ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 16: 17–27 (2017)© by the Fukui Prefectural Dinosaur Museum

ON THE DENTARY IN THE FUKUI PREFECTURAL DINOSAUR MUSEUM OF
GORGOSAURUS LIBRATUS (THEROPODA: TYRANNOSAURIDAE) FROM THE
DINOSAUR PARK FORMATION (UPPER CRETACEOUS) OF ALBERTA, CANADA
Sebastian G. DALMAN and Spencer G. LUCAS
New Mexico Museum of Natural History and Science
1801 Mountain Road N. W. Albuquerque, NM 87104, USA
ABSTRACT
An isolated right dentary of the tyrannosaurid Gorgosaurus libratus in the Fukui Prefectural Dinosaur Museum foundin the Dinosaur Park Formation in southern Alberta, Canada, is described. The referral of the dentary to G. libratus isbased on several morphological features, including the lack of the posteroventral transitional point in the anterior step of
the lingual bar, the foramen intermandibularis oralis overlaps ventrally the anterior terminus of the Meckelian groove,and the anterior terminus of the Meckelian groove contacts the posterior margin of the dentary symphysis. The excellent
preservation of the dentary allows for detailed comparisons with other tyrannosaurids, especially with its close relative
Albertosaurus sarcophagus with which G. libratus is often synonymized. The two taxa can be differentiated based on
the overall morphology of the anterior step of the lingual bar, position of the foramen intermandibularis oralis withrespect to the anterior terminus of the Meckelian groove, the posterior step of the Meckelian fossa, and the articulation
facet and notch for the splenial. This work provides further support for recognizing G. libratus as a taxon distinct fromand not a synonym of Albertosaurus.
Key words : Gorgosaurus, Albertosaurus, Albertosaurinae, Tyrannosauridae, Theropoda, Upper Cretaceous, Dinosaur
Park Formation, Canada

INTRODUCTION

hoglundi, Teratophoneus curriei, and Tyrannosaurus rex.
Specimens currently assigned to Daspletosaurus and Gorgosaurus
collected from the Campanian deposits of the Judith River andTwo Medicine formations in Montana most likely represent new
species (Currie et al. 2005; Stein and Triebold, 2013; Dalman andLucas, 2015). Similarly, fragmentary specimens of tyrannosaurids
recovered from the San Juan Basin in northwestern New Mexico,
the Aguja and Javelina formations of Texas, and isolated teeth
from the Corral de Enmedio and Lomas Coloradas formations ofSonora, Mexico most likely represent previously unrecognizedtaxa, which provide further support for the morphological andtaxonomic diversity of these iconic predators (e.g., Jasinski etal., 2011; Lehman and Wick, 2012; Serrano-Brañas et al., 2014,2017).
Although tyrannosaurids are the most studied carnivorous
dinosaurs, their species-level taxonomy is still not clearly
resolved. Gorgosaurus is one of the most commonly recovered
tyrannosaurids in North America, and the majority of specimens
have been found in the Campanian deposits of the Dinosaur Park
Formation (76.9–75.8 Ma) in southwestern Alberta, Canada, andthe Judith River (80–75 Ma) and the Two Medicine (83.5–74.9 Ma)
formations in Montana, USA (Lambe, 1917; Currie, 2003, 2005;
Dalman and Lucas, 2015). Among tyrannosaurids, Gorgosaurus

Tyrannosaurid dinosaurs were large-bodied predators,
which, for most of the Campanian-Maastrichtian dominated theterrestrial ecosystems in Asia and North America (e.g., Russell,
1970; Currie, 2003; Holtz, 2004; Hone et al., 2011; Dalman,2013). This highly specialized clade of carnivorous dinosaurs
is characterized by massive skulls and short didactyl forelimbs(e.g., Osborn, 1905; Lambe, 1917; Brochu, 2003; Holtz, 2004).
Since the discovery of the first skeleton of Tyrannosaurus rex
over a century ago in the Lance Formation in Wyoming, several
new taxa have been recovered from the Upper Cretaceous
rocks throughout the western part of North America. Currently
eight North American species have been identified representing
two subclades: Albertosaurinae, which includes Albertosaurus
sarcophagus and Gorgosaurus libratus, and Tyrannosaurinae,
which includes Bistahieversor sealeyi, two species of the genus
Daspletosaurus, D. horneri and D. torosus, Lythronax argestes,
the highly controversial Nanotyrannus lancensis, Nanuqsaurus
Received April 30, 2017. Accepted September 28, 2017.
Corresponding author ― Sebastian G. DALMAN
E-mail: sebastiandalman＊yahoo.com
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SEBASTIAN G. DALMAN AND SPENCER G. LUCAS

