Nanotyrannus, Bakker et al., 1988

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 : 11-26

3.2. Morphology of Nanotyrannus and Tyrannosaurus

The two hypotheses make different predictions about the morphology of Nanotyrannus specimens and Tyrannosaurus specimens. Both hypotheses predict that the two will show distinct forms—either a distinct Nanotyrannus morphology and a distinct Tyrannosaurus morphology or a distinct juvenile Tyrannosaurus and adult Tyrannosaurus morphology. If Nanotyrannus is a juvenile, however, the Nanotyrannus morphology and Tyrannosaurus rex morphology must be linked by intermediate forms [ 107]. These intermediates should show character states intermediate between the Tyrannosaurus state and the Nanotyrannus state and/or mosaicism, with mixtures of juvenile Nanotyrannus characters and adult T. rex characters. If, however, Nanotyrannus is a distinct taxon, such intermediates will be nonexistent; variation should be discrete, not continuous.

3.2.1. Characters Differentiating Nanotyrannus and Tyrannosaurus

The following list of 158 characters ( Table 4 ) is assembled from previous studies of Nanotyrannus , including the original description by Gilmore [ 27], as well as Bakker et al. [ 30], Witmer and Ridgely [ 43], Larson [ 40], Schmerge and Rothschild [ 45] phylogenetic analyses by Loewen et al. [ 56] and Brusatte and Carr [ 1], and new characters found during this study. The number of characters differentiating the two is remarkable. Diagnostic characters occur in every bone in the skull examined; multiple characters typically diagnose each bone. Even more striking is the absence of clear intermediate fossils linking the two morphotypes: tyrannosaurs either exhibit the Nanotyrannus character states or the Tyrannosaurus character states but never combinations of these states, strongly arguing that they represent two distinct species.

This list is not comprehensive. Some characters were subtle or variable and so excluded pending further study. The list also focuses on cranial characters because casts, specimens, and descriptions are more readily available, but the postcrania also show marked differences, which require more study. Selected characters are illustrated ( Figures 4–13 ) but are not meant to exhaustively catalog all diagnostic characters found.

Character List: Characters differentiating Nanotyrannus and Tyrannosaurus . Ontogenetically stable characters = †, Ontogenetically labile characters = *. Nanotyrannus characters are plesiomorphies unless denoted otherwise.

We were able to assess just over half of these characters (80/158) for their stability over the course of development in Tyrannosaurini by examining their expression in juvenile Tarbosaurus [ 54] versus adult Tarbosaurus [ 53, 101] (or for the frontals, juvenile versus adult Tyrannosaurus : see below). Not all characters are known for juveniles of Tarbosaurus , and a handful of the characters that diagnose Tyrannosaurus do not occur in Tarbosaurus , but of the 50% of characters that could be assessed, just under half (36 characters, 45%) changed over ontogeny, and slightly more than half (44 characters, 55%) were stable, being visible in young juveniles and adults. It is possible that some characters scored here as ontogenetically stable are absent in very young T. rex , but they do not change over the ranges of size relevant to the Nanotyrannus problem. Characters related to the premaxilla, maxilla, and dentition tended to be stable; characters related to the orbit, cranial ornament, and skull roof tended to show ontogenetic change.

This is meant to be a preliminary study; more specimens and a more thorough analysis of the problem are required. However, if some differences could conceivably be explained by ontogeny, not all can be. Furthermore, there is the issue of whether the distribution of characters in the specimens is consistent with this hypothesis.

3.2.2. Character Distribution and Clustering Analysis

The hypothesis that the Nanotyrannus morphology and the Tyrannosaurus morphology represent endpoints of a “growth series” makes a testable prediction about the distribution of the characters distinguishing the two. The “growth series” hypothesis predicts that morphological intermediates must exist between the endpoints of the small Nanotyrannus morphology and the large T. rex morphology. If Nanotyrannus is a juvenile T. rex , then the species should progressively pick up T. rex -like characters. It should exhibit traits intermediate between the two (e.g., a maxilla taller than in Nanotyrannus but lower than in T. rex ), or exhibit a mosaic of traits of the two, or some combination of intermediate traits and mosaicism.

