Tyrannosaurus, Osborn, 1905

Longrich, Nicholas R. & Saitta, Evan T., 2024, Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea) - A Distinct Taxon of Small-Bodied Tyrannosaur, Fossil Studies 2 (1), pp. 1-65 : 43-46

3.5. Existence of Juvenile Tyrannosaurus Refutes Identification of Nanotyrannus as Juvenile Tyrannosaurus

The hypothesis that Nanotyrannus is a juvenile Tyrannosaurus predicts that the two forms should not overlap in size; that is, all Nanotyrannus will be small, and all Tyrannosaurus will be big. No small Tyrannosaurus should exist. Conversely, if Nanotyrannus is a distinct species, then small juveniles of Tyrannosaurus —approaching the size of Nanotyrannus or smaller—must exist. Juvenile dinosaurs tend to be extremely rare; however, potential juveniles of Tyrannosaurus are known, including a partial skull.

The smallest unambiguous Tyrannosaurus skeleton known is LACM 28345 [ 17]. This specimen exhibits diagnostic features of T. rex, including broad, posteriorly tapering nasals, short nasal processes of the frontals, loss of the cornual process of the lacrimal, a reduced antorbital fossa of the lacrimal, and reduced exposure of the antorbital fossa on the maxilla [ 17].

The skull of LACM 28345 is an estimated 800 mm long. This is 40% longer than the holotype of Nanotyrannus lancensis ( CMNH 7541) [ 30], which measures 570 mm [ 44] but only about 12% larger than the estimated skull length for Jane BMRP 2002.4.1, which measures ~ 710 mm. LACM 28345 is unfortunately incomplete but exhibits the Tyrannosaurus morphology in almost all characters for which it can be coded [ 17].

Although it is conceivable that the differences in morphology seen could rapidly develop as the animals mature at this size, it seems unlikely. The apparent absence of smaller Tyrannosaurus has been considered evidence that Nanotyrannus represents a juvenile T. rex . However, isolated specimens document individuals comparable to or smaller than Nanotyrannus in size.

One such specimen is UCMP V84133 from the Hell Creek Formation ( Figure 29 ). The specimen is a small right frontal. It differs from the frontals of the Nanotyrannus lancensis holotype CMNH 7541 and DDM 334.1 [ 27, 30, 44] in several respects ( Figure 30 ). First, the nasal process is narrow, half the width of the frontal or less; the nasal processes are more than half the width of the frontal in Nanotyrannus . Second, the lacrimal is broadly extended inward to constrict the skull table and inserts into deep depressions on the lateral surface of the frontal, approaching the condition seen in T. rex [ 7, 138]. Third, the orbital margin is not visible, with the postorbital and the lacrimal contacts approaching one another so they would have contacted, excluding the frontal from the orbit.

Furthermore, the postorbital process is dorsoventrally extended, again resembling T. rex ; this feature is absent in Tarbosaurus and is, therefore, an autapomorphy of Tyrannosaurus [ 139]. The posterior end of the frontal is deflected downward relative to the skull table, another feature characteristic of T. rex . The bone is dorsoventrally thickened, a feature of tyrannosaurids. The frontals also become thinner where the sagittal crust approaches the midline, showing the development of the double-ridge condition characterizing T. rex [ 17]. None of these features are seen in Nanotyrannus [ 44].

Although the frontal differs from adult specimens of Tyrannosaurus rex in its shape and proportions, it can be connected with other specimens to form a growth series ( Figure 30 ), suggesting it represents a young T. rex .

The paired frontals would have been about 80 mm across, suggesting an animal about 60% larger than the smallest known Tarbosaurus [ 54], implying a skull length of around 465 mm —significantly smaller than the type specimen of Nanotyrannus . Assuming skull length was around 12% of body length, this would imply a total length of ~ 4 m.

Phylogenetic analysis of this specimen ( Figure 31 ) recovers it as a derived tyrannosaurine, but unresolved with respect to Tyrannosaurus , Tarbosaurus , and Zhuchengtyrannus . This is consistent with its referral to Tyrannosaurus rex . This result is recovered in the dataset 3.6. Phylogenetic Analysis Suggests Nanotyrannus Is Not a Member of Tyrannosauridae

If Nanotyrannus is not Tyrannosaurus , then what is it? Gilmore [ 27] concluded that Nanotyrannus lancensis was a species of Gorgosaurus . This would place it in the family Tyrannosauridae , in the subfamily Albertosaurinae .

Bakker et al. [ 30], however, argued Nanotyrannus lay outside of the split between Tyrannosaurinae and Albertosaurinae , and represented a primitive side-branch of tyrannosaurs. This puts Nanotyrannus outside of Tyrannosauridae (depending on the precise definition used). This hypothesis was not tested using a morphological phylogenetic analysis. Surprisingly, little attempt has been made to test the phylogenetic position of Nanotyrannus since the work of Bakker et al.

