NANOTYRANNUS ISN’T REAL, REALLY.

A cast of the Cleveland skull in the Hall of Horns and Teeth at the Museum of the Rockies (Bozeman, MT), where it is presented as a juvenile Tyrannosaurus rex, next to a cast of the subadult MOR 1125.

Introduction

The impending auction of the ‘Dueling Dinosaurs’ has brought Nanotyrannus back into the pop culture spotlight. I repeat here the evidence that I published in support of the hypothesis that the Cleveland skull is a juvenile Tyrannosaurus rex (Carr, 1999). A critical evaluation of the diagnosis for Nanotyrannus lancensis follows this, and I close with evidence documented by Bakker et al. (1988) that supports the specimen’s referral to T. rex.

What was Bakker et al.’s (1988) characterization of Nanotyrannus? They considered the skull to represent a fully adult individual (skull length: 572 cm) of the most primitive (basalmost) tyrannosaurid from the Late Maastrichtian Hell Creek Formation of Montana, which was sympatric with T. rex. They considered Nanotyrannus to be convergent with T. rex upon an identical and extensive suite of adaptations for stereoscopy and striking behavior (i.e., head-neck kinematics).

Isn’t the Cleveland skull from an adult? No – the basis for adulthood by Bakker et al. (1988) was based on the mistaken notion of suture closure in the skull (Carr, 1999). In fact, very few skull sutures actually close during the ontogeny of tyrannosaurids; regardless, every observable suture in the Cleveland skull is open, aside for the internasal and interparietal sutures, which are closed in other tyrannosaurid juveniles (1999).

Also, the surfaces of the bones in the Cleveland have a distinct striated texture that is typically seen in juvenile dinosaurs (Carr, 1999). A similar texture is seen in adults, but its distribution is patchy and the striae are less dense and distinct than the condition seen in small specimens.

Therefore, based on the absence of suture closure (beyond what is usually seen in tyrannosaurid juveniles), the presence of immature bone grain, and its small size, the Cleveland skull is unquestionably a juvenile tyrannosaurid.

Is the Cleveland skull similar in any way to other juvenile tyrannosaurids? Yes – in virtually every detail; this table summarizes the salient features (Carr, 1999):

