Re: Megalosaurus and Titanosaurus

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

George Olshevsky (Dinogeorge@aol.com) wrote:

<In my opinion, a species may be declared a nomen dubium if and only if
one  can produce two other distinct species that cannot be distinguished
from it. For example, Deinodon horridus is based on a type series of teeth
that cannot (presently) be distinguished from the teeth of Albertosaurus
libratus or Albertosaurus torosus. This is what makes it a nomen dubium.
(It may become possible in the near future to tell whether Deinodon
horridus teeth are more like those of A. torosus or A. libratus, and this
would help remove the "dubium" from the "nomen" Deinodon horridus.)
Trachodon mirabilis is a nomen dubium because the type tooth could have
belonged to any of a number of different lambeosaurid genera.>

  This is a good principle, but the practicle side usually applies to just
one other genus as one way to determine a nomen dubium. This is acheived
by the simple matter of whoever is doing the taxonomy, and there is no set
rule in declaring nomina dubia aside from the ICZN's broad conditions.
Their condition is where it is possible a taxon cannot be compared to any
other _or_ if it may be synonymous with another, it can be a nomen dubium.

<With regard to Megalosaurus bucklandii, no one has yet produced two
distinct theropod species from Middle Jurassic England that each have
dentaries indistinguishable from the Megalosaurus bucklandii type
dentary.>

  This does not have to be a "Bajocian" or "Englsh" only condition. Any
_two_ as in your tyrannosaur example which spans the Campano-Maastrichtian
(the Deinodon teeth are likely Tyrannosaurus rex, but are smaller and
typical of erosion that shows a morphology comparing well to most
tyrannosaurids, and come from Lancian beds; whereas A. libratus is from
the older Campanian Dinosaur Park Fm. and Daspletosaurus is from the less
older Campano-Maastrichtian Two Medicine Fm.). Comparatively, the lower
jaw of *Megalosaurus* is distinct. Several decades of comparison have
shown this, but it is a stand-alone jaw, possibly to include as a hypodigm
the other lower jaws (yet none have been explicitly compared yet,
demonstrating the assumption), and the material of *P. bucklandii* lacks a
jaw, indicating the types cannot be combined. Absence of unequivocal
synonymy with the postcrania of the Stonesfield Quarry reduces their use
in comparing *Megalosaurus* to anything without a lower jaw. Other species
with lower jaws differ by various degrees, and it should be practical to
remove all *M.* species but *M. bucklandii* from *M.*, and accept as
temporary the validity of *Megalosaurus nethercombensis* (aside from it
being French in provenance).

<So Megalosaurus bucklandii is >not< a nomen dubium, merely a species
defined on scanty material. Just because someone publishes the notion that
a species is a nomen dubium, perhaps because there is not enough material
for a good diagnosis, doesn't make it a real nomen dubium. A species may
be  undiagnosable on known material without necessarily being a nomen
dubium.>

  If it cannot be diagnosed, how does one compare it? We're talking crap
here. Who knows where in the series various titanosaur caudal vertebrae or
*Paronychodon* teeth come from, despite morphological uniqueness? If we
were to find a series (big if) of a titanosaur tail with every sector of
tail (proximal, medial, distal, terminal) having a different morphology,
as in some saltasaurines, then each other species would be less likely to
be real based on sheer lack of value of a morpholgical paradigm compared
to other taxa. 

<For another example, Majungasaurus crenatissimus was declared a nomen
dubium "in favor of" Majungatholus atopus, but I have no doubt that the
former is a senior subjective synonym of the latter. The recently
discovered topotype good skull bridges the type material of the two
species almost perfectly.>

  Actually, I can find a variety of differences between the two lower
jaws, where *M. crenatissimus* has a greater arc of the jaw and a higher
Meckelian canal and the medial wall of the dentary is similarly wider.
This is similar to *Carnotaurus*, and it is not likely the two Mascarene
taxa are synonymous. Despite this, *M. crenatissimus* lacks apomorphies
that I can define, any way, that are not carnotaurine in general or shared
with *Carnotaurus*, and this may be just as well.

<Once a species is defined, zoologists are (or should be) obliged to bend 
over backward to retain the species, not blow it off as a nomen dubium at
the first opportunity.>

  Why? I see no practicality in this. You even disfavor this treatment in
considering *Daspletosaurus* a synonym of *Albertosaurus*. Now hey, I'm
all in favor that a genus is not real; I like the idea of a singular
taxonomic binomen, for one species, and the finite taxonomic unit as a
population. Once a binomen is coined, why synonymize it with another to
favor placing a species under someone else's genus? In this, no matter how
close they are, or far, *Gorgosaurus*, *Albertosaurus*, *Daspletosaurus*,
*Tarbosaurus*, and *Tyrannosaurus* are all perfectly legitimate.
*Nanotyrannus* is something of a fun thing, and certainly worth a
reeeeeaaallly good look at.

<A species may be retained by, for example, discovery of better-preserved
topotype specimens, or of more material from the same horizon/locality.
This is how Troodon formosus was "saved" from the nomen dubium dustbin.>

  Actually, as I understand it, the authors seriously thought the material
better catalogued under one nomen, and Currie was wary of this, and
explained in very clear terms why *Stenonnychosaurus* was dropped to favor
*Troodon*. However, Currie has always voiced the possibility of several
valid troodontids in the Judithian sequences, and the idea that only one
troodontid is present because all the teeth are from the same horion is
the same mistake that got a lot of teeth referred to various *Deinodon*,
*Megalosaurus*, *Trachodon*, *Monoclonius*, *Troodon* (as *Stegoceras*),
and *Aublysodon* species. This is plain unlikely. The teeth of both
species of *Saurornithoides* are virtually identical, and may have been
assumed to be ontogenetic, were it not for the remainder of the skeleton,
including the jaw.

<I definitely find fault with the contempt some modern paleontologists
evince toward the work of their predecessors, who for the most part did
very well with the material available to them.>
  This doesn't make many of these workers any more right, or the claim of
contempt any more real because present day work and theory on systematics
and taxonomy is different than 25, or 50, or 100 years ago.

  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.

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