Nanotyrannus Nomore!?

Nanotyrannus is everyone’s favorite tyrannosaur, as long as it’s right behind the big guy, Tyrannosaurus, itself. It’s been lovingly depicted and aggressively championed by such notable figures as Robert Bakker, and supported less enthusiastically but no less masterfully by Philip Currie. Nanotyrannus was the name given to Gorgosaurus lancensis (Gilmore, 1946) by Bakker, Williams and Currie in 1988 for an elongated, narrow-snouted skull (CMNH 7541). 40 years later, the peculiar morphology of the skull, with its broad boxy posterior end and the thin snout, slender and narrow blade-like teeth, and greater numbers of them, implied that it might be very distinguishable from Gorgosaurus, and in fact from Tyrannosaurus to which it was compared favorably, resulting in the “tiny tyrant,” Nanotyrannus being given to replace Gorgosaurus, thus Nanotyrannus lancensis.

The Nanotyrannus Debate

Skull of Nanotyrannus lancensis, CMNH 7541 (holotype), from Carr (1999).

Debate sprung up almost immediately advocating that this skull might just well be a juvenile Tyrannosaurus rex (they were both from Hell Creek Formation), but much of this debate was anecdotal due to the absence of clear growth stages among theropod dinosaurs, and especially among tyrannosaurids. While some authors were debating the issue using specimens from other taxa (Carr, 1999 approached the topic from Gorgosaurus libratus, Currie, 2003 and Currie, Hurum and Sabath, 2003 from Tarbosaurus bataar) few authors approached it from the perspective of Tyrannosaurus rex itself. Virtually all of the specimens clearly Tyrannosaurus-y were adults or nearly so, and quite a bit larger than CMNH 7541, thus impairing the issue from resolution.

Of course, recent work on a specimen currently in the Burpee Museum of Illinois, nicknamed “Jane,” BMR P2002.4.1, represents another Hell Creek tyrannosaurid specimen close to the size of CMNH 7541 which might either bolster or sunder the argument that Nanotyrannus lancensis is distinct from Tyrannosaurus rex, but this work hasn’t been published yet.

This leaves the question to other species, and whether they can support the work.

Dale Russell’s 1970 landmark analysis of North American tyrannosaurids distinguished Albertosaurus sarcophagus, Gorgosaurus libratus and Daspletosaurus torosus from the later Tyrannosaurus rex into a determinate sequence, but it also included an inference based on reverse projecting ontogenetic trends from adult to subadult Gorgosaurus specimens, and one of the cutest, but hypothetical, concepts of a juvenile. This was a lanky, small-bodied, large-headed, and tiny armed animal, a tyrannosaur “teenager,” only relatively much younger.

From this point, speculation about potential ecological segregation of various taxa at different ages become more common. This segregation, such as what occurs in crocodilians and varanid lizards, where the adults feed and hunt in a manner quite disimilar from the juvenile, where their prey items are entirely different, and where communal hunting can be more or less common as they age, is ontogenetic ecological partitioning, and it become the primary reason to support tyrannosaurids with vastly different jaw anatomy when young. This allowed researchers to more easily see Nanotyrannus lancensis as merely a younger, more svelte Tyrannosaurus rex youngster (Carr, 1999; Currie and Dong, 2001; Holtz, 2003; Horner and Padian, 2004).

The Ontogenetic Debate

But this did not settle the issue of whether or not CMNH 7541 was a juvenile Tyrannosaurus rex. Instead, it forced a series of papers that argued that only strict observational changes in the skull could clearly determine this transformation (Currie, 2003; Currie et al., 2003) and thus concretely determine ontogeny in Tyrannosaurus rex. It did not help that Tyrannosaurus rex is identified in most census as the only tyrannosaur from Hell Creek Formation deposits or equivalent latest-Maastricthian formations, although in many cases this may be due to an identification bias (see Horner et al., 2011 and my discussions of this topic here and here).

