Re: Dryptosaurus species specifics

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

Mickey Mortimer (Mickey_Mortimer111@msn.com) wrote:

<The two taxa are very similar, and as I've said before, Sereno et al.
distinguished them by only three characters, two of which were due to his
misidentification of the distal pubis as an ischium.  The other is the
narrower ilial peduncle of Bahariasaurus' ischium.  Given the amount of
individual variation in theropod species (eg. Tyrannosaurus rex,
Coelophysis bauri), this is hardly an adequate criterion to base a new
species on. This wouldn't be tolerated for distinguishing other taxa, and
there's no reason to continue supporting Deltadromeus either.>

  Rather, support multiple species based on various shape differences in
teeth alone? Relative DSDI variation is diagnostic for a named species or
genus without question, or that affinity is based not on number of
characters, but on value, whereas another on the opposite distinction?
Could I easily adequately support a species whose only distinction is a
combination of non-unique features, without comparative positioning of the
element in a series (various tooth-based taxa or caudal vertebrae assumed
to have diagnostic value, such as the New Mexico/Arizona/Texas teeth
collection named by Hecker/Lucas/Spencer, etc or Krantz' support for an
Arundel vertebra). Even given the single supporting or two characters for
a species diagnosis, this is still sufficient grounds for species
identifications by historical and practical reasons. For instance, the
skull of *Nodocephalosaurus* is defined by two autapomorphies (shapes of
the squamosal and quadratojugal horns); so is *Chirostenotes sternbergi*
(height of the intercotylar crest of the articular). These species are
diagnostic, however.

  So, on the contrary, the valid support for the diagnosis of many species
on one or two characters can be considered adequate means for support. In
fact, mammalian teeth from the Mesozoic tend not to be supported so much
by autapomorphies but by their complex of shared features in unique ways.
This same quality can be adequately supported simialrly for other species,
if it weren't for the general complexity of mammalian teeth over
reptiles'. On the other hand, the partial pelvis and vertebrae that
comprises the type of *Bahariasaurus* vary from *Deltadromeus,* and until
support for intra-specific variation for *Deltadromeus* can be supported
by more material than fossils over 2000 miles away and with insecure
geological correllation laterally or vertically, these forms must be
considered distinct, with *B. ingens* as a nomen dubium, as a possible
senior synonym of the must more strongly supportable *Deltadromeus*;
similarly, distinctions between the two holotypes DO exist, however
synonymous they may be, and in some forms (e.g., *C. bauri* vs *M.
rhodesiensis*) the variation between species can be just as dramatic as
variation between genders, or less so.

<Differences also exist between the holotype of Carcharodontosaurus and
the new incomplete skull (nasal rugosities extend further anterior in
holotype; maxillary teeth more symmetrical mesiodistally in holotype;
maxillary body higher in holotype; etc.), but you would hardly recommend
keeping them as separate species.>

  Actually, I would reject both skulls from *Carcharodontosaurus* on the
provision that the holotype is a tooth, not a skull, and a tooth found
isolated. That carcharodontosaurid teeth have been found throughout of a
type virtually indestinguishable from the type makes it difficult to
assess the consistency of referral. Variation of the type noted above also
exists between *Allosaurus* skulls, but less so in their postcrania, and
this, as in my second paragraph, can be shown as either inter- or
intraspecific variation. After all, it is one or two small variations in
the postcrania of *Saurophaganax* that distinguishes it from *Allosaurus*,
but skulls show more. The issue is relative, and should not be considered
that one (too much) or the other (too little) variation is very diagnostic
of species. Rather, it is consistency among specimens that supports
apomorphies, and this is lacking in *Deltadromeus* versus *Bahariasaurus*,
so for the moment, I would argue caution in conjoining the two, until
better data is known. Considering the PENN expeditions to Egypt and
Baharija, we may get lucky in the next few years.

  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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