment. My work here is theoretical. Given the variable recovery apart
from the holotypes and lack of articulation or association of nearly any
element makes the taxonomic hypodigms questionable, and until better
material can be found from Baharija to show the association of the
referred material to the type, the absence of which cannot affirm
Stromer's complex.

<After all, who knows what the small amounts of variation indicate?>

  All the reason to err on the side of caution than to start forming
diagnoses and codings to employ loads of data from material that may or
may not apply, as we are getting from the so-called properly associated
*Patagosaurus* hypodigm. These days, when researchers in the Zuni basin
are finding material like disassociated ceratopsian material, even if
affirmed as a single taxon, only a single element is a type, and the
apparent lack of referral of one "obvious" element, such as a squamosal
that was an ischium, can change one's perspective of the hypodigm as a
whole and to look more closely. This is also why the type of
*Nedcolbertia*, depsite the complex of its hypodigm being found in a very
small area, is a single bone.

<Indeed, all non-holotypic specimens of any fossil would be unassignable
for the same reasons.>

  Or the holotype itself.

<Obviously, such reasoning doesn't work for other taxa, and we shouldn't
use it as an excuse to erect a new taxon either.>

  But such reasoning DOES work for other taxa. Note the *Ornithodesmus*
argument. Or *Echinodon*, a little closer to home (at least for me).

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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