Being made yet more complicated, a new series of taxa were named between 1970 and 2010 that illustrated apparently juvenile to adult theropods, later noted to be juvenile or subadult tyrannosaurids, including Shanshanosaurus huoyanshanensis (Dong, 1970; see Currie and Dong, 2001), Alioramus remotus (Kurzanov, 1976; see Brusatte et al., 2009, also for Alioramus altai), and Raptorex kriegsteini (Sereno et al., 2009). Intrinsic in these works have been the observation of a distinct ontogenetic sequence in one taxon (Gorgosaurus libratus, Carr, 1999) which has been extrapolated onto other taxa. The value of this “mapping” has been questioned to some degree (e.g., Currie, 2003) largely due to the fact that not all taxa grow the same way. I think personally that Carr’s work is a benchmark, rather than a map against which taxa should be enforced, but that requires us having a more comprehensive suite of fossils for other taxa.

Yesterday, I mentioned new work in regards to Nanotyrannus lancensis … which is a paper (Tsuihiji et al., 2011) that attempts to resolve the youngest ontogenetic stage of Tarbosaurus bataar, MPC-D 107/7[n1]. This specimen

Juvenile tyrannosaurid skull, referred to Tarbosaurus bataar, MPC-D 107/7, from Tsuihiji et al. (2011). Shown is the skull in left lateral, right lateral, dorsal and posterior views. Scar bar equals 10cm. See below for more.

Above is MPC-D 107/7 (at least, the skull), modified from Tsuihiji et al. (2011), showing several ontogenetic features (arrows) as noted by Carr (1999). These are as follows:

1, narrow premaxillae, forming a “tighter” V-shaped angle, rather than a broader U-shaped “arch” as occurs in later ontogeny;
2, thickness of the maxilla (and “roughness” of the lateral surface) relatively low, increasing with age;
3, promaxillary fenestra exposed in lateral view, with a portion of the antorbital fossa separating the maxillary fenestra from the rostral margin of the external antorbital fenestra (arrow points to this distance), while with age this margin decreases so that the maxillary fenestra is partially overlapped;
4, “maxillary body” is shallow, increasing in height relative to maxillary length with age;
5, internal or medial antorbital fenestra is relatively long, more triangular than rounded, as it shortens with age;
6, maxillary and dental tooth rows decrease in count with age;
7, nasal bears a lateral process that articulates with the lachrymal, which disappears with age;
8, lachrymal ornamentation develops with age, from a smooth, slightly rounded or flat margin into an arched or pointed process of lateral protrusion or “boss”;
9, orbit rounded, broadly ovate or near-circular, increasing in height relative to length with age, as well as increasing in eccentricity;
10, suborbital process of the postorbital (as well as occassionally the lachrymal) projects into the orbital margin with age, but is small or absent in youth;
11, sagittal crest of the parietal is shallow, but increases and extends onto the frontal with age;
12, posterior surangular foramen large, similar in size to anterior surangular foramen, decreasing in size with age.

The literature has expanded in the last decade with purportedly grown theropod specimens being regarded as juveniles because of several “general” features indicating juvenility, including a relative large, round orbit, a short, triangular snout, and the lack of ornamentation and the like. But unsubstantiated for the most part has been the literature regarding dental ontogeny, where the tooth shape itself changes during age; most work (e.g., Russell, 1970; Carr, 1999; Currie, 2003) have focused rather on the relative number of teeth, rather than their morphology, unless it is to deal with the premaxillary/mesial maxillary teeth, which are “D-shaped” or “incisiform” (see Holtz, 2001; Currie et al., 2003). It has been argued since Russell (1970) that tyrannosaurids developed fewer teeth with age (Carr, 1999; Currie et al., 2003), but have done little to assess the variation in ontogeny for the teeth themselves. Currie et al. (2003) note merely that allometric growth produces broader (“thicker”) teeth (labiolingual width) with age, but this variation is not apparent in other large theropod dinosaurs (e.g., Carcharodontosaurus saharicus; see Brusatte and Sereno, 2007).

