Re: Dryptosaurus species specifics

["Mickey Mortimer" <Mickey_Mortimer111@msn.com>]

Jaime Headden 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'.

Most of what you wrote is irrelevent.  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.  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.  Similar arguments could be applied
to my "Capitalsaurus" comparisons, which were done with other caudals in
similar positions (around the fifth or sixth centrum in the series).

> 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.

They only vary in one character Sereno was able to find, which you have yet
to defend the importance of.  You also have yet to list any other
differences.  I don't believe any further "support" for individual variation
of this minor level needs to be found in Bahariasaurus, as related taxa show
that much variation and more.  You yourself stated the Baharija material was
referrable to the same taxon as Deltadromeus yesterday, and both Sereno and
I agree.  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.

>> 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.

No, the holotype of Carcharodontosaurus is a partial maxilla, teeth, nasal,
parietal, cervical vertebrae, caudal vertebrae, manual ungual, ilium, pubis,
femur and fibula.  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).  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).  After all, who knows what the small amounts of
variation indicate?  Indeed, all non-holotypic specimens of any fossil would
be unassignable for the same reasons.  Obviously, such reasoning doesn't
work for other taxa, and we shouldn't use it as an excuse to erect a new
taxon either.

Mickey Mortimer