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Taphonomy (was Tarbosaurus)


We are talking without data here.  That is generally not a good idea...

Taphonomy (the study of the incorporation of organisms into the rock record)
is a science.  It requires field observations, measurments, analysis,
hypothesis testing, the works.

At present, these data are NOT available for the Nemegt.  For that matter,
there's only limited published on the Hell Creek and its equivalents (or for
that matter the Morrison, or the Judith River Group, or the Chinle Group,
or...). Historically, most of the people interested in vertebrate taphonomy
have been Cenozoic workers; Fiorillo and Rogers and Eberth and a few others
have done some good studies in the Mesozoic, though.

As such, we cannot make anything more than generalized statements yeah or
nay about how the taphonomic conditions of the Nemegt vs. the Hell Creek
compare: yes, both are fluviolacustrine systems, but the latter is a lowland
system, and the former probably higher land (and thus we might expect more
in the way of braided vs. meandering streams, crevasse splays, whatever).

Be that as it may, we have to remember that ANY concentration of vertebrate
bones is a remarkable phenomenon: these are the rare exceptions, not the
rules.  (The mention of Dinosaur National Monument was a good case in point:
yes, there are wonderful spots within the Morrison for bones, but there are
vast tracts of land without bonebeds).

Now, does this have a bearing on the taxonomic and phylogenetic status of
Tarbosaurus.  Not really, despite the sound and fury going on here over the
weekend.  Yes, it is odd that (TEMPORARILY assuming that all the big tyrant
dinos of the Nemegt represent a single species) that we have some better
samples of smaller ones than the big ones, whereas in the Hell Creek  we
have better skeletons of the big ones.  Yes, on *sample sizes of a couple
dozen*: not exactly the kind of statistical data that installs confidence!!
There does seem to be a difference: is it a statistically significant
difference?  That work has not been done.

What might be more productive is to take the data we DO have (the morphology
of the specimens), and see if there are consistent anatomical differences
between two different subsets of the Nemegt tyrant material.  If so, are
differences better explained as sexual dimorphism or by separate species?
This again requires some actual measurements, observations, and other data:
analyzed and plotted out.

At present, the only published version of this (which looked only at limb
lengths) was Rohzdestvensky's paper.  Carr has presented a morphometric
analysis of skull forms of tyrants, but had only a limited sample of Nemegt
specimens at that time (they fell among _T. rex_, by the way).  In his look
at Judith River tyrant ontogeny, he observed that the differences Maleev and
Carpenter used to separate out one or more subset of the hypodigm were
consistent with ontogenetic variation in _Gorgosaurus_.  On the other hand,
Olshevsky has put forth a few potential characters supporting separation of
_bataar_ and _efremovi_.  All these are good starting points for study, but
the results aren't in.

So, where does that leave us?  As I said back in my initial response, "By
priority, the name of the big Mongolian tyrannosaur (assuming it is a single
species) is _Tyrannosaurus bataar_.  _Tarbosaurus efremovi_ was used for a
more complete, smaller specimen: most (but not all) workers consider this
specimen a subadult of the first one."  Note I DID mention George's
hypothesis in that text.

Now the anatomy of both _T. rex_ and _T. bataar_ will be much better known,
as excellent new material and reprepared old material is described.  Then we
can start getting closer to some answers.

Hope this helps.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843