Albertosaur Systematics

From: "D.W.Naish" [dwn194@soton.ac.uk]

> Which brings up a question I've been meaning to ask for a while(although
> I realize this is drifting into a semi-hated thread, I promise not to
> stay too long). I've noticed in various representations of tyrannosaurids
> a disparity in the length or overall size of these lacrimal horns.
> Just how large were these and how much are they overrrepresented in
> current illos and models?

If anything, albertosaur lacrimal horns have long been ignored in restorations. It is evident that, in life, they really did exist as a rostral-pointing, sharp-tipped horn that, as evident in Paulian and other restorations, are quite apparent. Notably, Vladimir Krb's diorama does have noticeable horns, but prior to this tyrannosaurs are generally given flat, straight areas above the eye. In life, such bone structures could be enlarged by keratin by as much as two or three times, but renditions that realise such possibilities generally appear to go over the top.

A recent tendency (started by Mark Hallett I reckon) has been to give Tyrannosaurus big, triangular preorbital horns too, but tyrannosaurs appear to have more of a blunt, rounded boss here than a horn.

Of course, this all has a phylogenetic perspective, as George will tell you.

From: pharrinj@PLU.edu

Also, has anyone renamed the weird-looking "Albertosaurus" yet? I mean the one with the low, rectangular orbital horn, big teeth, shallow jaw, and rectangular skull, represented by several good specimens (I don't have the designations with me, but if I can get my hands on my copy of _PDW_...). To me it looks less similar to the type of A. libratus than does Daspletosaurus or Dinotyrannus, so it needs a new generic title (and certainly a new trivial name; no way is it A. libratus).

I suggest Albertogorgon lambei, but as I have no Ph.D., no one has to listen to me. "Gorgosaurus" sternbergi also belongs in this genus.

From: Dinogeorge@aol.com

Actually, this morph ("big-toothed gorgo" as Bakker calls it) has much in common with the type specimen of Gorgosaurus libratus, whose skull is mashed but shows similar large maxillary teeth. The thing is, no way is Gorgosaurus referable to Albertosaurus. (At least one worker thinks Albertosaurus is a nomen dubium, by the way.)

From: pharrinj@PLU.edu

Ah, yes. As I recall, the type species of Albertosaurus was the badly-preserved A. sarcophagus, correct? However, Paul (1988) writes that the type skull of G./A. libratus is of the daspletosaur-like morph, indicating that it is the more primitive, big-toothed form that needs a new name. As I have never seen the specimens personally, however, I defer to those more in-the-know. At any rate, either the big-toothed form or the daspletosaur-like forms (the rest of A. libratus, along with A. arctunguis--possibly referable to Dinotyrannus?) need a new appellation.

From: Dinogeorge@aol.com

Yes, well, Daspletosaurus happens to be a large-maxillary-toothed albertosaurinid (i.e., tyrannosaurine with well-developed lacrimal horn) as well as Gorgosaurus. After reading the literature and examining the figures, I concluded Gorgosaurus was phyletically intermediate between Albertosaurus and Daspletosaurus in relative size of maxillary teeth and dentary tooth count. Other interpretations, such as Tom Carr's that there was only one species of albertosaurinid with lots of random individual/ontogenetic variation, remain possible, of course. Dinotyrannus has no lacrimal horn and belongs with Nanotyrannus and Tyrannosaurus in tribe Tyrannosaurini. As with tribe Albertosaurini, the three included genera may all(!) be ontogenetic variants, but of the genus Tyrannosaurus.

From: Stang1996@aol.com

OK, I've got _PDW_ in my hands (I love the way the glue smells in this book by the way...) and it is turned to the Albertosaur page.

We have got Albertosaurus sarcophagus which seems now to be the only valid species of A., right?

Then we have got Gorgosaurus libratus which is only the skull AMNH 5458 (in the book anyway).

Then we have Gorgosaurus sternbergi which is much more gracile than G. llibratus and represented by AMNH 5664 and FMNH PR308.

Next is Gorgosaurus arctanguis which is maybe represented by TMP 81.10.1.

Now Dasplateosaurus torosus is recognised as an Albertosaurinid, and Dinotyrannus (=Albertosaurus) megagracilis and Nanotyrannus lancensis as Tyrannosaurinids.

Is this right? Is this making any sense?

I do _not_ like the name _Gorgosaurus_ AT ALL by the way.

Oh, also, that giant Morrison Megalosaur that Bakker was blathering on about could be Brontoraptor too (watch him be a lurker on the list...).

From: 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.