has most often been synonymized with Albertosaurus and referred
to as Albertosaurus libratus (Russell, 1970; Carr, 1999, 2010;
Carr and Williamson, 2000; Carr et al., 2017). In the past decade,new specimens of Albertosaurus sarcophagus and Gorgosaurus
libratus have been recovered from the Upper Cretaceous ofAlberta, Canada. These specimens allowed Currie (2003) tofind numerous morphological features in the skull that clearly
differentiate these taxa. In his study, however, Currie (2003) only
briefly described the morphological features of the dentaries ofAlbertosaurus and Gorgosaurus.
This work presents the first detailed osteological descriptionof an isolated dentary of Gorgosaurus libratus (FPDM-V
8062) found in the Dinosaur Park Formation in southwestern
Alberta, Canada. FPDM-V 8062 was purchased by FPDM
from a Japanese fossil collector who is now deceased. Detailed
comparisons with the dentaries of Albertosaurus sarcophagus andother tyrannosaurids provide further evidence of the taxonomic
distinctiveness of A. sarcophagus and G. libratus.
Institutional abbreviations
BHI: Black Hills Institute of Geological Research, Hill City,
South Dakota, USA; BMNH: British Museum of Natural
History, London, UK; BMR: Burpee Museum of Natural History,
Rockford, Illinois, USA; FPDM: Fukui Prefectural Dinosaur
Museum, Fukui, Japan; IGM: Institute of Geology (currently
Institute of Paleontology and Geology), Ulaan Baatar, Mongolia;
NMMNH: New Mexico Museum of Natural History and Science,
Albuquerque, New Mexico, USA; TMP: Royal Tyrrell Museumof Palaeontology, Drumheller, Alberta, Canada; UMNH: Natural
History Museum of Utah, Salt Lake City, Utah, USA; ZCDM:
Zhucheng Dinosaur Museum, Zhucheng, China; ZPAL: Instituteof Palaeobiology, Polish Academy of Sciences, Warsaw, Poland.

TYRANNOSAURIDAE Osborn, 1906
ALBERTOSAURINAE Currie et al., 2003
GORGOSAURUS LIBRATUS Lambe, 1914
Material.―FPDM-V8062, a complete right dentary with teeth,
described here for the ﬁrst time (Fig. 1).
Locality and age.―Dinosaur Provincial Park, southern
Alberta, Canada. Upper Cretaceous (Campanian) Dinosaur Park
Formation.
DESCRIPTION
Dentary.―The length of the preserved dentary FPDM-V8062
is 54 cm (Fig. 1). The dentary is mediolaterally thick along thetooth row; however, it gradually narrows at the posterior end. The
dentary has 15 well-preserved teeth. Nearly all of the teeth are
fully erupted, except for d5, d8, d11, and d14, which are partially
erupted. The alveoli are nearly circular, except for alveolus 1,
which is mediolaterally compressed and has an elliptical shape.
The dorsoventral depth of the dentary is not uniform throughout
its length. The dorsoventral depth of the dentary is 10 cm atalveolus 5 and increases more posteriorly, so that, it is 11 cm atalveolus 9 and, is 15 cm at alveolus 15.
The lateral surface of FPDM-V8062 is perforated by several
mental foramina. The foramina are arranged in two parallelrows: alveolar and ventrolateral rows. Two anteriormost mental
foramina in the alveolar row are situated directly ventral to thealveolar margin of the first alveolus. These foramina are nearly
1 cm apart from each other. The first foramen is approximately
0.5 cm ventral to the alveolar margin of the dentary, whereas thesecond foramen is 1 cm ventral to the alveolar margin, so that the

SYSTEMATICSFPDM-V8062, a complete right dentary with teeth in the
Fukui Prefectural Dinosaur Museum, is assigned to Gorgosaurus
libratus based on several key characters, such as the lack of theposteroventral transitional point in the anterior step of the lingualbar, the foramen intermandibularis oralis overlaps ventrally
the anterior terminus of the Meckelian groove, and the anterior
terminus of the Meckelian groove contacts the posterior margin of
the dentary symphysis. Furthermore, the surface of the inﬂection
point where, the ventral and the anterior margins of the dentary
meet, is strongly rugose and projects ventrally beyond the ventral
margin of the dentary. The posterior step in the dentary is welldeﬁned, whereas the notch for the articulation with the splenial
is diminutive. The anterior portion of the facet for the splenial isdorsoventrally wide.
DINOSAURIA Owen, 1842
SAURISCHIA Seeley, 1887
THEROPODA Marsh, 1881
COELUROSAURIA von Huene, 1914

FIGURE 1. Right dentary of Gorgosaurus libratus (FPDM-V8062). A,
lateral view; B, medial view; C, dorsal view. Abbreviations are shown inAppendix 1. Bar = 5 cm.

