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Distribution of the dentary groove of theropod : Implications for theropod phylogeny and the validity of the Nanotyrannus Bakker et al., 1988

Article in Research · June 2016 Impact Factor: 1.9 · DOI: 10.1016/j.cretres.2015.12.016

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Distribution of the dentary groove of theropod dinosaurs: Implications for theropod phylogeny and the validity of the genus Nanotyrannus Bakker et al., 1988

* Joshua D. Schmerge a, , Bruce M. Rothschild b a Biodiversity Institute, University of Kansas, 1345 Jayhawk Blvd., Lawrence, KS, 66045, USA b Department of Vertebrate , Carnegie Museum, 4400 Forbes Ave., Pittsburgh, PA, 44272, USA article info abstract

Article history: This study examines the phylogenetic distribution of a morphologic character, described as a groove Received 3 September 2015 containing pores, on the lateral surface of the dentary bone in theropod dinosaurs. The of this Received in revised form groove is a feature unique to theropods. Of the 92 theropod taxa examined for the presence and absence 27 December 2015 of this feature, 48 possessed and 44 lacked this feature. Distribution of this character was compared Accepted in revised form 27 December 2015 to published phylogenetic analyses of theropods, in order to evaluate the utility of the dentary groove Available online xxx as a diagnostic feature. 80% of pre-Tyrannoraptoran theropods possessed the dentary groove, with only 6 reversals in theropod clades. Theropods with beaks or edentulous jaws all lacked a dentary Keywords: groove. is marked by mosaic distribution of this character. Among tyrannosauroids, þ Tyrannosaurinae the dentary groove occurs only in and the ( ). Albertosaurinae Nanotyrannus lancensis, sometimes described as representing juvenile Tyrannosaurus rex, also possesses Neurovascular foramina this groove, unlike the remainder of the Tyrannosaurinae. Nanotyrannus lancensis was included in a phylogenetic analysis of Tyrannosauroidea and was recovered within Albertosaurinae. We recommend that Nanotyrannus stand as a valid taxon nested within the Albertosaurinae, based on the presence of this groove, as well as other features of the skull. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction occurrence of the dentary groove in theropod dinosaurs, and to discuss its phylogenetic and behavioral inferences. The past decade has witnessed increased attention to the Recognition and description of the dentary groove has varied in anatomy, ontogeny, and phylogeny of theropod dinosaurs, espe- previous studies. As such, there is no standard nomenclature to cially tyrannosaurids, with speculation as to their physiology and describe this feature found in some theropods. Various descriptions behavior (Brochu, 2003; Erickson et al., 2004; Weishampel et al., used to describe grooves on the lateral surface of the dentary 2004; Snively et al., 2006; Larson and Carpenter, 2008; Brusatte include “longitudinal groove that supports the nutritious foramina” et al., 2010; DePalma et al., 2013; Parrish et al., 2013). Theropod (e.g. Calvo and Coria, 1998), “dentary sulcus” (e.g. Novas et al., 2005; phylogeny based on cladistic analysis has shifted attention to large Sampson and Witmer, 2007; Eddy and Clarke, 2011), “lateral ridge” character datasets (e.g., Rauhut, 2003; Brusatte et al., 2010; Carrano with a “row of foramina above the ridge” (e.g. Coria and Currie, et al., 2012; Turner et al., 2012), with less consideration of func- 2006), “groove” (Novas et al., 2009), “groove for nutrient tional morphology. Such seems to be the case for a groove on the foramina” (e.g., Benson, 2010), “neurovascular groove” (Brusatte lateral aspect of the dentary, herein called the dentary groove, et al., 2012a), and “mental groove” (Carrano et al., 2012). Previous which appears to have functional and phylogenetic utility for the detailed consideration has apparently been limited to the Troo- Tyrannosauroidea. The purpose of this paper is to provide an dontidae, in which the groove bears neurovascular foramina that anatomical definition of the theropod dentary groove, trace the are connected with the inferior alveolar canal (e.g. Barsbold et al., 1987; Currie, 1987; Makovicky and Norell, 2004). The dentary groove was not included in reported cladistic analyses of