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Re: Albertosaurus



At 07:14 PM 1/30/97 -0500, you wrote:
>I'm completely confused about Albertosaurus, Gorgosaurus, and other 
>members of the Tyrannosauridae family.  Which animals would have been 
>contempories of T Rex?  What's the family tree look like now?

Well, I can update a bit of VERY current information, which may be confirmed
or rejected based on some additional studies (to be conducted later this
year, hopefully).

First off, there are some early forms (Late Jurassic Stokesosaurus, Early
Cretaceous Siamotyrannus, early Late Cretaceous Itemirus and teeth in Utah)
which may represent tyrannosaurid ancestors.

Within the diagnostic Tyrannosauridae proper, there are a mess of primitive
forms and a clade of very closely related forms.  The primitive forms
("aublysodontsines" or "shanshanosaurines") may or may not form a
monophyletic group: their monophyly is supported soley by lack of serrations
on their premaxillary teeth.  (Given that the immediate outgroup to
tyrannosaurids ALSO lack serrations on their premaxillary teeth, there is
some question as to the polarity of this character state).  These primitive
forms include:

Alectrosaurus olseni: early Campanian (ealier studies placed it earlier in
the Late Cretaceous), eastern Asia.

Shanshanosaurus huoyanshanensis: early Maastrichtian, eastern Asia.

the "Aublysodon" complex: mostly teeth in North America, ranging from the
Cenomanian to the latest Maastrichtian.  Aublysodon molnari (aka
"Stygivenator") is a diagnostic skeletal-based form from the late Maastricthian.

So, dependant on the character polarity, the above form either an unresolved
polytomy with Tyrannosaurinae, or a monophyletic Aublysodontinae with
unresolved internal structure.

The Tyrannosaurinae: a very well supported clade.  In my latest analysis,
Alioramus branched off first from this clade, although some have suggested
it is much more derived.

Alioramus remotus: early Maastrichtian, eastern Asia.

Above the level of Alioramus, the structure is dependant on whether or not
several small forms ("Gorgosaurus sternbergi", "Maleevosaurus novojilovi",
"Nanotyrannus lancensis") are valid taxa or not.

If a valid taxon, "Gorgosaurus" sternbergi is the next to branch off from
the other Tyrannosaurinae.

"Gorgosaurus" sternbergi: late Campanian, western North America.

If the character states for G. sternbergi are included in G. libratus, than
things follow pretty much as below:

Depending on the character polarity of two ectopterygoid features (I'll be
spending a lot of time with ectopterygoids this year, I fear), there may (or
may not) be a monophyletic bi-specific "Albertosaurus".  If these states are
considered derived, than Albertosaurus sarcophagus and Gorgosaurus libratus
(even if it includes G. sternbergi) form a monophyletic pair.

Gorgosaurus libratus: late Campanian, western North America.

Albertosaurus sarcophagus: early Maastrichtian, western North America.

If these states are considered primitive, then these forms and
Daspletosaurus form an unresolved polytomy with respect to the Tyrannosaurus
complex (see below).

Daspletosaurus falls out in most trees as the sister taxon to all remaining
tyrannosaurines.

Daspletosaurus torosus: late Campanian (?to early Maastricthian, although
this may be a new species and/or genus), western North America.

The remain structure is contingent on whether "Maleevosaurus" and
"Nanotyrannus" are considered valid taxa.  If so, they form a basal polytomy
with respect to the big boys.

Maleevosaurus novojilovi: early Maastrichtian, eastern Asia.

Nanotyrannus lancensis: late Maastricthian, western North America.

Note that the features which unite the final forms to the exclusion of these
smaller guys are ADULT features.  Therefore, there remains the possibility
that either or both of the above are actually just juveniles of the Asian
and North American Tyrannosaurus populations, respectively.

Okay, here's the interesting part, and something I will be spending a lot of
professional time on the next few years (dependant on grant money).

Having examined the skull of Tyrannosaurus bataar, there is nothing
preserved in this specimen which forbids it from being EITHER a specimen of
Tyrannosaurus rex OR a specimen of "Tarbosaurus" efremovi.  It is lacking
those few portions of the skull by which rex and efremovi can be distinguished.

Therefore, if you code them as three different taxa, there is a trichotomy
at this level: rex + bataar + efremovi.  Alternatively, you can code bataar
and efremovi as a single taxon, and you find rex + bataar.  Adding
"Gorgosaurus" lancinator doesn't do much to add to the situation: it (at
best) tends to unite bataar and efremovi.

In any case, there really isn't any good support for separating
Tyrannosaurus and Tarbosaurus at a generic level: I'd almost feel confident
enough to follow Jim Farlow's suggestion (in the 1993 paper on theropod
energetics) and just call them all T. rex (although there are a few features
by which they can be distinguished).

Tyrannosaurus rex: late Maastrichtian, western North America.

Tyrannosaurus bataar: early Maastricthian, eastern Asia.

Tyrannosaurus efremovi (?= T. bataar): early Maastrichtian, eastern Asia.

Tyannosaurus lancinator (probably = T. bataar, = T. efremovi): early
Maastrichtian, eastern Asia.

Of course, the addition of new taxa (YES!!), new characters, new character
states in previously poorly known basal forms (YES!!), and so forth may
alter the tree topologies discussed here.

For those who planned on going to Chicago just to hear my SVP talk (as
if...), you can save your air fare... :-)  (Of course, you miss the slides
this way, and I reserve the right to update my analysis for October!).

Any discussion of particular character states will have to wait until SVP.
I've got to leave SOMETHING new to say.

Later, all,


Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"To trace that life in its manifold changes through past ages to the present
is a ... difficult task, but one from which modern science does not shrink.
In this wide field, every earnest effort will meet with some degree of
success; every year will add new and important facts; and every generation
will bring to light some law, in accordance with which ancient life has been
changed into life as we see it around us to-day."
        --O.C. Marsh, Vice Presidential Address, AAAS, August 30, 1877