Unfortunately, while Tsuihiji et al. (2011) assess the teeth of a juvenile tyrannosaurid like no analysis of teeth before it, little is said of the ontogeny aside from tooth count — tooth morphology is limited to the premaxillary dentition and serration density and size. However, the paper has some beautiful photos of the teeth in several ways that underscore how thin these teeth are in juveniles, and their qualified variation is particularly distinct. This is backed up with CT data, so this information is pretty robust.

Juvenile tyrannosaurid teeth, referred to Tarbosaurus bataar, MPC-D 107/7, from Tsuihiji et al. (2011). Third maxillary tooth (largest in the upper jaw) is highlighted in orange to indicate the extent of enamel.

Almost certainly important in this debate has been the sheer number of taxa named for supposedly or actually young individuals that have been argued to be synonyms of one sort or another.

Shanshanosaurus huoyanshanensis was described by Dong (1977) as a close ally of the Tyannosauridae (in Shanshanosauridae), but later recognized to match other known juvenile and subadult specimens of Tarbosaurus bataar (Currie and Dong, 2001).

Alioramus remotus was named by Kurzanov (1976) as the closest relative of Tarbosaurus bataar (after synonymy of all the other species he had named of Tarbosaurus was taken into account), but of a particularly unique character, due to the morphology of the nasal expressing “adult” features. Currie (2003), following speculation on the synonymy of Shanshanosaurus huoyanshanensis with Tarbosaurus bataar, used information drawn from Carr (1999) to speculate that not only was Alioramus remotus a subadult, but also that it was an intermediate between Shanshanosaurus huoyanshanensis with Tarbosaurus bataars in morphology, and thus likely a synonym of the latter. This was later refuted by Brusatte et al. (2009) who, although they agreed on the ontogenetic aspects, identified a number of features of the skull that could not be accounted for ontogenetically, including the unique form of the nasals, and used this information to erect a new taxa (Alioramus altai) which differed further from Alioramus remotus. Several commenters (such as Mickey Mortimer, here) have affirmed that any variation among the species is individual in nature, but based on a sample size of two, I fail to see how such an argument can be supported.

Raptorex kriegsteini (Sereno et al., 2009) was named for a specimen virtually identical in size to that of MPC-D 107/7, and in general is close to identical. Several blatantly “juvenile” features are generally argued as being unique to the species, as histology indicates the specimen is a “teenager,” around 6 years of age. A phylogenetic analysis by Sereno et al. (2009) supported the unique identity of Raptorex kriegsteini by placing it basally within the Tyrannosauroidea, far from other tyrannosaurids known from central Asia. However, Carr (1999) used several phylogenetic analyses to place subadult and juvenile specimens apart from their adult counterparts, and found that they become separated: juveniles and subadult act as unique taxa in a phylogenetic analysis, and are more basal than their adults, a result supported by Tsuihiji et al. (2011), which placed CMNH 7541 and MPC-D 107/7 in the analysis and found them to clade close to one another separate from adults of the same purported taxa. This underscores the peril of treating juveniles as adults without safeguards in the analysis; it would seem that Tsuihiji et al. regard that Sereno et al. did not provide these safeguards.

Juvenile tyrannosaurids. A, MPC-D 107/7 (Tarbosaurus bataar), original (1) and interpretive drawing (2); C, IVPP V4878, holotype of Shanshanosaurus huoyanshanensis, taken from Wikipedia; C, LH PV18, holotype of Raptorex kriegsteini, from Sereno et al., 2009. Images are not to relative size.

So What About Nanotyrannus?

I’ve spent only a few paragraphs on Nanotyrannus lancensis so far. Setting the stage, so to speak. While the ontogeny of tyrannosaurs is important, and the identity of various named taxa was rendered in doubt, the same analysis has produced further doubt against the case for a unique Nanotyrannus lancensis. Prime among this work is that Tsuihiji et al. highlight the different conclusions of Carr (1999) and Currie (2003) in which taxa are compared to either Tarbosaurus bataar or Tyrannosaurus rex (with CMNH 7541 placed in as a “young” version for comparison), against which the more secure ontogenies of Gorgosaurus libratus and Albertosaurus sarcophagus are known. The most important feature used in arguing ontogenetic variation has been tooth count (Kurzanov, 1976), but as noted by Tsuihiji et al. (2011), MPC-D 107/7 indicates that, unlike Gorgosaurus libratus and likely Tyrannosaurus rex, tooth count does not vary considerably with age, merely morphology. Tyrannosaurus rex exhibits substantive ontogenetic transformation in adult and very old stages above that of Tarbosaurus bataar, concretely separating the two samples. While the reasoning of this difference has received little attention, that the projected growth curve of Tyrannosaurus rex exceeds that of any other tyrannosaurid (Erickson et al., 2007) affirms that projecting growth patterns among species is frought with difficulties. It may then be diagnostically viable to consider North American tyrannosaurine growth separately from Asian tyrannosaurine growth, enough perhaps to lump taxa from each province into unique taxa, but few of them (i.e., Tyrannosaurus rex and Tarbosaurus bataar).

Asian versus North American “tyrannosaurine” tyrannosauroids. In the middle are all named taxa, while on the right is the most condensed proposed model, while also supporting the two provinces as having separate taxa. Note: “huoyanshanensis” is misspelled above as “houyanshanensis,” but the former is intended.

So, really, what about Nanotyrannus? Previous work has fallen in one of two camps: Those advocating for unique taxonomic identity (as a binomial epithet always), and those advocating doubt and questionable separation of the taxon from coeval Tyrannosaurus rex. Despite the segregation of these “sides,” no analysis has argued that Nanotyrannus lancensis is securely a junior synonym of Tyrannosaurus rex without use of anecdotal or speculative work. Tsuihiji et al. (2011) affirm that despite the similarities, using a growth series of one taxon may not be useful in concretely defining another. Tsuihiji et al. nonetheless affirm the doubts of the situation, coming firmly to the conclusion that there is little conclusion to come to save doubt. Extensive work describing, redescribing, and re-CT-scanning the skull of CMNH 7541 (Witmer and Ridgely, 2009), has failed to produce firm data that places Nanotyrannus lancensis as a junior synonym of Tyrannosaurus rex. Moreover, Tsuihiji et al. affirm previous work separating Asia taxa, rather than lumping them as implied by other work and despite the doubt of certain Thomases (of course, I am referring to Mickey, and I jest). Tsuihiji et al. conclude (pg. 515):

[W]e tentatively conclude that, in accordance with its reportedly very different stratigraphic age and provenance, R. kriegsteini is not a juvenile T. bataar. Perhaps of greater general significance, the impact of MPC-D 107/7 on debates surrounding the status of R. kriegsteini, “S. huoyanshanensis,” and CMNH 7541 highlights the potential difficulties in discriminating plesiomorphic from juvenile characters and the importance of taxonomically unambiguous juvenile specimens such as MPC-D 107/7[.]

So far from affirming synonymy, the authors affirm the inverse, but leave the final conclusion based on a single, apparently non-variable feature of the postcranium. What does this leave to the skull for taxonomic utility? More is yet to come, as the results of the Burpee Museum Tyrannosaurus symposium has yet to be published. What we know now is quite a lot, but seemingly also not that much. Certainly, among the best we know now is that ontogenetic features are poor proxies for phylogenetic features, even were they useful for phylogeny; greater care should be taken in treating analyses in which they are included.

[n1] MPC is the abbreviation for Mongolian Paleontological Center, and now houses all specimens formerly placed under the acronym GIN. Thus, while originally labeled GIN 107/7, the new specimen is now MPC-D 107/7. Thus also, GIN 100/42 (the “Zamyn Kondt oviraptorine”) would be MPC 100/42. The “-D” appellation is a subcollections designation, as is the specimen prefix “107“.

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