Nanotyrannus lancensis (Gilmore, 1946)

Currie, Philip J., 2003, Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada, Palaeontologica Polonica 48 (2), pp. 191-226 : 223-224

Nanotyrannus lancensis .

— N. lancensis ( Gilmore, 1946) , originally described as Gorgosaurus lancensis , was proposed as a separate genus by Bakker et al. 1988. It is represented by the only known skull ( CM 7541 ), which is less than 600 mm long, and is smaller in most dimensions than the immature G. libratus (TMP 91.36.500). However, the back of the skull is consistently wider in the Maastrichtian genus, with a maximum width across the postorbitals of 210mm. Nanotyrannus lancencis is almost certainly an immature tyrannosaurid that is closely related to Tyrannosaurus , but the higher number of maxillary teeth suggest that it might be easily distinguishable from the latter genus if mature specimens were found. Carr (1999) has demonstrated that the holotype of Nanotyrannus is an immature individual. However, many of the characters he cites as indicators of immaturity (such as the elongate proportions of the antorbital fenestra, the length of the orbital notch, and the height of the dentary) are in fact simply a consequence of the small size of the specimen and may not have any real ontogenetic significance. The presence of immature bone grain is in itself sufficient to indicate that the specimen was a juvenile. Rozhdestvensky (1965), Carpenter (1992), Carr (1999), and others have suggested that N. lancensis is an immature T. rex . The thorough analysis done by Carr (1999) makes it clear that most of the characters used to distinguish Nanotyrannus from Tyrannosaurus ( Bakker et al. 1988) are simply size−related or ontogenetic differences. However, most of the characters (on page 509) used to demonstrate that Nanotyrannus and Tyrannosaurus are synonymous are also characters of Tarbosaurus and Daspletosaurus . Dealing with the characters one at a time:

– The nasal processes of the premaxillae are tightly appressed throughout their entire length in Tyrannosaurus , and form a single tapering tip wedged between the nasals. However, most of the lengths of the nasal processes are not preserved in Nanotyrannus .

– The jugal only forms a small part of the margin of the antorbital fenestra in Tyrannosaurus , Nanotyrannus , and the other tyrannosaurines. It is variable enough that there is at least one specimen of G. libratus (TMP 91.36.500) in which the character can be coded differently on the left and right sides of the skull.

– The nasal makes a small but variable contribution to the antorbital fossa in all tyrannosaurids.

– The jugal pneumatopore is transversely wide in Nanotyrannus , Tarbosaurus bataar (PIN 551−1), and Tyrannosaurus .

– The sagittal crest extends onto the frontal to the same degree in Alioramus , Daspletosaurus , Nanotyrannus , Tarbosaurus , and Tyrannosaurus .

– The basal tubera are short and strongly divergent in A. sarcophagus (NMC 5600, TMP 85.98.1), Alioramus remotus (PIN 3141−1) Daspletosaurus torosus (NMC 8506), Nanotyrannus , Tarbosaurus ( Maleev 1974) , and Tyrannosaurus . They seem more elongate and vertical in G. libratus (TMP 86.144.1, 94.12.602) and the juvenile specimen of Daspletosaurus ( Fig. 26B ), but this character needs to be quantified in some way.

– The lower part of the occiput is oriented to face posteroventrally in Alioramus , Daspletosaurus , Nanotyrannus , Tarbosaurus , and Tyrannosaurus .

– The subcondylar recess is shallow in Alioramus remotus (PIN 3141−1), Daspletosaurus torosus (NMC 8506), Nanotyrannus , Tarbosaurus bataar (PIN 551−1, 553−3), and Tyrannosaurus .

– The basisphenoid plate is deep anteroventrally but the basisphenoid recess is broader laterally than anteroventrally long in Albertosaurus , Alioramus , Daspletosaurus , Nanotyrannus , Tarbosaurus , and Tyrannosaurus .

– The ectopterygoid is more inflated in Daspletosaurus and Tarbosaurus than it is in Nanotyrannus .

– The external surface of the anterior plate of the surangular is strongly convex in Daspletosaurus sp. (TMP 85.62.1), Nanotyrannus , Tarbosaurus bataar (GIN 100/65, 107/2), and Tyrannosaurus .

– The narrow snout and broad temporal region is characteristic of Nanotyrannus and Tyrannosaurus , although Tarbosaurus and to a lesser extent Daspletosaurus are intermediate between these taxa and albertosaurines.

– The jaw is no deeper in Tyrannosaurus and Nanotyrannus than it is in Daspletosaurus and Tarbosaurus ( Fig. 17 ).

In short, almost all of the 13 characters define a broader taxonomic unit than just Tyrannosaurus and Nanotyrannus . Nanotyrannus lancensis is closer to Tyrannosaurus rex than to any other tyrannosaurid in that it is relatively broader (compared to the snout width) behind the orbit than albertosaurines. Daspletosaurus and Tarbosaurus are intermediate.

There is at least one way in which N. lancensis is different from Tyrannosaurus . The number of maxillary teeth is either 11 or 12 in Tyrannosaurus , whereas the holotype of Nanotyrannus lancensis has at least 14 ( Gilmore 1946), but possibly 15 ( Bakker et al. 1988) maxillary teeth. Carr (1999) speculated that the number of teeth may be reduced ontogenetically in tyrannosaurids, but the evidence for this is very weak (Currie 2003). There is no indication that any other theropod did this, and the counts always vary within one or two teeth. Given the fact that Nanotyrannus is difficult to distinguish from an immature Daspletosaurus (an animal that survived into early Maastrichtian times), and that the tooth counts are intermediate between Daspletosaurus and Tyrannosaurus , it would be more conservative to retain Nanotyrannus as a distinct genus at this time. Ultimately, more specimens may resolve the problem, and tooth counts and stratigraphic position may turn out to be a valid way to distinguish N. lancensis from T. rex at the generic or species level.

Figures

Fig. 26. Daspletosaurus sp. (TMP 94.143.1). Braincase in right lateral (A), posterior (B), and ventral (C) views.

Fig. 17. Graph comparing the logarithms of the length of the dentary tooth row (X−axis) versus the minimum height of the dentary, which is close to the middle of the tooth row (Y−axis). Minimum height was used rather than maximum height because it is less susceptible to distortion during the fossilization process as this is normally the thickest part of the dentary. The Albertosaurus clade includes Albertosaurus and Gorgosaurus, whereas the Tyrannosaurus clade includes Daspletosaurus, Shanshanosaurus, Tarbosaurus, and Tyrannosaurus. The trendlines for both clades virtually coincide, showing that increased jaw depth is not characteristic of just Tyrannosaurus.