However, the traits are strongly clustered and show a discrete rather than continuous distribution, with no clear intermediates known. This distribution of traits is inconsistent with the hypothesis of a growth series.

To visualize these patterns, we performed a clustering analysis. We analyzed the morphological data using a UPGMA clustering analysis ( Figure 14 ) using PAUP * 4.10 b10 to analyze a matrix of 158 anatomical characters coded for Nanotyrannus and Tyrannosaurus . Because a UPGMA tree shows branch lengths as proportional to similarity, it serves to visualize the overall difference between the specimens. If the two represent a growth series, they should form a continuum. Instead, specimens show two discrete clusters, consistent with two separate lineages. A similar pattern ( Figure 15 ) emerges using principle coordinates analysis (PCoA).

The discrete clusters found here are not a result of our choice of characters but reflect the highly dissimilar anatomy of the fossils. This can be shown by repeating the same analysis using the dataset of Carr [ 10]. This character–taxon dataset is meant to capture ontogenetic changes but recovers a similar pattern to the one found with our dataset. This pattern is seen with UPGMA analysis ( Figure 16 ) and PCoA ( Figure 17 ). The lone exception is BMRP 2006.4.4, which clusters with Tyrannosaurus rex . This does not seem to result from a strong character signal because (i) the animal lacks cranial material, (ii) the femur was not coded, and (iii) the matrix includes a very large number of subtle characters of the pedal phalanges which (as the material has not been described) we could not verify, but which may drive this pattern. We suspect the placement of BMRP 2006.4.4 is a coding artifact, but further study of the characters and material is needed.

Again, variation is discrete; the Tyrannosaurus morphs cluster to the exclusion of the Nanotyrannus morphs without intermediates. This pattern is consistent with two distinct evolutionary lineages rather than a growth series.

Finally, we studied the variation of a discrete, multi-state character, tooth count, versus size ( Figure 18 ), using dentary toothrow length as a size proxy [ 40]. Nanotyrannus has more teeth than Tyrannosaurus . Although tooth count has been hypothesized to change as the animals grow, when the Tyrannosaurus tooth count is plotted against toothrow length, the slope is almost horizontal, with no clear correlation between toothrow length and maxillary tooth count ( R 2 = 0.0123) or dentary tooth count ( R 2 = 0.0077). This suggests that tooth count, while variable between individuals, does not change markedly as animals grow. Nanotyrannus shows a slight increase in tooth count with size, but the sample size is small. These results suggest that the difference in tooth count between Nanotyrannus and Tyrannosaurus does not result from differences in size and age of the animals.

Figures

Figure 4. Left maxilla of Tyrannosaurus rex CM 9380 and right maxilla (reversed) of Nanotyrannus BMRP 2002.4.1 in lateral view, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 5. Left maxilla of Tyrannosaurus rex CM 9380 and right maxilla (reversed) of Nanotyrannus BMRP 2002.4.1, medial view, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 6. Nasals of Tyrannosaurus RSM P2523.8 and nasals of Nanotyrannus BMRP 2002.4.1, dorsal view, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 7. Right lacrimal of Tyrannosaurus rex holotype CM 9380 and left lacrimal Nanotyrannus BMRP 2002.4.1, lateral view, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 8. Right postorbital of Tyrannosaurus rex LACM 150167 and left postorbital Nanotyrannus BMRP 2002.4.1 in lateral view, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 9. Left jugal of Tyrannosaurus RSM P2523.8 and left jugal of Nanotyrannus BMRP 2002.4.1 in lateral view, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 10. Right squamosal of Tyrannosaurus CM 9380 and left squamosal (reversed) Nanotyrannus BMRP 2002.4.1 in ventral view, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 11. Left ectopterygoid of Tyrannosaurus RSM P2523.8 and right ectopterygoid (reversed) of Nanotyrannus BMRP 2002.4.1 in lateral view, showing anatomical characters differentiating the two. Scale = 5 cm.

Figure 12. Left dentary of Tyrannosaurus rex CM 9380 and right dentary (reversed) Nanotyrannus BMRP 2002.4.1 in lateral (top) and medial (below) views, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 13. Left surangular of Tyrannosaurus CM 9380 and right surangular (reversed) Nanotyrannus BMRP 2002.4.1 in lateral views, showing anatomical characters differentiating the two. Scale = 10 cm.

Figure 14. Unweighted pair group with arithmetic mean (UPGMA) tree showing distinct clustering of Tyrannosaurus and Nanotyrannus, with no intermediate morphotypes using characters identified in this study (Supplementary Information S2). Of 158 potentially diagnostic morphological characters, almost all are invariant, exclusively found in either Nanotyrannus or Tyrannosaurus.

Figure 15. Principal coordinates analysis (PCoA) showing discrete clusters of Tyrannosaurus and Nanotyrannus using characters identified during this study (Supplementary Information S2). The first principal coordinate explains 60.138% of the variation and drives most of the separation between Tyrannosaurus and Nanotyrannus; the second explains 10.921% of the variation.

Figure 16. Unweighted pair group with arithmetic mean (UPGMA) tree showing distinct clustering of Tyrannosaurus and Nanotyrannus, using the Carr [10] dataset.

Figure 17. Principal coordinates analysis (PCoA) showing the first two principal coordinates using the Carr [10] dataset. Tyrannosaurus and Nanotyrannus form discrete clusters with the exception of Nanotyrannus BMRP 2006.4.4, which lacks cranial material. The first principal coordinate explains 29.348% of the variation; the second principal coordinate explains 6.1163%.

Figure 18. Tooth count versus dentary toothrow length in Nanotyrannus and Tyrannosaurus. Top, maxillary tooth count versus dentary toothrow length; bottom, dentary tooth count versus dentary toothrow length. Tooth count shows no correlation with toothrow length in Tyrannosaurus, suggesting that tooth count does not change with size. Data from Larson [40] and maxilla HRS 0438.

Tables

Table 4. Morphological characters differentiating Nanotyrannus and Tyrannosaurus.

Premaxilla
1.	Premaxilla, anterior margin of premaxilla sloped posterodorsally in lateral view
	( Nanotyrannus ) versus vertically oriented ( Tyrannosaurus ) †;
2.	Premaxilla, ventral margin distinctly upturned in lateral view ( Nanotyrannus ) or horizontal
	( Tyrannosaurus ) † (autapomorphy).
3.	Premaxilla, articulated premaxillae form a long, narrow U in dorsal or ventral view
	( Nanotyrannus ) or very short, and broad transversely ( Tyrannosaurus );
4.	Premaxilla, subnarial process faces anterolaterally ( Nanotyrannus ) or anteriorly
	( Tyrannosaurus ) †;
Maxilla
5.	Maxilla, sculpture of maxilla weakly developed, simple rugosity ( Nanotyrannus ) or forming
	ridges and deep depressions on lateral surface of maxilla ( Tyrannosaurus ) *;
6.	Maxilla, subnarial fossa and foramen exposed in lateral view (Nanotryannus) or obscured in
	lateral view ( Tyrannosaurus ) †;
7.	Maxilla, maxillae narrow in dorsal view ( Nanotyrannus ) versus muzzle broad in dorsal view
( Tyrannosaurus );
8.	Maxilla, maxilla long and low, more than twice as long as tall ( Nanotyrannus ), versus short
	and tall ( Tyrannosaurus ) †;
9.	Maxilla, ventral margin of maxilla weakly convex or straight ( Nanotyrannus ), or strongly
	curved, projecting strongly downwards relative to jugal process ( Tyrannosaurus ) *;
10.	Maxilla, nasal closely approaches anterodorsal margin of antorbital fossa ( Nanotyrannus ) or
	maxilla broadly separates nasal from antorbital fossa ( Tyrannosaurus ) *;
11.	Maxilla, antorbital fossa shallow ( Nanotyrannus ) or deep ( Tyrannosaurus ) †;
12.	Maxilla, promaxillary fenestra visible in lateral view ( Nanotyrannus ) or concealed in lateral
	view ( Tyrannosaurus ) †;