OSTEOLOGY OF GORGOSAURUS LIBRATUS DENTARY

foramina are not parallel to the alveolar margin of the dentary.
The third foramen is posteroventral to the second foramen, 1
cm apart from the latter and 2 cm ventral to the alveolar marginof the dentary. The consecutive mental foramina parallel thealveolar margin. At the level of alveoli 2 and 4, the foramina aretightly spaced, approximately 1 cm apart from each other and2 cm ventral to the alveolar margin of the dentary. At the levelof alveolus 5, the foramina become gradually larger and are
more widely separated from each other. The distance between
each consecutive foramen increases from 1.5 cm to 4 cm. In theposterior region, the foramina are situated within an elongate,
fossa-like depression. At the level of alveolus 12, the fossa-like
depression is 4 cm long and 0.5 cm wide. The two consecutive,
more posterior fossae are 2 cm and 3 cm long, and 0.3 cm wide.
The ventral row of mental foramina in FPDM-V8062 parallels
the alveolar row and the ventral margin of the dentary. The
foramina are elongate and widely spaced. The lengths of theforamina are between 2 cm and 3 cm. The distalmost foramen islarge and has a circular shape. Associated with this foramen is anelongate, shallow, fossa-like depression that is directly posterior
to it. The fossa-like depression is 3.2 cm long and 1 cm wide.
Another row of mental foramina is situated on the anterior
surface of the dentary. The row extends dorsoventrally,
uninterrupted to the inflection point (= transition point of Carret al., 2005) where the ventral and the anterior margins of thedentary meet. The foramina are large, circular and deep. A portionof the ﬁrst anterodorsal foramen is visible in lateral view, whereas
all other foramina in the series are only visible in anterior view.
The inﬂection point is convex and projected ventrally.
The medial surface of the dentary is ﬂat. The interdental plates
are well preserved. The ﬁrst interdental plate is entirely coveredby the anterior step of the lingual bar (Dalman and Lucas, 2015;
Fig. 2). Most of the second interdental plate is also covered by
the lingual bar, with only a small posterior portion being visible.
The third through seventh interdental plates are the tallest in
the series. Their anterior and posterior surfaces are vertical and19parallel to each other. The dorsal surface of each interdental plateforms a low triangle; idp8 and idp9 have the same morphology as
more anterior interdental plates, however, they are progressively
smaller. Each consecutive interdental plate in the posterior regionof the dentary, idp10 through idp15, gradually decreases in size
but maintains a triangular shape. As in other tyrannosaurids, theinterdental plates in FPDM-V8062 are unfused to each other;
they ﬂare out medially, and they are separated by a groove for thedental lamina ventral to the tooth row. In the anterior region of thedentary, the groove for the dental lamina curves anterodorsally,
and, in the posterior region it curves posterodorsally.
In life, the interdental plates in tyrannosaurids and other nonavian theropods were covered by the ossified supradentary/coronoid (e.g., Molnar, 1991; Brochu, 2003; Currie, 2003; Hurumand Sabath, 2003; Dalman and Lucas, 2015). FPDM-V8062 does
not preserve the supradentary. However, based on the overall
height of the interdental plates and the outline of the dorsal surfaceof the lingual bar, the supradentary in FPDM-V8062 would likely
have covered the posterior portion of the second interdental plate
and all other interdentary plates lingually. The first interdental
plate and the anterior portion of the second interdental plate arecovered by the anterior step of the lingual bar (Dalman and Lucas,
2015; Fig. 2). In FPDM-V8062, the dorsal surface of the anterior
step of the lingual bar parallels the alveolar row (= tooth row).
The dorsal surface of the step extends posteriorly and terminates
directly anterior to the second alveolus. This terminal point is
identiﬁed here as the posterodorsal transition point of the anterior
step of the lingual bar. Directly posterior to the posterodorsaltransition point, the lingual bar gently slopes ventrally andcontinues posteriorly and terminates approximately 8 cm posterior
to alveolus 15.
The lingual bar is elongate with a flat medial surface,underlying the interdental plates and overlying the Meckeliangroove. Overall length of the lingual bar is 43 cm. In the anterior
region, the lingual bar is slightly ﬂared and dorsoventrally deepand shallow throughout most of the anteroposterior length. The

FIGURE 2. Right dentary of Gorgosaurus libratus (FPDM-V8062). A, medial view; B, close up of the anterior end in medial view; C, close up of the posterior
end in medial view. Abbreviations are shown in Appendix 1. Bar = 5 cm.