We added Nanotyrannus to a previously published character–taxon matrix by Loewen et al. [ 56], updated by Wolfe et al. [ 64] and Dalman et al. [ 52], and ran a phylogenetic analysis using equal weights parsimony under PAUP * 4.10 b10 [ 48]. The analysis recovered the two most parsimonious trees ( Figure 32 ).

These results corroborate Bakker et al. in recovering Nanotyrannus just outside the Tyrannosaurinae– Albertosaurinae split, although our analysis differs from theirs in placing Alioraminae in a more basal position, below Nanotyrannus . We found almost no character support for the placement of Nanotyrannus in Tyrannosaurinae or Tyrannosauridae . Although characters can change through ontogeny, the near-total absence of any T. rex -like features in animals exceeding 1000 kg is striking.

We repeated our phylogenetic analysis using another dataset, the Brusatte and Carr matrix [ 1], updated by Wolfe et al. [ 64]. This matrix produces similar results ( Figure 33 ). Nanotyrannus emerges below Alioraminae but with Albertosaurinae further down the tree. This would make Nanotyrannus a basal member of the Tyrannosaurinae. We note, however, numerous discrepancies between specimens and codings in this matrix, particularly miscodes that appear to force Bistahieversor outside of Tyrannosauridae . This raises issues of whether the backbone of the tree is properly reconstructed; we suspect that the first topology better reflects tyrannosauroid phylogeny.

Because derived features appear late in ontogeny, immature animals may be artificially pushed down the tree, as seen by the inclusion of “ Raptorex kriegsteini ”, a juvenile tyrannosaurine [ 55] in phylogenetic analysis [ 113]. We, therefore, undertook a second series of analyses, with Nanotyrannus only coded for characters that are ontogenetically stable in tyrannosaurines, i.e., present both in young juveniles and adults. As discussed above, we assess ontogenetically stable characters as characters that are coded identically in juveniles [ 54] and adults [ 53] of Tarbosaurus baatar .

When this is done, Nanotyrannus still emerges as a non-tyrannosaurid ( Figure 34 ), with an identical position as found in the previous pair of analyses. This is because those characters of Tyrannosaurus , Tyrannosaurinae , and even Tyrannosauridae that are predicted to be present even in very young Tyrannosaurus (based on their occurrence in Tarbosaurus ) are conspicuously absent in Nanotyrannus , causing it to fall outside of Tyrannosauridae . Unless Tyrannosaurus developed in a way completely unlike Tarbosaurus (or any tyrannosaurid), these results refute the idea that Nanotyrannus is a juvenile Tyrannosaurus .

Figures

Figure 29. Frontal of juvenile Tyrannosaurus UCMP V84133 in (A), dorsal, (B), ventral, (C), lateral, and (D), medial view. Abbreviations: cer, cerebral fossa; Fr, frontal facet; Lac, lacrimal, Nas, nasal; olf, olfactory tract; orb, orbital fossa; Po, postorbital facet; Prf, prefrontal facet, stf, supratemporal fossa.

Figure 30. (A,B), frontals of juvenile Tyrannosaurus compared with Nanotyrannus DDM 334.1 (Scale bar = 5 cm); (C), growth series connecting UCMP V84133 Tyrannosaurus juvenile to the larger juvenile LACM 23845 and, finally, adult MOR 1125. Scales = 5 cm (A,B); 10 cm (C).

Figure 31. Phylogenetic analysis of the isolated frontal UCMP V84133. (A), analyzed using the Dalman et al. dataset [52], strict consensus of 14 trees (; Tree length = 1787; Consistency index (CI) = 0.3788; Retention index (RI) = 0.7484) and (B) the dataset based on Brusatte and Carr [1,64], strict consensus of 105 trees (Tree length = 760; Consistency index (CI) = 0.5592; Retention index (RI) = 0.8149).

Figure 32. (A) Phylogenetic placement of Nanotyrannus as a non-tyrannosaurid member of Tyrannosauroidea based on equal-weight parsimony Nanotyrannus based on the Dalman et al. matrix [52] (Tree length = 1808; Consistency index (CI) = 0.3744; Retention index (RI) = 0.7492); (B) closeup focused on derived tyrannosauroids.

Figure 33. (A) Phylogenetic placement of Nanotyrannus based on the Brusatte and Carr matrix [1], updated by Wolfe et al. [64]. Strict consensus of 21 trees (Tree length = 778; Consistency index (CI) = 0.5463; Retention index (RI) = 0.8084) (B) closeup focused on derived tyrannosauroids.

Figure 34. Phylogenetic analysis using only ontogenetically stable characters using (A) the Dalman et al. matrix [52], strict consensus of 2 most-parsimonious trees (Tree length = 1793, Consistency index (CI) = 0.3776, Retention index (RI) = 0.7484) and (B) the Brusatte and Carr matrix [1,64], strict consensus of 21 trees (Tree length = 767; Consistency index (CI) = 0.5541; Retention index (RI) = 0.8119).