Gorgosaurus libratus (Currie and Dong, 2001)

Currie, Philip J., 2003, Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada, Palaeontologica Polonica 48 (2), pp. 191-226 : 221-223

Gorgosaurus libratus .

— G. libratus Lambe, 1914 is known from more than twenty skeletons from southern Alberta. Good specimens of Gorgosaurus in the Museum of the Rockies and the Indianapolis Children’s Museum have also been recovered from Montana, although these have not been studied closely enough to be sure that they are the same species. Russell (1970) distinguished “ Albertosaurus ” libratus from A. sarcophagus because of differences in the times when these animals lived, and because of differences in relative lengths of the dentary tooth row (compared with the fourth metatarsal), the scapulocoracoid (compared with the femur) and the tibia plus astragalus (also compared with the femur). However, differences in relative proportions are not very reliable for taxa in which growth is allometric for most cranial and appendicular dimensions. This is confirmed by plotting tooth row versus metatarsal measurements against each other for Albertosaurus (four specimens) and Gorgosaurus (seven specimens). This particular calculation and others done by Currie (2003) fail to show any significant differences between the regression curves of these genera.

The most detailed comparison ( Bakker et al. 1988) used braincase characters to distinguish various tyrannosaurid genera, including Albertosaurus and Gorgosaurus . The basal tubera were observed to be wide and thick in Gorgosaurus , but reduced and thin in Albertosaurus , presumably reflecting a reduction in the size of tendonous muscle attachments. Paired pneumatic foramina in the ceiling of the basisphenoid recess are larger in Albertosaurus than Gorgosaurus , and the occipital condyle is more ventrally oriented in Albertosaurus .

Holtz (2001) distinguished Albertosaurus and Gorgosaurus on the basis of 14 characters, two of which he considered unique in Gorgosaurus amongst tyrannosaurids. These include differences in the nasal−frontal suture (his character 48), orientation of the lacrimal horn (52), presence of the suborbital process (57), orientation of the occiput (65), size of the basal tuber (67), number of foramina on the ventral surface of the palatine (71), number of incisiform maxillary teeth (78), expansion of the distal end of the scapula (82), size of deltapectoral crest (83), relative size of manual phalanx I−1 (85), position of the promaxillary fenestra (93), postorbitallacrimal contact (94), nature of the anterior margin of the suborbital process (95), and position of the foramina in the basisphenoidal recess (96). Although this long list should be enough to convince anyone of the distinctness of the two genera, many of the characters apply only to specific specimens. There is enough variability in anatomy in Gorgosaurus that any single individual is unlikely to show more than half the differences from Albertosaurus . Using the entire suite of character states of Bakker et al. (1988), Holtz (2001) and Currie et al. (2003), reasonably complete specimens of Gorgosaurus can be distinguished from Albertosaurus . However, it is likely that some of the characters will not code in the expected way, or will not code at all. Of the cranial characters, those associated with the braincase seem to be the most reliable for separating the two albertosaurine genera.