Albertosaurus lancensis ( Gilmore) 1946

Carpenter, K., 1992, Tyrannosaurids (Dinosauria) of Asia and North America, Aspects of Nonmarine Cretaceous Geology, Beijing: China Ocean Press, pp. 250-268 : 258-260

? Albertosaurus lancensis ( Gilmore) 1946

Fig. 2F

= Gorgosaurus lancensis Gilmore 1946

Holotype: CMNH 7 541 skull and lower jaws from the Hell Creek Formation of Montana, U.S.A .

Diagnosis: (Cranial) skull long and low;maxillary fenestra small and separated from antorbital fenestra by a wide bar of the maxillary; antorbital fenestra longer than tall;nasals do not form, a wedge between the frontals as in A. libratus and A. sarcophagus , in dorsal view, muzzle well differentiated from cranium so that there was stereoscopic vision. (Postcranial) fourth trochanter more proximal than in A. li ­ bratus; proximal articular face of tibia almost triangular, rather than quadrangular as in most tyrannosaurids (except for Maleevosaurus ) proximal expansion of fibula is symmetrical relative to long axis of the shaft.

Stratigraphic and Palaeobiogeographic Distribution: Hell Creek Formation of Montana, U.S.A.

Discussion: I agree with Russell (1970) that A. arctunguis is a junior synonym of A. sarcophagus despite the supposed differences between the tarsus of A. sarcophagus and A. arctunguis as noted by Welles and Long (1974). I do not consider these differences significant because Parks (1928) stated that part of the astragulus of A. arctunguis was damaged, and that both the astragulus and calcaneum were displaced. New skeletons of A. sarcophagus presently under study by Phil Currie are expected to verify these observations.

Russell (1970) synonymized Gorgosaurus with Albertosaurus claiming that the holotype and paratype skulls closely resemble those referable to Gorgosaurus libratus . However,both the holotype and paratype skulls of A. sarcophagus are incomplete and crushed making such comparisons suspect. AMNH 5222, on the other hand,is a partial, disarticulated skull. Nevertheless,it shares with the holotype and paratype skulls a number of features, such as the short, broad premaxillaries, a pro-maxillary fenestra that is visible in lateral profile and widely separated from the maxillary fenestra, and in having a narrow bar of the maxillary separating the maxillary fenestra from the antorbital fenestra.

AMNH 5222 has been rearticulated and gives us an undistorted view of the skull of A. sarcophagus . This shows that the skull is short and deep, as in Tyrannosaurus , rather than long and low as in A. libratus (compare Fig. 2D with Figs. 2E ). A. sarcophagus differs from A. libratus in a number of other points as well. These include a pro-maxillary fenestra clearly visible in lateral profile, a broad bar of the maxillary separating the pro-maxillary from the maxillary fenestra, and a narrow bar of the maxillary separating the maxillary fenestra from the antorbital fenestra. There is also some indication that the postorbital extended into the orbit in A. sarcophagus in a manner similar to that seen in adult Tyrannosaurus . Postcranially, A. sarcophagus differs from A. libratus in having more gracile limbs, and in having a short, Tyrannosaurus- like trunk (compare Figs. 5D and E ).

The differences between A. sacrophagus and A. libratus seem extreme enough to suggest that two distinct genera are present. However, pending a complete review of A. sarcophagus by P. Currie based on several new skeletons,Russell’s synonymy of Gorgosaurus with Albertosaurus is tentatively accepted.

Albertosaurus libratus is the best known tyrannosaurid in North America, being represented by nine skulls and partial skeletons. Among this wealth of material includes juveniles permitting Russell (1970) to document ontogenetic changes. Not surprisingly, many of these changes paralleled those seen in Tyrannosaurus bataar ( Rozhdestvensky,1965) .

A. lancensis is a problematical species despite it being represented by a complete skull. In profile, the skull most closely resembles that of A. libratus , especially in the development of the muzzle (compare Figs. 2E and F ). Thus,it is understandable why Gilmore assigned it to Gorgosaurus . However, when the skull is viewed dorsally, the muzzle is well differentiated from the cranium, a feature only known in Tyrannosaurus . In addition,as shown with T. bataar , juvenile Tyrannosaurus skulls are Atbertosaurus- like. in profile. Therefore,is A. lancensis a juvenile T. rex? Gilmore (1946) stated that many sutures are coalescent, indicating an adult individual. This condition is exaggerated, as the only completely coalescent bones are the parietals; both the frontals and nasals are separate anteriorly. The coalescence of cranial bones is known to be variable in dinosaurs ( Brown and Schlaikjer, 1940), making its usability to ‘age’ individuals suspect.