theropods, until Brusatte et al. (2010) study of Tyrannosauroidea and Eddy and * Corresponding author. E-mail address: [email protected] (J.D. Schmerge). Clarke (2011) study of . Neither study posited a http://dx.doi.org/10.1016/j.cretres.2015.12.016 0195-6671/© 2015 Elsevier Ltd. All rights reserved. J.D. Schmerge, B.M. Rothschild / Cretaceous Research 61 (2016) 26e33 27 function for the feature. Eddy and Clarke (2011) determined three Institutional AbbreviationsdLACM, Los Angeles County character states: absence of a groove and either a foramina- Museum, Los Angeles, CA; BHI, Black Hills Institute of Geological containing groove on the posterolateral portion of the dentary or Research, Inc., Hill City, SD; BMR, Burpee Museum, Rockford, IL; one running the length of the dentary. Brusatte et al. (2010) and BSP, Bayerische Staatsammlung für Palaontologie€ und historische Carrano et al. (2012) utilized a binary (presence vs. absence) coding Geologie, Munich, Germany; CMNH, Cleveland Museum of Natural of this character in their respective phylogenies, which is the History, Cleveland, OH; FMNH, Field Museum of Natural History, approach adopted here. This was one of more than 350 other Chicago, IL; KUVP, University of Kansas Museum of Natural History characters utilized in the aforementioned studies. Whether the and Biodiversity Institute, Lawrence, KS; NMMNH, groove was a phylogenetically-informative character was not Museum of Natural History, Albuquerque, NM; TCM, The Children's discussed. Museum, Indianapolis, IN. The phylogenetic distribution of the lateral dentary groove was investigated to determine the polarity of the character, to interpret 3. Results the implication of its losses, and to assess its potential as a diag- nostic feature for re-classifying the position of “Jane” (BMR The dentary groove is a unique structure with multiple pores P2002.4.1) and other specimens labeled Nanotyrannus within aligned within the depression and sub-equally spaced. The groove Tyrannosauroidea. “Jane” is clearly a tyrannosaurid (Brusatte et al., originates near the anterior portion of the bone from a position 2010), but its taxonomic affiliation has been highly disputed approximately underneath the 2nd to 4th dentary alveolus. It (Erickson et al., 2006; Snively and Russell, 2007; Larson, 2008; extends caudally along the lateral surface, terminating near the Henderson and Harrison, 2008; Peterson et al., 2009; Larson, end of the tooth row (Fig. 1; see also figure 1a of Sampson and 2013a). The debate over “Jane” is a smaller part of a larger debate Witmer, 2007). This groove does not extend beyond the dentary about the validity of Nanotyrannus lancensis as a separate genus in examined theropods. This contrasts with the sauropodomorphs (Bakker et al., 1988). Three specimens have taken center stage in and Panphagia, wherein a groove extends onto the sur- this debate: “Jane”, the specimen of N. lancensis (CMNH face of the surangular and terminates in a large foramen. Multiple 7531), and the theropod described as one of the “Dueling Di- pores were noted along the length of the groove in theropod nosaurs” (BHI 6437; Larson, 2013b). The debate presently rests on dinosaurs. three competing hypotheses, either that 1) Nanotyrannus stands as The dentary groove was present on 48 of 92 sampled theropod a valid taxon (Currie, 2003a; Larson, 2013a); 2) these individuals taxa (see supplemental data). No groove was observed in the represent the taxon Tyrannosaurus lancensis (Currie et al., 2005); or crocodylomorph Shuvosaurus or the early Herrerasaurus, 3) these individuals are juvenile Tyrannosaurus rex (Carr, 1999; strengthening the hypothesis that this feature is unique to Brochu, 2003; Carr and Williamson, 2004; Holtz, 2004). A goal of theropod taxa. The groove was also not observed in this study is to attempt to clarify the relationship of N. lancensis or Tawa, the most primitive true theropods that we sampled. with other tyrannosaurids using the dentary groove and other , , , , , cranial characters (Larson, 2013a). and were the only observed pre-Tyrannoraptoran taxa that reverted to the primitive state (Fig. 2). 2. Materials and methods The single most parsimonious tree recovered by our phyloge- netic analysis of is reported in Fig. 3A. The mini- We investigated 92 theropod taxa for presence or absence of a mum tree length was found after 5 replicates, and the most groove on the lateral surface of the dentary (Fig. 1). We also parsimonious tree has a length of 561. Nanotyrannus was recovered examined photographs of the crocodylomorph Shuvosaurus, the as the sister taxon to the Albertosaurinae. We recovered the same early dinosaurs Herrerasaurus and Staurikosaurus, and the sau- topology as reported by Brusatte et al. (2010), except that the ropodomorphs Eoraptor, Leyesaurus, Pampadromaeus, and Pan- Proceratosauridae was recovered as a polytomy. The most parsi- phagia for comparison. Sampled taxa and reviewed published monious tree we produced resulted in a polytomy, regardless of records are delineated in the supplemental data. We relied on de- whether Nanotyrannus was included or not. Alternative non- scriptions of whenever possible, to avoid controversy as parsimonious tree topologies based on our result but reflecting to taxonomic assignment. Some sampled specimens are the subject the proposed relationships of Nanotyrannus among the Tyranno- of debate (e.g. “Jane”) and the attribution of some unpublished sauroidea sensu Currie (2003a) and Carr (1999) are presented for specimens is controversial (e.g., assignment of specimens to Spi- comparison in Fig. 3B and C respectively. nosaurus aegypticus). When such published descriptions did not We investigated undisputed Tyrannosaurus rex material per- include photographs of the specimen(s) or presence or absence of taining to the different life stages (i.e., juvenile, subadult, and adult the groove was not unambiguously depicted in an illustration, we of Carr and Williamson, 2004). The dentary of the smallest known examined images of additional congeneric specimens, as available. juvenile of T. rex (LACM 28471) lacks the dentary groove. Although Phylogenetic analyses were conducted using TNT v1.1 (Goloboff its dentary is fragmentary, the anterior portion is preserved, and et al., 2008). We utilized the methodology and character matrix of examination failed to reveal the presence of a dentary groove. Brusatte et al. (2010), and modified it only by adding Nanotyrannus None of the T. rex specimens examined (LACM 28471, LACM 23845, to the character matrix and coding the dentary groove as present in LACM 150167, KUVP 155809, FMNH PR2081) possessed the Dryptosaurus, without modification of any other characters. The groove. character matrix is included in the supplemental file. We coded the character matrix for Nanotyrannus based on personal observations 4. Discussion of skull of “Jane” at the Burpee Museum. Presence of the groove is character 176 of Brusatte et al. (2010) and character 124 of Carrano 4.1. Discussion of presence of dentary groove et al. (2012). We traced the most parsimonious occurrence of this character on the branches of these previously published clado- The dentary groove is present in 33 of 41 (80%) of the pre- grams to evaluate its use as a diagnostic character in theropods and Tyrannoraptoran theropods sampled here. We interpret only 6 re- to evaluate competing phylogenetic hypotheses (Figs. 2e3). The versals responsible for groove losses (Fig. 2). The presence of this most parsimonious result of this study is reported in Fig. 3A. groove is therefore a character primitive for nearly all early 28 J.D. Schmerge, B.M. Rothschild / Cretaceous Research 61 (2016) 26e33

Fig. 1. Lateral views of theropod skulls demonstrating presence or absence of the dentary groove. The dentary groove is present in the primitive theropod (A) bauri (NMMNH P-42200), as well as in the derived theropod (B) longipes (BSP AS I 563). The dentary groove is present in the tyrannosaurids (C) Gorgosaurus libratus (TCM

2001.89.1) and (D) Nanotyrannus lancensis (“Jane”; BMR P2002.4.1). The dentary groove is absent in both (E1) young (LACM 28471) and (E2) adult (“Sue”; FMNH PR2081) Tyran- nosaurus rex. Arrows indicate the position of the groove, when present. Scale bars equal 5 cm. J.D. Schmerge, B.M. Rothschild / Cretaceous Research 61 (2016) 26e33 29

Fig. 2. Cladogram of modifed from Carrano et al. (2012). Thickened branches indicate lineages possessing the dentary groove, thin branches indicate lineages in which the groove is absent. Grayed-out braches marked with dashed lines indicate taxa without a known dentary. Circled numbers indicate sequence of losses of the dentary groove assuming maximum parsimony.

theropods. Given the absence of the groove in the theropod clades middle of the tooth row and extends nearly to the posterior end of which include Daemonosaurus and Tawa, the groove first evolved in the dentary. We interpret this difference as truly morphological and the earliest common ancestor of the . do not consider the groove found in the to be ho- Within highly derived theropods, the dentary groove is present mologous with the dentary groove of other theropods. Currie in some members of the Maniraptora. A similar foramina-bearing (1987) and Makovicky and Norell (2004) have interpreted a neu- groove has been considered a synapomophy of rovascular function for this troodontid groove and its pores, an (Makovicky and Norell, 2004). As such, it is present on all sampled interpretation that we do not challenge. troodontids, but limited to the posterior portion of the dentary. A Few tyrannosauroids retain this groove. “Jane” clearly has this groove was also observed in the dromaeosaurids Acheroraptor, groove (Fig. 1D) as does BHI 6437. The condition of the Nano- Austroraptor and Buitreraptor. In Maniraptora, the groove appears to tyrannus holotype is unclear, as its jaws are occluded and the increase in height posteriorly (i.e., grows closer to the tooth row), groove-bearing portion of the dentary obscured. The only other creating a wedge-shaped appearance, rather than the linear shape tyrannosauroids that possess this groove are Dryptosaurus aqui- of the groove in other theropod dinosaurs. In the troodontids, lunguis and the Albertosaurinae: Gorgosaurus libratus and Alberto- Acheroraptor, and Austroraptor, the groove originates beneath the saurus sarcophagus. 30 J.D. Schmerge, B.M. Rothschild / Cretaceous Research 61 (2016) 26e33

Fig. 3. Proposed phylogenetic relationships of Nanotyrannus within Tyrannosauroidea with distribution of the theropod dentary groove on trees indicated with thickened bars. (A) Most parsimonious cladogram proposed by this study placing Nanotyrannus as sister to the Albertosaurinae. (B) Relationship sensu Currie (2003a) placing Nanotyrannus as sister to Tyrannosaurus. This tree requires 5 more independent losses of the dentary groove than the tree proposed in this study. (C) Relationship proposed by Brusatte et al. (2010) placing Nanotyrannus as a juvenile Tyrannosaurus. This tree requires 4 more independent losses than the tree proposed in this study and a loss of the dentary groove through ontogeny in Tyrannosaurus.

4.2. Possible implications of beak formation and dentary groove oviraptorosaurs (Caudipteryx and Chirostenotes) and all sampled absence lack the dentary groove. The jaw structure of has been completely modified into a beak and the dentary of Limu- There are several dentary groove absences that occur simulta- saurus even lacks any foramina on its lateral surface. Simply neously with the presence of a beak or edentulous jaws. The beaked modifying the mandible into a beak, however, does not account for ceratosaur Limusaurus, ornithomimids ( and Shenz- this absence of the groove. Only the anterior of the upper jaws were housaurus), therizinosaurs (Erlikosaurus and Jiangchanosaurus), beak-like in Jiangchanosaurus, in which only the premaxillae were J.D. Schmerge, B.M. Rothschild / Cretaceous Research 61 (2016) 26e33 31 considered to have been covered by a rhamphotheca (Pu et al., ripping large portions (e.g., limbs) from the body of the prey. Since 2013). Jiangchanosaurus does not have a groove. The toothed the groove conducted a nerve along the surface of the dentary, a birds Archaeopteryx, Hesperornis, Ichthyornis, and Parahesperornis powerful bite may also have been intrinsically painful. The nerve also lack a dentary groove, despite being beakless or only having an may have been reduced and the groove therefore eliminated, as incipient beak. more powerful biting force evolved in tyrannosaurids. A biome- Adoption of herbivory is a reasonable hypothesis for the chanical explanation for the loss of the groove is appealing, because evolutionary loss of serrated teeth from the jaws in favor of eden- large carcharodontosaurids were stratigraphically contempora- tulous jaws or beaks. One explanation is that that the dentary neous with tyrannosaurines and, in some cases, were larger than groove played a role in predatory behavior and that the groove was tyrannosaurines. Thus increase in size alone is not sufficient to lost in these theropods because carnivory was abandoned. Orni- explain the loss of the groove. thomimids (Kobayashi et al., 1999; Norell et al., 2001) and ther- The variable distribution of the groove among tyrannosaurids izinosaurs (Zanno et al., 2009) are both postulated to have been may explain the seemingly incompatible cohabitation (Farlow and herbivorous. Oviraptorosaurs may have been predominately her- Pianka, 2003)ofDaspletosaurus (in which a groove is absent) with bivorous (Ji et al., 1998; Xu et al., 2002), although they may have Gorgosaurus and Albertosaurus (in which the groove is present). taken prey or scavenged under rare circumstances (Norell et al., may have been adapted to eating prey that required 1994). Analysis of jaw mechanics in theropod dinosaurs clearly powerful bites to subdue or consume, whereas the albertosaurines demonstrates the dissimilarity of oviraptosaurs and predatory were more lightly built predators that perhaps preferred less robust theropods (Brusatte et al., 2012b). The lack of the dentary groove is prey. This is in keeping with Russell's (1970) hypothesis that taken as further evidence that they may not have been active albertosaurines preyed on hadrosaurs, whereas Daspletosaurus predators. The possibility that the common ancestor of Ornitho- specialized on ceratopsians. Tyrannosaurines are known to have mimosaurs, Oviraptorosaurs, and Therizinosaurs also lacked a taken hadrosaurs as prey items (Varricchio, 2001; DePalma et al., dentary groove cannot be overlooked, especially given the absence 2013). Rather than invalidate our hypothesis, this merely is evi- of a groove in most other maniraptorans. While the presence of a dence that tyrannosaurines were large, powerfully built predators beak does not necessarily indicate herbivorous lifestyledas squid, that were capable of taking any suitable prey items in their envi- placoderm fish and birds of prey all utilize beaks in predatory ronment. This explanation is consistent with the mechanical hy- behaviordwhy a well-adapted predator like a theropod would pothesis we pose here. evolve a beak to bolster a predatory lifestyle is unclear, and an A potential flaw in this hypothesis is the absence of the groove in interpretation of herbivory in this context is more parsimonious. many smaller tyrannosauroid genera, such as , Dilong, and . While an examination of the bite force of these taxa is 4.3. Absence of groove in tyrannosauroids beyond the scope of the present