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Re: Albertosaurus or Gorgosaurus?

At 03:51 PM 3/11/99 +0100, Thomas Hammann wrote:

>Hello all,
>yesterday I bought a dinosaur-tooth of an animal which was called
>libratus_  but I can't find such a species in the literature, there only
>a _Gorgosaurus libratus_. So which one is the right "libratus"? What are the 
>differences between _Albertosaurus_ and _Gorgosaurus_ (and especially between 
>their teeth)? 

I am very, very astonished that you couldn't find _Albertosaurus libratus_
in the literature: between 1970 and now that was the accepted name of that

You can look up the dinosaur list archives for many, many postings on this
topic: some with pretty much the same subject line.

However, in brief: _Albertosaurus sarcophagus_ and _Gorgosaurus libratus_
are two closely related North American tyrannosaurids.  In 1970 Dale Russell
suggested (based on what is now regarded as shared primitive characters)
that _libratus_ and _sarcophagus_ belonged in the same genus, for which the
name _Albertosaurus_ has priority.

In recent years, however, some (such as myself) have suggested that
_Gorgosaurus_ and _Albertosaurus_ may not form a monophyletic group to the
exclusion of all other tyrannosaurids, and so the old generic name for
_libratus_ should be restored.

As to telling them apart from their teeth: at present, no one has
demonstrated that you can tell _Gorgosaurus_ from _Albertosaurus_ from their
teeth.  Tyrannosaurid teeth change allometrically (different shapes at
different sizes), and for the most part tyrannosaurs with teeth the same
size also have teeth the same shape (ugh).  Certain persons (not me) on this
list are planning to investigate theropod tooth shape in greater and more
precise detail, and maybe they can come up with something.

Until then, though, a useful rule-of-thumb: if the specimen is from the
Horseshoe Canyon Formation, it is _Albertosaurus_.  If it is from the Judith
River, it is either _Gorgosaurus_ or _Daspletosaurus_.  If it is from the
Lance or Hell Creek, it is probably _Tyrannosaurus_.  Chances are (given the
great exposure of the Judith River) the specimen is a _Gorgosaurus_.  If it
is not from the Late Cretaceous of North America, it isn't _Albertosaurus_
OR _Gorgosaurus_! (Or it is, and is a major discovery!)

On even a more general level: beware rock shop labels!  It isn't as if these
stores or the companies they purchase from actually get professional
paleontologists to identify all their specimens.  For common or distinctive
species they can be pretty accurate, but not always.

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