Albertosaurus libratus juvenile condition	Is it seen in the Cleveland skull?
Premaxilla, narrow, rostral view	Yes!
Premaxilla, lateral margin, concave, rostral view	Yes!
Premaxilla, alveolar region, shallow, rostral and lateral views	Yes!
Maxilla, mediolaterally narrow, lateral and rostral views	Yes!
Maxilla, first tooth, incisiform, all views	Yes!
Maxilla, teeth except the first, labiolingually narrow	Yes!
Maxilla, ventral jugal process, not breached by neurovascular sulcus	Yes!
Maxilla, lateral surface along rostral end of the antorbital fossa, not strutlike, lateral view	Yes!
Maxilla, promaxillary fenestra, tall and not recessed, lateral view	Yes!
Antorbital fenestra, longer than tall, lateral view	Yes!
Maxilla, maxillary fenestra, size, small, lateral view	Yes!
Maxilla, maxillary fenestra, position, midway between the rostral margins of the antorbital fossa and fenestra, lateral view	Yes!
Lacrimal, rostroventral ala, rostral margin, concave, lateral view	Yes!
Lacrimal, rostroventral ala, contact with jugal, exceeds that of the ventral ramus, lateral view	Yes!
Jugal, maxillary ramus, dorsoventrally shallow and long, lateral view	Yes!
Jugal, pneumatic recess, rostrally restricted slit, lateral view	Yes!
Jugal, postorbital joint surface approaches the orbit floor, lateral view	Yes!
Jugal, base of the postorbital process, convex, lateral view	Yes!
Jugal, caudal margin of the postorbital process, convex, lateral view	Yes!
Jugal, caudal rim of the joint surface for the lacrimal, subvertical, lateral view	Yes!
Postorbital, laterodorsal margin, vertically oriented, dorsal view	Yes!
Postorbital, cornual process, absent, lateral view	Yes!
Postorbital, subocular prong, absent, lateral view	Yes!
Frontal, lacrimal notch, long and narrow, dorsal view	Yes!
Frontal, paired bones are longer than wide, dorsal view	Yes!
Frontal, dorsotemporal fossa, shallow, dorsal view	Yes!
Frontal, dorsotemporal fossa, indistinct rostral margin	Yes!
Parietal, nuchal crest, delicate in form	Yes!
Parietal, nuchal crest, low in height	Yes!
Ectopterygoid, jugal ramus, not inflated, ventral view	Yes!
Ectopterygoid, muscle scar on jugal ramus, caudolateral in position	Yes!
Supraoccipital, dorsal process, narrow, caudal view	Yes!
Basioccipital, occipital condyle, shape of an inverted triangle, caudal view	Yes!
Basioccipital, occipital condyle, form of caudoventral surface, flattened, caudal view	Yes!
Basioccipital, basituberal web, flat ventral surface, ventral view	Yes!
Basioccipital, basituberal web, ventral surface is dorsally arched, caudal view	Yes!
Basioccipital, subcondylar recess, deeply excavated, caudal view	Yes!
Basisphenoid, pneumatic foramina are small, caudoventral view	Yes!
Basisphenoid, pneumatic foramina are situated ventrally within the basisphenoid recess, caudoventral view	Yes!
Basisphenoid, oval scar, smooth and lateroventrally oriented, caudoventral view	Yes!
Dentary, dorsoventrally shallow, lateral and medial views	Yes!
Dentary, mediolaterally narrow, ventral view	Yes!
Surangular, dorsoventrally shallow, lateral view	Yes!
Surangular, surangular shelf, horizontally oriented, lateral view	Yes!
Prearticular, dorsal margin is restricted caudally, medial view	Yes!
Prearticular, caudal ramus is dorsoventrally shallow, medial view	Yes!
Prearticular, rostral ramus is dorsoventrally shallow and straplike, medial view	Yes!
Prearticular, rostral ramus, caudodorsal surface is smooth, medial view	Yes!

Therefore, every bone in the Cleveland skull is identical to the condition seen in small juvenile tyrannosaurids, not adults (Carr, 1999). Therefore, we can add the details of the bone structure to the stack of evidence regarding its immaturity.

What about the diagnosis of Nanotyrannus lancensis? Doesn’t it adequately define the new genus and species? This is the heart of the matter because the validity of a taxon rests on its diagnosis. A diagnosis is a list of the anatomical features that set a new taxon apart from its closest relatives; presumably the diagnosis of Nanotyrannus includes indisputably unique features. For Nanotyrannus, let’s consider each character (Bakker et al., 1988) one at a time:

1) “muzzle width greatly constricted to only one fourth the width of the temporal region” (Bakker et al., 1988:3)—This feature is not unique; a narrow snout and wide temporal region is also seen in T. rex. Also, the snout is not as narrow in the Cleveland skull as the authors claim, a condition that has  been exaggerated by mediolateral crushing. This can be seen in the plate of the specimen in dorsal view (1988:28), where  fractures extend across the right jugal and maxilla, and the base of the snout extends rostrally at an unnaturally sharp angle.

2) “extraordinarily wide basicranium between basitubera and basipterygoid processes” (Bakker et al., 1988:3)—This is also seen in adult T. rex (Bakker et al., 1988).

3) “two large pneumatic foramina, placed one behind the other, near the midline of the basisphenoid” (Bakker et al., 1988:3)—This condition is the result of damage (Carr, 1999).

4) “Differs from Daspletosaurus, Gorgosaurus and Alioramus and agrees with Tyrannosaurus, Albertosaurus and Tarbosaurus spp. in having the derived features of a marked expansion of the width across the temple, relative to the skull length…”(Bakker et al., 1988:3)—These are not a diagnostic characters since they are seen in other taxa; there is no reason to think that it didn’t evolve once in the common ancestor of Tyrannosauridae and later became lost in several species.

5) “…and basitubera that are displaced forward towards the basipterygoid processes” (Bakker et al., 1988:3)—The same issue applies here as in (4).

6) “Differs from all other tyrannosaurids, except Alioramus, in retaining the primitive characters of a long, low snout…”(Bakker et al., 1988:3)—A long, low snout is typical of juvenile tyrannosaurids, and it has recently been shown (Brusatte et al., 2012) that the extremely long and shallow depth of the snout seen in Alioramus is derived beyond the norm in tyrannosaurids; i.e., the Cleveland skull does not have the condition seen in Alioramus, its snout depth is normal for a tyrannosaurid of its size.

7) “…and maxillary teeth that are strongly compressed side-to-side” (Bakker et al., 1988:3)—Narrow teeth are typical of juvenile tyrannosaurids, and the Cleveland skull does not have unusually narrow teeth in comparison with them (Carr, 1999).

8) “Differs from all other tyrannosaurids in lacking strong ridges and striae along the dorsal surface of the nasal” (Bakker et al., 1988:3)—It turns out that this is not true; a smaller and less mature juvenile T. rex (LACM 28471) has smoother nasals than are seen in the Cleveland skull, indicating that the nasals coarsened with maturity in T. rex (Carr and Williamson, 2004). In other tyrannosaurids, such as Albertosaurus, the nasals are coarse relatively earlier in growth (Carr, 1999).

9) “Differs from Tyrannosaurus in retaining the primitive character of a smooth depressed area for muscle origin on the anterior-ventral corner of the lachrimal” (Bakker et al., 1988:3)—This is the antobital fossa, a pneumatic surface and not an origin for muscle, which is seen in adult T. rex, except that this region is swollen by the internal inflation of the bone by the antorbital air sac system (Carr, 1999). This is a simple growth change that is seen in all tyrannosaurids (Carr, 1999).

Why then is the Cleveland skull referable to T. rex?

To answer this we don’t need to go any further than Bakker et al. (1988), who provided a long list of characters that are only seen in T. rex and the Cleveland skull:

1) “In Tyrannosaurus the basituberal width is enormously expanded, reaching 19% of the skull length…In Nanotyrannus the basituberal web is fully 20% of skull length…”

In other tyrannosaurids the ratio is much lower,  12%-15% (Bakker et al., 1988: 9).

2) “in Albertosaurus, Tyrannosaurus and Nanotyrannus [the main tuberous process of the basal tuber] is reduced in bulk” In other tyrannosaurids, the process is a “swollen development” (Bakker et al., 1988: 9).

Given the modern understanding of tyrannosaurid phylogeny, the reduced condition would have almost certainly have evolved twice, once in Albertosaurus, and again in Tyrannosaurus.

3) “In Nanotyrannus the ventral floor of the basisphenoid is nearly flat between the basituberal and basisphenoid webs, a condition seen in Tyrannosaurus” (Bakker et al., 1988: 12).

In other tyrannosaurids the “ceiling of the central basisphenoidal cavity is very tall and the cavity is very deep top-to-bottom” (1988:12). Although the authors are inconsistent in using the terms “ceiling” and “floor”, they are discussing the same feature, namely, the basisphenoid recess.

4) “The occipital condyle…in Nanotyrannus…faces…strongly downward…agreeing with the condition in Tyrannosaurus” (Bakker et al., 1988: 12).

In other tyrannosaurids the condyle presumably faces more caudally than downward.

5) “Moreover, the articular surface on the condyle in Nanotyrannus has been displaced downwards and around onto the ventral surface of the [occipital condyle] neck…again agreeing with [the condition seen in] Tyrannosaurus. Clearly the head was flexed far more sharply downwards in Nanotyrannus and Tyrannosaurus than in more primitive [i.e., all other] tyrannosaurids” (Bakker et al., 1988: 12).

In other tyrannosaurids the articular surface presumably does not extend ventrally onto the neck of the occipital condyle.

6) “…in Tyrannosaurus and Nanotyrannus, the basitubera are displaced forward and project less strongly below the floor of the basicranium” (Bakker et al., 1988:13).

In other tyrannosaurids “the basitubera are located far aft, beneath the occipital condyle, and project strongly downward” (1988:13).