study, it may be the case that these smaller tyrannosauroids were also exerting relatively high bite There are numerous tyrannosauroids that lacked the dentary forces on their prey or otherwise needed to strengthen their jaws groove. Assuming maximum parsimony for the distribution of the for successful food acquisition. dentary groove, we must interpret several of these as independent losses. One potential explanation for the absence of the groove in 4.4. Interpreting the identity of “Jane” and the validity of certain taxa might be that the nerves and blood vessels accom- Nanotyrannus modated by the groove occurred superficial to the dentary rather than along its surface. Another potential explanation for the loss of The mosaic distribution of the dentary groove within Tyranno- the groove in various tyrannosaurids is to enhance jaw strength. sauroidea allows an opportunity to use the occurrence of this The groove could potentially serve as a point of weakness: force- character to determine the relationship of the embattled Nano- modeling experiments on the mandible of Erlikosaurus demon- tyrannus to other tyrannosaurs. Tyrannosaurus rex lacks this groove, strate higher stress levels along the line of mandibular foramina whereas it is a distinct character in “Jane” and the other Nano- during simulated bites (Lautenschalger et al., 2013). The groove was tyrannus lancensis specimens in which the dentary is visible. The likely eliminated as the cortical bone of the dentary was thickened occurrence of the dentary groove in N. lancensis, but not in T. rex,is to accommodate strong bite forces (Therrien et al., 2005). Loss of itself independent evidence of separation of these two taxa. The the groove would therefore be advantageous from a mechanical presence of the groove stands with more than 30 other skeletal perspective for the largest tyrannosaurids. Snively et al. (2006) characters as evidence to separate Nanotyrannus from Tyranno- hypothesized that tyrannosaurines had stronger bite forces saurus (Larson, 2013a). compared to albertosaurines and carnosaurs, due to vaulting of the A frequent point of contention in the literature for using skeletal skull, enhanced fusion of the cranial bones and skull kinematics. characters to differentiate Nanotyrannus and Tyrannosaurus is the Tyrannosaurines likely consumed greater amounts of bone relative possibility of ontogenetic variation, the chance that characters to smaller theropods (Fiorillo, 1991; Erickson and Olson, 1996; become altered as an individual matures into adulthood. While the Snively et al., 2006). That makes sense, given that (1) observation size, orientation, and depth of cranial fossa and such openings as of modern ecosystems demonstrates that predators with stronger the orbits in tyrannosaurs, as well as tooth count and morphology, bite forces tend to consume more portions of the prey (Van have been interpreted to be ontogenetically variable (Carr, 1999), Valkenburgh, 1996) and (2) the presence of prey items (e.g., cera- these interpretations have been questioned (Currie, 2003a; Larson, topsians) in the Cretaceous which possess bony frills. Large tyran- 2013a). The argument that the dentary groove is an ontogenetically nosaurids used a “puncture and pull” feeding strategy (Bakker, variable character is unappealing for several reasons. First, if this 1986; Molnar and Farlow, 1990; Erickson and Olson, 1996) which feature corresponds to a system of nerves enervating the mandible, required that the teeth penetrate bone, in order to effectively an- a dramatic change (e.g., metamorphosis) would need to be invoked chor the jaws into the prey. This feeding behavior has been inter- to explain the loss of this feature through maturation. Second, there preted from bite marks on ceratopsians (Erickson and Olson, 1996). are other undisputed T. rex specimens, representing juvenile (LACM Other theropods (e.g, Allosaurus, ) may have instead 28471, LACM 23845) and subadult (LACM 150167, KUVP 155809) relied on cranial kinesis and thin, slicing teeth to wound their prey stages, that lack this groove. The adult form (e.g., FMNH PR2081) (Paul, 1988; Rayfield et al., 2001; Therrien et al. 2005), instead of lacks this groove. As all Tyrannosaurus rex specimens we 32 J.D. Schmerge, B.M. Rothschild / Cretaceous Research 61 (2016) 26e33 investigated lack a dentary groove, even in juvenile and subadult feature. As 80% of basal