Albertosaurus - or is it?

[pharrinj@PLU.edu]

I found the specimen designations for the known skulls of _Gorgosaurus 
libratus_  and the tyrannosaur I refer to as "Albertogorgon".

_G. libratus_           "Albertogorgon"
type (NMC 2120)         FMNH PR308
TMP 85.62.1             AMNH 5336
AMNH 5458               USNM 12814 (juv.?)
                        AMNH 5664 (juv.?)
                        ROM 1247 (juv.?)

(courtesy _Predatory Dinosaurs of the World_, p. 329)

"Albertogorgon lambei" is distinguished from _G. libratus_ by the 
following features, among others:

1)  rostrocaudally long, low, rostrally directed lacrimal horn with a 
prominent foramen

2)  broader postorbital bar?

3)  smaller, slitlike mandibular fenestra; surangular without significant 
embayment for the mandibular fenestra

4)  deeper skull immediately rostral to lacrimal horn

5)  larger surangular foramen

6)  straight angular, forming a smooth curve with the ventral margin of 
the dentary.  This contrasts with the form of the mandible in _G. 
libratus_, _G. arctunguis_, and even _Daspletosaurus torosus_, in which 
the lower end of the angular bends sharply rostrally, forming a sharp 
angle between the body of the angular and the dentary.

7)  angle between angular and dentary approximately 150 degrees 
(approximately 135 degrees in _G. libratus_)

8)  shallower mandible at the point of the articulation between the 
angular and dentary, measured across the broadest part of the surangular.

9)  thicker rostral process of the nasal bone

10)  jugal boss less than halfway from the jaw joint to the 
dentary-surangular articulation

11)  smaller dentary teeth?

12)  less prominent process on the caudal end of the surangular for the 
depressor mandibulae

13)  no invasion of the orbit by the postorbital.

With further studies, including more skulls, some of these differences 
may prove to be due to individual variation.  However, I am confident 
that the two species can be reliably separated.  In fact, there are more 
similarities between _Gorgosaurus_ and _Daspletosaurus_ in the form of 
the lacrimal horn and mandible than there are between _Gorgosaurus_ and 
"Albertogorgon".  A new generic title is therefore in order.

     Hope this clears up some confusion!
     Comments and questions welcome.

     Nick Pharris
     Pacific Lutheran University
     Tacoma, WA