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RE: tyrannos - the fossil record is still pretty scarce, no?

Having just read Larson's chapter in the T.rex volume, in analysing the
dimorphism of T.rex both Nanotyrannus and "Tyrannosaurus x" came out as
separate species (if species is the correct term now).

-----Original Message-----
From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On Behalf Of
Thomas R. Holtz, Jr.
Sent: 06 August 2008 13:56
To: hammeris1@bellsouth.net; dinosaur@usc.edu
Subject: RE: tyrannos - the fossil record is still pretty scarce, no?

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] 
> On Behalf Of Hammer
> In general, how many T.rex, "nano", Daspleto's etc. would be 
> needed to get a good feel on what the experts really need to 
> make a positive distinction on species and anatomy?

Anatomy we know pretty well, although not all growth stages. Tyrannosaurus
rex, Tarbosaurus bataar, and Gorgosaurus libratus are known from rather many
fairly complete and complimentary individuals; Albertosaurus sarcophagus and
Daspletosaurus torosus have (at present) somewhat fewer specimens but still
quite good skeletons.

However, don't think this is simply a sample size issue. After all,
"species" and "genus" distinctions are not discrete in nature, and
consqeuently have fuzzy boundaries. Zoologists disagree about the number of
modern species of ourangutan, gorilla, African elephant, manta ray,
Galapagos tortoise, etc., or the genus status of the Galapagos tortoise, the
blue whale, and so on.

So don't expect there to be a final answer. Ever.

> Or is it just getting one really complete skeleton of a baby, 
> juvenile, and adult for these guys?

Bingo! As I've said before, and will again, the case for a separate status
for Nano-T would be more convincing if we found either:

A) An adult tyrannosaurid that is distinct from Tyrannosaurus rex but shares
unique features with the type Nanotyrannus and/or "Jane"


B) A juvenile tyrannosaurid OF THE SAME ONTOGENETIC STAGE as Nanotyrannus
and/or "Jane" which is clearly distict from those two but shares unique
features with adult T. rex. (The "same ontogenetic stage" part is important:
a T. rex a few years older, for instance, would have shifted allometrically
from the proportions and details of Nano-T).

> The big more-or-less complete Rex female recently found sort 
> of covers the ground for the adult female Rex, but you need 
> one of the same quality for a male, no?

Of course we don't know that "Sue" is a female. Indeed, if we use Peter
Larson's chevron argument, than Sue is actually a male (since the supposed
male-form chevron was found later in preparation). However, as Erickson et
al. (with Larson being one of the "al.s") showed
0&_userid=961305&md5=8de3e28c623c59d12f30f5dd5d3b31d5) the chevron argument
doesn't even hold for modern crocodilians.

The only T. rex whose gender status is fairly secure is "Bob" (MOR 1125),
assuming the medullary bone is indeed medullary bone. Using Larson's (2008,
in the T. rex volume) plots, MOR 1125 is a robust form. Assuming the
robust/gracile divisions represent sex, than "Sue" does fall out as a robust
form. In this case, MOR 555 and BHI 3033 ("Stan") represent male (or at
least gracile) individuals of nearly the same completeness as "Sue".

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Fax: 301-314-9661               

Faculty Director, Earth, Life & Time Program, College Park Scholars
Fax: 301-405-0796

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA