Albertosaurus sarcophagus, Osborn, 1905

Currie, Philip J., 2003, Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada, Palaeontologica Polonica 48 (2), pp. 191-226 : 220-221

Albertosaurus sarcophagus .

— A. sarcophagus Osborn, 1905 is only known with certainty from the Horseshoe Canyon Formation of Alberta, Canada. There are ten skulls and skeletons known, plus a bonebed that includes at least twelve individuals of different ontogenetic stages ( Currie 2000b). Russell (1970) synonymized the more ancient Gorgosaurus with this genus because he was unable to find any significant differences between the two forms. However, this was largely because the only cranial material of Albertosaurus that was available to study at that time consisted of several partial skulls that had “not been satisfactorily characterized” ( Gilmore 1946). In the absence of any derived characters in the specimens available at that time, when cladistic methodology was not in widespread use in palaeontology, the synonymy of the two genera was a legitimate approach. The recovery of additional specimens has made it possible to do a more thorough comparison. Albertosaurus is a slightly larger animal than Gorgosaurus , and consequently adult specimens tend to be more robust. Compared with all other tyrannosaurids, most specimens of Albertosaurus have more numerous, deeper pits in the ventral surfaces of the maxillary palatal shelves to accommodate the tips of the dentary teeth. The occipital condyle is oriented more ventrally than in Gorgosaurus , although not to the same degree as in the tyrannosaurines. The braincase box ( Bakker et al. 1988) is mediolaterally wider than anteroposteriorly long, in contrast with Gorgosaurus where the dimensions are the opposite. In Albertosaurus , the nasal−frontal suture is more complex than that of Gorgosaurus , and the paired midline processes of the nasals expand posteriorly (rather than taper) and extend farther backwards than the posterolateral process of the nasal. In contrast, the prefrontal seems to have very limited dorsal exposure in Albertosaurus , and the lacrimal did not plug into a socket in the frontal, which is more similar to T. rex (RSM 283.2) than Gorgosaurus . Finally, Carr (1999) pointed out that Albertosaurus differs from other tyrannosaurids in having an angular suture between the exoccipital and basioccipital in the occipital condyle.

There are enough morphological differences to distinguish Albertosaurus from Gorgosaurus , although it is still an arbitrary decision as to whether the distinction is generic, specific or even subspecific. With the recovery of more albertosaurine specimens from New Mexico in the south to Alaska in the north, specimens are being found that have anatomical characters not seen in either of the Alberta albertosaurines. Generic distinction of Albertosaurus and Gorgosaurus gives more latitude for the assessment of relationships of these new specimens. Furthermore, there are as many anatomical differences between Albertosaurus and Gorgosaurus as there are between Daspletosaurus , Tarbosaurus and Tyrannosaurus . At this time, with our present state of knowledge and understanding, it is advisable to maintain generic distinction between Albertosaurus and Gorgosaurus .