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SEBASTIAN G. DALMAN AND SPENCER G. LUCAS

posterior portion of the lingual bar (= lappet of Brusatte et al.,2012) covers the dorsal portion of the Meckelian fossa medially.
The medial prong for the articulation with the surangular is notpreserved, whereas the lateral prong is preserved and nearlycomplete. A sharp notch separates the lateral prong from thedorsal margin of the Meckelian fossa.
The anteroventral surface of the lingual bar is slightly
inclined anterodorsally and makes direct contact with theinterdentary symphysis and with the dorsal portion of the foramenintermandibularis oralis (Fig. 2). For most of its anteroposterior
length, the ventral surface of the lingual bar is straight. As a result,the dorsal and the ventral surfaces of the lingual bar are parallel toeach other.
Directly ventral to the lingual bar is the Meckelian groove.
It is an elongate and well-defined feature. The anterior portionof the Meckelian groove at the level of alveoli 2 through 6 issituated at the mid-height of the dentary. At the level of alveoli 10
through 15, the Meckelian groove is inclined posterodorsally, sothat it is dorsal to the mid-height of the dentary. Throughout its
length the Meckelian groove has a uniform dorsoventral depth ofapproximately 0.5 cm. At the level of alveolus 3, the Meckeliangroove contacts directly the foramen intermandibularis oralis(Fig. 2). Anterior to the foramen, a small terminal end of the
Meckelian groove at the level of alveolus 2 extends anteriorly
and contacts the posterior margin of the dentary symphysis. The
posterior end of the Meckelian groove terminates at the levelof alveolus 14 and directly dorsal to the posterior step for thecontact with the splenial. The posterior step emerges from theposterodorsal surface of the ventral portion of the dentary andlaterally envelops the intermandibular vascular foramen (Fig. 2).
The anterior end of the step is concave and situated at the levelof alveolus 12. The ventral portion of the step is significantly
shorter than the dorsal one. It slopes posteroventrally andterminates at the level of alveolus 13. Directly ventral to it is ashallow notch for the articulation with the splenial (Fig. 2). The
anterior and ventral portions of the notch form a 90 °angle. A
single, well-deﬁned intermandibular vascular foramen is situatedslightly dorsal to the ventral portion of the posterior step (Fig.
2). The foramen is circular and deep. A characteristic fossa-likestructure is associated with the foramen. The fossa slightly ﬂares
posteriorly and merges with the Meckelian fossa. The surface ofthe Meckelian fossa is smooth and ﬂat.
A ﬂat articular facet for the splenial extends along the ventral
margin of the Meckelian fossa (Fig. 2). The facet is extensiveat the anterior end. Its dorsal surface is strongly convex anddorsoventrally wide. The convexity extends approximately 8
cm and terminates at the level of and posterior to alveolus 15.
Immediately posterior to this level, the facet becomes signiﬁcantly
narrower.
Anteriorly, the ventral and the anterior margins of the dentary
meet at the inﬂection point ventral to the level of alveoli 2 and 3
and brace the dentary symphysis medially (Fig. 2). In this region,
the dentary is convex and strongly rugose. The rugosity extends
anteroposteriorly along the convex surface of the inﬂection point.
Along its anteroventral margin the rugosity consists of several

short and well-deﬁned grooves and ridges. The medial surface of
the inﬂection point is slightly raised.
The dentary symphysis in FPDM-V8062 is long and, flatand has an elliptically-shaped smooth surface, which terminates
posteroventrally at the level of alveolus 3 (Fig. 2). Alongthe anteromedial margin of the dentary symphysis extends acharacteristic rugosity. The rugosity consists of short grooves andridges, which are parallel to each other. However, this rugosity
does not extend throughout the entire length of the anteromedial
margin of the dentary symphysis. It originates nearly 1 cm ventral
to the anteriormost end of the anterior step of the lingual bar andterminates at the level anterior to the foramen intermandibularis
oralis (Fig. 2). The remaining portion of the anterior margin of thedentary symphysis between the terminal end of the rugosity andthe inﬂection point is smooth.
A single foramen intermandibularis oralis is present at the levelof alveolus 4 (Fig. 2). The foramen overlaps ventrally the anterior
portion of the Meckelian groove. However, it does not contact thedentary symphysis. Instead, a characteristic fossa anteroventral to
the foramen and associated with it contacts the posterior marginof the dentary symphysis.
DISCUSSION
Russell (1970), in his revision of the North Americantyrannosaurids, synonymized Albertosaurus and Gorgosaurus.
At the time, Russell (1970) was unable to ﬁnd any morphologicaldifferences between these genera because of the lack of wellpreserved specimens. However, the discovery of new specimensof Albertosaurus and Gorgosaurus in the past decade allowed
Currie (2003) to ﬁnd many morphological features in the skullsof these genera that clearly differentiate them. In his study, Currie(2003), however, only brieﬂy described the morphological featuresof the dentaries of Albertosaurus and Gorgosaurus as well as
those of other tyrannosaurids. Despite a detailed osteologicaldescription by Currie (2003), especially of Albertosaurus andGorgosaurus, some other workers continued to synonymize thesegenera (e.g., Carr, 1999, 2010; Carr and Williamson, 2000, 2010;
Carr et al., 2011, 2017). Both genera are classified as membersof the Subfamily Albertosaurinae. Holtz (2004) suggested that
Appalachiosaurus montgomeriensis (Carr et al., 2005) may also
be a member of Albertosaurinae. Albertosaurus was a slightly
larger and more robust animal than Gorgosaurus (Currie, 2003).
When compared with other tyrannosaurids, Albertosaurus andGorgosaurus have narrower skulls in the posterior region, andmore numerous pits in the ventral surfaces of the maxillary
palatal shelves for the accommodation of the dentary teeth
than do tyrannosaurines (Currie, 2003). The occipital condyleis oriented ventrally in Albertosaurus and Gorgosaurus, but
not to the same degree as in the tyrannosaurines (Currie,2003). Finally, both genera have a single peneumatic foramen(pneumatopore) in the palatine, a morphological feature theyshare with Appalachiosaurus montgomeriensis and Bistahieversor
sealeyi, whereas the derived tyrannosaurines (e.g., Tyrannosaurusrex) have two pneumatic foramina (Carr et al., 2005; Carr andOSTEOLOGY OF GORGOSAURUS LIBRATUS DENTARY

Williamson, 2010).
The referral of the isolated right dentary, FPDM-V8062, toGorgosaurus libratus is supported by several morphologicalfeatures and also by its stratigraphic position. Gorgosaurus isfound exclusively in the Campanian deposits in the Dinosaur
Park Formation, whereas Albertosaurus is found in the lateCampanian and early Maastrichtian deposits of the Horseshoe
Canyon Formation (Currie, 2003, 2005; Eberth and Currie, 2010;
Bell and Currie, 2014). FPDM-V8062 can be differentiated fromthe dentaries of its closest relative, Albertosaurus sarcophagus,
based on the following characters. In A. sarcophagus, theanterior step of the lingual bar is diminutive, especially its dorsal
surface that parallels the alveolar row (Fig. 3). The posterodorsaltransition point of the anterior step of the lingual bar is notwell defined. As a result, the supradentary in A. sarcophagus
covers the posterior portion of the first alveolus/interdental
plate, whereas in G. libratus it does not reach the ﬁrst alveolus/
interdental plate. Furthermore, the foramen intermandibularis
oralis in A. sarcophagus is ventral to the anterior terminus of the
Meckelian groove, whereas in G. libratus the foramen overlaps the
Meckelian groove ventrally. In both taxa the anterior terminus of
the Meckelian groove contacts the posterior margin of the dentary
symphysis. However, A. sarcophagus lacks the characteristic
anterior fossa associated with the foramen intermandibularis
oralis, whereas in G. libratus the fossa contacts the posterior
margin of the dentary symphysis.
The lingual bar in Albertosaurus sarcophagus is dorsoventrally
wider than it is in Gorgosaurus libratus and not flared in theanterior region. Interestingly, the flaring of the anterior lingual
bar is absent in juvenile, subadult and adult individuals of A.
sarcophagus (Fig. 4). In G. libratus, the ﬂaring of the anterior endof the lingual bar is present in individuals of all ontogenetic stages(Fig. 4).