theropods possess this groove, it could be individuals, and all Nanotyrannus specimens we investigated considered a defining character of theropod dinosaurs, lost in only possess a dentary groove, this seems to suggest that the groove is a handful of lineages. While loss of the groove did not appear to not variable in a single species and that it does not change during result from the evolution of beaks in different theropod clades, it ontogeny. The presence or absence of a groove may further be may have been one of many evolutionary changes that occurred sufficient to diagnose one taxon from another taxon. The most which strengthened the jaws of tyrannosaurids. This feature has reasonable explanation for this data is that N. lancenesis is a distinct also proved useful for interpreting relationships among tyranno- taxon and not a juvenile form of T. rex. sauroids, which have a mosaic distribution of this character. The dentary groove is such a phylogenetically conservative Nanotyrannus clearly presents the dentary groove, whereas character (i.e., only five losses outside of Tyrannoraptora) that the Tyrannosaurus rex lacks this feature. Without a modern analog presence or absence of the groove in tyrannosaurids seems a that demonstrates the loss of such a groove through maturation, useful taxonomic character. Nanotyrannus has been suggested to accepting that this feature is lost through ontogeny is speculative. be similar to Gorgosaurus, based on the presence of numerous “Jane”, and the other specimens referred to Nanotyrannus, there- cranial and dental characteristics (Larson, 2013a). Interestingly, fore would not be examples of juvenile Tyrannosaurus,andstand CMNH 7531 was originally described as a species of Gorgosaurus alone as a distinct genus. Nanotyrannus should further be (Gilmore, 1946). The presence of the dentary groove in Nano- considered sister to the Albertosaurinae, rather than the Tyran- tyrannus seems to further confirm its affinity with Albertosaur- nosaurinae, as they are the only other clade of large tyrannosaurs inae rather than Tyrannosaurinae. Albertosaurinae is defined as to possess this groove. This feature, in addition to many other the tyrannosaurs possessing: an antorbital cavity that reaches the cranial characters, makes an alignment of Nanotyrannus with the nasomaxillary suture, lateral surface of nasal excluded from the albertosaurines preferred. antorbital vacity, and a dorsally-oriented, triangular corneal pro- cess of the lacrimal (Holtz, 2004). Currie (2003b) defined Alber- Acknowledgements tosaurinae as possessing, in contrast to Tyrannosaurinae, short and low skulls, shorter ilia, longer tibiae, longer metatarsals and We thank D. Burnham and V. Naples for access to photographs longer , a definition Nanotyrannus meets. Larson (2013a) and for discussions and feedback, P. Larson and A. Falk for access to presented more than 30 skeletal characters that separate Nano- additional photographs and specimens, and the staff of the Burpee tyrannus from Tyrannosaurus, with the following characters Museum of Natural History Museum for facilitating access to “Jane”. possibly uniting Nanotyrannus and the albertosaurines: greater We thank R. Molnar and an anonymous reviewer for comments dentary tooth counts, contact of the maxillary fenestra with the that improved the manuscript. rostral margin of the antorbital fossa, a medial post-orbital fossa and an ectopterygoid pneumatic foramina bounded by a thick lip. References Albertosaurines may also be regarded as having more circular than ovoid orbits. We propose that the presence of the dentary Bakker, R.T., 1986. The Dinosaur Heresies. William and Morrow, New York, New groove be added to the list of diagnostic albertosaurine characters, York, 481 pp. Bakker, R.T., William, M., Currie, P., 1988. Nanotyrannus, a new genus of pygmy as it is a character that is lost in Tyrannosaurinae. We therefore tyrannosaur, from the Latest Cretaceous of . Hunteria 1, 1e28. propose that Nanotyrannus should be regarded as a member of Barsbold, R., Osmolska, H., Kurzanov, S.M., 1987. On a new troodontid (Dinosauria, Albertosaurinae. Theropoda) from the of . Acta Paleontologica Polonica 32, 121e132. Fig. 3 shows three potential interpretations of the phylogeny of Benson, R.B., 2010. The osteology of Magnosaurus nethercombensis (Dinosauria, Tyrannosauroidea. 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