Besides the Albertosaurus- like profile, two other features of the? A. lancensis skull that seem to differ from T. rex are the oval-shaped orbit and the shape of the antorbital fenestra. The shape of the orbit changes ontogenetically in T. bataar as may be seen in Figure 4 . In the smallest skull the orbits are almost oval in shape, whereas in the larger skulls the orbit becomes progressively taller and is subdivided by the orbital process of the postorbital. Considering that the skull of A. lancensis is smaller than the smallest T. bataar skull, and by extending backwards ontogenetically the changes that occur in T. bataar skulls, the oval shape of the orbit in A. lancensis might be a juvenile character.

The shape of the antorbital fenestra in A. lancensis , however, seems to exceed that expected ontogenetically based on T. bataar . As may be seen in Figure 4 ,the anterior edge of the fenestra remains rounded throughtout life in T. bataar . In T. rex this area is variable, being rounded, as in T. bataar , to squared ( Carpenter,in press). In? A. lancensis , this region is more acute ( Fig. 2F ). But because the A. lancensis skull is considerably smaller than the smallest known T. bataar skull, it is possible that area undergoes early ontogenetic change (this hypothesis is partially inferred from the early ontogenetic changes in the hadrosaur Maiasaura ;Horner in preparation).

A single fragmentary skeleton of A. lancensis is known,and identification is tentative ( Molnar, 1980). Molnar does describe a partial lacrymal,which,if it belongs to? A. lancensis would indicate the absence of a horn. In the holotype skull this region is either damaged or restored,but enough of the horizontal ramus exists on the left side to indicate that at a small horn was present. Because Molnar did not figure the element,it is not possible to verify his identification. Molnar’s identification of the material to A. lacensis is due to its small size and more gracile appearance. However, as discussed above,a juvenile A. libratus (AMNH 5664) is more gracile than the adult (AMNH 5458) and the small size is might also indicate a young animal. One feature Molnar (1980) noted about the fragmentary skeleton that might differentiate it from T. rex is the bisymmetrical proximal expansion of the fibula. However, this is moe apparent than real and is based upon the orientation of the specimen. At this time there is nothing about the fragmentary skeleton to prevent its being a juvenile T. rex .

Figures

Figure 2. Comparison of tyrannosaurid skulls: A-Tyrannosaurus rex AMNH 5027 (modified from Osborn, 1912); B-Tyrannosaurus bataar holotype PIN 551-1, with missing areas restored after PIN 551-3 and GI SPS 100/59 (from photographs,Maleev, 1974 and Barsbold,1983);C-Maleevosaurus novojilovi holotype PIN 552-2 (modified from Maleev, 1974);D-Albertosaurus sarcophagus AMNH 5222, with lower jaws based on NMC 5600 and NMC 5601 (modified from photographs and Lambe, 1904);E-Albertosaurus libratus(modified from Russell, 1970); F-? Albertosaurus lancensis holotype CMNH 7541, composite of the left and right side (modified from photographs);G-Daspletosaurus torosus(modified from Russell, 1970 and photographs of NMC 8506). All drawn to scale;scale beneath Tyrannosaurus rex = 10 cm .

Figure 5. Comparison of tyrannosaurid skeletons: A-Tyrannosaurus rex(based on drawings and photographs of AMNH 5027 and CM 9380);B-Tyrannosaurus bataar (based on photographs of PIN 552-1,a young adult);C- Maleevosaurus novojilovi (modified from Maleev, 1974);D-Albertosaurus sarcophagus (skeleton based on photographs and drawings of ROM 807,with the skull based on photographs and drawings of AMNH 5222,NMC 5600 and NMC 5601); E- Albertosaurus libratus (skeleton modified from Lambe, 1914,and skull modified from Russell, 1970);F-Daspletosaurus torosus (modified from Russell, 1970). All drawn to scale;height of scale beneath Tyrannosaurus rex tail = 1 m .

Figure 4. Growth series for skulls of Tarbosaurus bataar: A-PIN 551-1 (holotype) with missing areas reconstructed from PIN 551-3 and GI SPS 100/59 (from photographs,Maleev, 1974,and Barsbold 1980);B-PIN 551-3 (holotype of Tyrannosaurus bataar) (from photographs and Maleev, 1974; the posterior margins of the squamosal and quadratojugal are apparently incomplete or curved away from the viewer, hence the different shape of these bones compared to the other skulls);C-GI SPS 100/59 (modified from Barsbold, 1980);D-PIN 552-2 (holotype of Gorgosaurus lancinator) (modified from Maleev, 1974). All skulls to scale;scale under A= 10cm .