21

The posterior step in the dentaries of Albertosaurus
sarcophagus and Gorgosaurus libratus is situated ventral to the
Meckelian groove (Figs. 2 and 3). In both taxa, the posterior step
is diminutive. In A. sarcophagus, however, it is not as widely
open as in G. libratus. In contrast, the notch for the articulation
with the splenial is much better deﬁned in A. sarcophagus thanin G. libratus (Figs. 2 and 3). In A. sarcophagus the notch iselongate, narrow, and nearly parallels the Meckelian groove. Also,in A. sarcophagus a shallow depression-like structure is directly
ventral to this notch, whereas in G. libratus this depression isabsent (Figs. 2 and 3).
The facet for the articulation with the splenial in Gorgosaurus
libratus is extensive, whereas it is an elongated structure, andits dorsal margin parallels the ventral margin of the dentaryin Albertosaurus sarcophagus (Figs. 2 and 3). However, one
exception is the isolated dentary TMP 1999.55.170, assigned toG. libratus by Currie (2003), from the Dinosaur Park Formation(Fig. 5). In TMP 1999.55.170, the facet for the articulation withthe splenial closely resembles in overall morphology that of
G. libratus (Fig. 2). However, there are several morphologicalfeatures present in TMP 1999.55.170 that are different from
Gorgosaurus, such as relatively high tooth count (17), the anterior
step of the lingual bar covers two instead of one alveolus, andthe foramen intermandibularis is situated directly at the anterior
terminus of the Meckelian groove (Fig. 5), which is characteristicof Daspletosaurus. Therefore, it is very likely that the TMP
1999.55.170 dentary was misidentiﬁed and most likely belongs toa tyrannosaurine tyrannosaurid, probably Daspletosaurus, and not
Gorgosaurus.
Among tyrannosaurids, the alioramines exhibit dentarymorphology similar to that of the albertosaurines. The holotype ofAlioramus altai, a member of Alioramini, includes exceptionallywell-preserved dentaries. The dentaries of A. altai were described

FIGURE 3. Left dentary of Albertosaurus sarcophagus (TMP 1999.50.40). A, medial view; B, close up of the anterior end in medial view; C, close up of theposterior end in medial view. Abbreviations are shown in Appendix 1. Bar = 5 cm.

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SEBASTIAN G. DALMAN AND SPENCER G. LUCAS

previously by Brusatte et al. (2012). However, some of themorphological features here described were not mentioned
previously for this taxon (Fig. 6). The anterior step of the lingual
bar in A. altai closely resembles that of Gorgosaurus libratus,
but differs from that of Albertosaurus sarcophagus. The step isdiminutive and covers only the ﬁrst alveolus/interdental plate. As
in G. libratus, the anterior step of the lingual bar in A. altai has
the posterodorsal transitional point, and its dorsal surface parallels
the alveolar row. The dorsoventral width of the lingual bar in A.
altai seems uniform throughout its length. The anterior terminalend of the Meckelian groove directly contacts the foramenintermandibularis oralis, but does not contact the posterior
margin of the dentary symphysis. At the level of alveolus 11, theMeckelian groove begins to widen dorsoventrally and continues
to widen as it progresses posteriorly. In both A. sarcophagus
and G. libratus, the Meckelian groove is a narrow structure.
However, in isolated juvenile dentaries of A. sarcophagus (TMP
2007.007.0001) and of G. libratus (TMP 86.144.01; Fig. 7), theposterior portion of the Meckelian groove is slightly wider than
the anterior end, suggesting that the wider posterior end of the

Meckelian groove is a juvenile characteristic. Indeed, Brusatte
et al. (2012) referred the holotype specimen of A. altai (IGM
100/1844) to a juvenile.
The posterior step in the dentary of A. altai is well defined(Fig. 6). It lies directly on the long axis of the Meckelian groove.
The dorsal and ventral surfaces of the step are angled slightly. In
contrast, in Albertosaurus sarcophagus, the dorsal surface of thestep is angled, whereas the ventral surface nearly parallels theposterior end of the Meckelian groove. In Gorgosaurus libratus,
the ventral surface of the step is strongly angled. However, in an
isolated dentary of G. libratus (TMP 1983.36.134), the ventral
surface of the posterior step parallels the Meckelian groove (Fig.
8). This indicates that this feature is variable and may also change
during ontogeny.
The notch for the articulation with the splenial is well deﬁned
in Alioramus altai (Fig. 6). It is a short and narrow structure thatis aligned with the ventral margin of the Meckelian groove. In
contrast, in Albertosaurus sarcophagus and Gorgosaurus libratus,
the notch is diminutive and is ventral to the Meckelian groove.
The facet for the articulation with the splenial in A. altai closely