7) “In Nanotyrannus the width of the…entire temporal region is greatly expanded relative to the snout…the breadth between the jaw joints…is 57% of the skull length…[In Tyrannosaurus] the quadratojugal width is 62% of skull length” (Bakker et al., 1988:14).

In other tyrannosaurids this ratio is much lower, ranging from ~33%-42% (1988:14).

8) “As a consequence of the temporal breadth/muzzle breadth ratio in Nanotyrannus and Tyrannosaurus, right and left eyes have a wide overlap of visual fields. Stereoscopy was probably present over a range of 30 degrees or more” (Bakker et al., 1988:15).

In other tyrannosaurids this overlap is not seen.

9) “the orbits face upwards as well as forwards in Tyrannosaurus…In Nanotyrannus, the upward orientation of the orbits is less marked” (Bakker et al., 1988:16).

Although the authors are making a contrast here, it is worth noting that both taxa have orbital fenestrae with an upward component, a condition that is not seen in other tyrannosaurids.

10) “The paroccipital processes of Nanotyrannus agree with those of Tyrannosaurus in being oriented nearly directly outwards and having much less of a backward orientation than that seen in gorgosaurs and daspletosaurs” (Bakker et al., 1988:16).

Arguably, the authors made a better case for referring the Cleveland skull to T. rex than I did (cf. Currie, 2003).

What did Bakker et al. (1988) make of these striking similarities between the Cleveland skull and T. rex? How could they think they had a new genus and species?

Here it is, in their own words:

“Whatever its evolutionary origins, Nanotyrannus seems to have evolved its stereoscopy independently of Tyrannosaurus. The low, long snout and primitive teeth of Nanotyrannus seem to preclude an ancestry from an animal as advanced as Gorgosaurus. The two stereoscopic genera, Nanotyrannus and Tyrannosaurus, are the only well known large predators from the Lancian Age, and it is noteworthy that both have achieved the highest degree of potential stereoscopy known among large theropods…Clearly natural selection was favoring greater precision of depth perception…Thus Nanotyrannus seems to have been shaped by an inexorable law of the evolution between predator and prey, a law that operated to produce ever greater finesse in the choreography of attack behavior” (Bakker et al., 1988:26).

The authors considered this suite of characters to have evolved by convergent evolution in Nanotyrannus and Tyrannosaurus, evidently at the same point in geological time and in the same geographic location. Clearly, that is not a parsimonious hypothesis.

Summary

Here’s my train of thought on the matter:

1) The Cleveland skull is of a juvenile tyrannosaurid.

2) The diagnosis of Nanotyrannus lancensis fails under scrutiny: the characters reflect the specimen’s relative maturity, damage to the skull, and characters shared with other tyrannosaurids. Consequently, N. lancensis is an invalid taxon; it entirely lacks defining features.

3) The Cleveland skull shares many specializations that are seen nowhere else except in adult T. rex, the species with which it happens to be sympatric.

4) Therefore, the Cleveland skull is a juvenile T. rex.

5) Ergo, the ‘dueling tyrannosaurid’ cannot belong to a taxon that does not exist.

So what species is the dueling tyrannosaurid?

It isn’t in a legitimate repository, so we can't know this at present. We’ll almost certainly never know if the auction goes ahead and the specimen is sold to a private individual – unless, as a colleague recently suggested to me, the people of the US want to intervene and rescue it for science under a successful argument for eminent domain.

References Cited

Bakker RT, Williams M, Currie PJ. 1988. Nanotyrannus, a new genus of pygmy tyrannosaur from the latest Cretaceous of Montana. Hunteria 1: 1-30.

Brusatte, S. L., T. D. Carr, and M. A. Norell. 2012. The osteology of Alioramus, a gracile and long-snouted tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia. Bulletin of the American Museum of Natural History 366:1-197.

Carr, T. D. and T. E. Williamson. 2004. Diversity of Late Maastrichtian Tyrannosauridae from western North America. Zoological Journal of the Linnean Society 142:479-523.

Carr, T. D.  1999.  Craniofacial Ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology 19:497-520.

Currie, P. J. 2003. Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica 48: 191-226.