FIGURE 4. Dentaries of albertosaurinae tyrannosaurids in medial view. A, Albertosaurus sarcophagus (subadult) (TMP 1999.50.40); B, Albertosaurus
sarcophagus (juvenile) (TMP 2007.007.0001); C, Gorgosaurus libratus (adult) (TMP 1983.36.134) (right dentary reversed); D, Gorgosaurus libratus (juvenile)(TMP 1986.144.01) (right dentary reversed). Abbreviations are shown in Appendix 1. Bar = 5 cm.

FIGURE 5. Left dentary (TMP 1999.55.170) previously assigned to Gorgosaurus libratus. A, medial view; A’, close up of the anterior end in medial view.
Abbreviations are shown in Appendix 1. Bar = 5 cm.

OSTEOLOGY OF GORGOSAURUS LIBRATUS DENTARY23

FIGURE 6. Left dentary of Alioramus altai (IGM 100/1844). A, medial view; B, close up of the anterior end in medial view; C, close up of the posterior end
in medial view. Abbreviations are in Appendix 1. Bar = 5 cm.

FIGURE 8. Right dentary of Gorgosaurus libratus (TMP 1983.36.134).
A, medial view; A’, close up of the posterior area in medial view.
Abbreviations are in Appendix 1. Bar = 5 cm.

FIGURE 7. Dentaries of juvenile albertosaurinae tyrannosaurids. A,
Albertosaurus sarcophagus (TMP 2007.007.0001); B, Gorgosaurus
libratus (TMP 1986.144.01) (right dentary reversed). Abbreviations are in
Appendix 1. Bar = 5 cm.

resembles that of A. sarcophagus. It is elongated, and its dorsal
margin parallels the ventral margin of the dentary.
The various morphological features in the dentaries oftyrannosaurines differ from those of Albertosaurus andGorgosaurus and from Alioramus. They also differ among taxa.
The anterior step of the lingual bar in all known tyrannosaurines,
such as Bistahieversor, Daspletosaurus, Lythronax, Nanotyrannus,
Nanuqsaurus, Teratophoneus, Tarbosaurus, Tyrannosaurus andZhuchengtyrannus, has both transitional points: posterodorsal andventral (Dalman and Lucas, 2015). The anterior step of the lingual
bar in some taxa is diminutive and covers only the ﬁrst alveolus/
interdental plate (e.g., Bistahieversor and Nanotyrannus; Fig. 9),
whereas in other taxa the step is pronounced and covers the ﬁrst
and the second alveoli/interdental plates (e.g., Daspletosaurus,
Lythronax, Tarbosaurus, Tyrannosaurus, and Zhuchengtyrannus;
Fig. 10). Furthermore, among tyrannosaurines the posterior
surface (towards the mouth) of the anterior step of the lingual bar,
particularly in Tyrannosaurus rex, is vertical, giving it a nearlysquare-shaped appearance (Dalman and Lucas, 2015), whereas
in all other taxa the posterior surface of the step is angled.
The dentaries of derived tyrannosaurines such as Tarbosaurus
and Tyrannosaurus lack the notch for the articulation with thesplenial, but they possess the posterior step (Fig. 11), which ispresent in albertosaurines, alioramines, and in other less derived
tyrannosaurines.

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SEBASTIAN G. DALMAN AND SPENCER G. LUCAS

The position of the foramen intermandibularis oralis intyrannosaurines varies among taxa. For example, in some taxa
(e.g., Bistahieversor, Daspletosaurus, Lythronax, Teratophoneus,
and Tyrannosaurus) the foramen intermandibularis oralis isat the anterior terminus of the Meckelian groove, whereas in

Tarbosaurus and Zhuchengtyrannus it is ventral to the anterior
terminus of the Meckelian groove (Dalman and Lucas, 2015).
The shape of the interdental plates in tyrannosaurines also
varies among taxa. In Daspletosaurus and Lythronax, theinterdental plates are triangular in shape, whereas in Tarbosaurus,

FIGURE 9. Dentaries of tyrannosaurinae tyrannosaurids in medial view. A, Bistahieversor sealeyi (NMMNH P-27469); A’, close up of the anterior end of thedentary; B, Nanotyrannus lancensis (BMR P2002); B’, close up of the anterior end of the dentary. Abbreviations are in Appendix 1. Bar = 5 cm.

FIGURE 10. Dentaries of tyrannosaurinae tyrannosaurids in medial view. A, Daspletosaurus torosus (TMP 2001.36.1); A’, close up of the anterior end of thedentary; B, Lythronax argestes (UMNH VP 20200); B’, close up of the anterior end of the dentary; C, Tarbosaurus bataar (ZPAL MgD-I/5); C’, close up of
the anterior end of the dentary; D, Tyrannosaurus rex (BMNH R7994); D’, close up of the anterior end of the dentary; E, Zhuchengtyrannus magnus (ZCDM
V0031); E’, close up of the anterior end of the dentary. Abbreviations are in Appendix 1. Bar = 5 cm.

25

OSTEOLOGY OF GORGOSAURUS LIBRATUS DENTARYFIGURE 11. Dentaries of tyrannosaurinae tyrannosaurids in medial view. A, Tarbosaurus bataar (ZPAL MgD-I/1); A’, close up of the posterior end of thedentary; B, Tyrannosaurus rex (BHI 3033); B’, close up of the posterior end of the dentary. Abbreviations are in Appendix 1. Bar = 5 cm.

Tyrannosaurus, and Zhuchengtyrannus the interdental plates arelarge and rectangular (Figs. 9, 10). Furthermore, the anterior endof the lingual bar in tyrannosaurines is more strongly ﬂared than it
is in albertosaurines and especially in Gorgosaurus libratus.
CONCLUSIONS
The dentary of Gorgosaurus libratus is similar to that ofAlbertosaurus sarcophagus and in some respects to othertyrannosaurids, especially the alioramines. In this research,the dentary of G. libratus was described in detail, and its
comparison with its closest relative, A. sarcophagus, shows
several morphological features upon which these taxa aredifferentiated. Furthermore, the dentary of Alioramus altai shares
many morphological features with both A. sarcophagus and G.
libratus, suggesting a closer affinity with these taxa than withtyrannosaurines. This research supports the distinctiveness of G.
libratus.
ACKNOWLEDGMENTS
We thank Dr. Teppei Sonoda (Fukui Prefectural Dinosaur
Museum) for access to the specimen and the permission to study
it. We thank Daniel Brinkman (Yale Peabody Museum of Natural
History) and Kate Wellspring (formerly at Beneski Museum of
Natural History) for access to specimens. We are grateful to Steve
L. Brusatte, Philip J. Currie, David W. E. Hone, Jørn H. Hurum,
Steven E. Jasinski, Mark A. Loewen, and Lawrence M. Witmer
for sharing photographs of specimens and the permission to usethem in our study. The paper beneﬁted also from reviews by W.
Scott Persons IV (University of Alberta) and Takanobu Tsuihiji(The University of Tokyo).

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OSTEOLOGY OF GORGOSAURUS LIBRATUS DENTARYAPPENDIX 1
Anatomical Abbreviations
alb, anterior lingual bar; alvr, alveolar row of mental foramina;
antr, anterior row of mental foramina; ar, anterior rugosity;
aslb, anterior step of the lingual bar; atmg; anterior terminus of
the Meckelian groove; avf, anteroventral fossa associated withthe foramen intermandibularis oralis; d1, d15, d17, dentary
tooth position; daslb; dorsal surface of the anterior step of
the lingual bar; dpst, dorsal surface of the posterior step; ds,
dentary symphysis; dsplf, dorsal margin/surface of the splenial
facet; dvsplf, dorsoventral depth of the posterior portion of thesplenial facet; fimo, foramen intermandibularis oralis; fossa, fossa

27

associated with intermandibular vascular foramen; gdl, groove
for dental lamina; idp, interdental plate; ip, inﬂection point of thedentary; ivf, intermandibular vascular foramen; lb, lingual bar;mf, Meckelian fossa; mg, Meckelian groove; paslb, posterior
surface (towards the mouth) of the anterior step of the lingual
bar; pdtp, posterodorsal transition point of the anterior step of
the lingual bar; pst, posterior step; sdc, supradentary contact;splf, splenial facet; spln, splenial notch; surl, lateral prong forarticulation with surangular; surnt, articular notch between
lateral and medial prongs for articulation with surangular; talvrf,
posterior terminal alveolar mental foramen; tvntf, posterior
terminal mental foramen; vent, ventral row of mental foramina;
vtp, ventral transition point of the anterior step of the lingual bar;vpst, ventral surface of the posterior step.