Tyrannosaurid theropod specimens in the San Diego Natural History Museum from the Dinosaur Park Formation (Campanian) of Alberta, Canada

Abstract

This paper documents previously undescribed appendicular skeletal elements of tyrannosaurid dinosaurs, which were collected in 1917 by Charles H. Sternberg in the Dinosaur Park Formation of Alberta, Canada. In this study, four metatarsals and one pedal phalanx are described and referred to Gorgosaurus libratus, and one pedal phalanx is referred to Daspletosaurus torosus based on several diagnostic characters including the position of the collateral ligament pit, form of the distal articulation surface, and the thickness of the shaft. These specimens are significant because they represent one of the first discoveries of these tyrannosaurids in Alberta, Canada. Lastly, several significant differences between metatarsals of Gorgosaurus and Daspletosaurus, such as the form of the distalmost margin of the third metatarsal as well as the shape of the distal articulation surface of the fourth metatarsal, are recognized.

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Lucas, S. G., Hunt, A. P. & Lichtig, A. J., 2021, Fossil Record 7. New Mexico Museum of Natural History and Science Bulletin 82.
TYRANNOSAURID THEROPOD SPECIMENS IN THE SAN DIEGO NATURAL HISTORY
MUSEUM FROM THE DINOSAUR PARK FORMATION (CAMPANIAN)
OF ALBERTA, CANADA
CHAN-GYU YUN
Biological Sciences, Inha University, Incheon 22212, Republic of Korea, email: changyu1015@naver.com
Abstract—Abstract-This paper documents previously undescribed appendicular skeletal elements of
tyrannosaurid dinosaurs, which were collected in 1917 by Charles H. Sternberg in the Dinosaur Park
Formation of Alberta, Canada. In this study, four metatarsals and one pedal phalanx are described
and referred to Gorgosaurus libratus, and one pedal phalanx is referred to Daspletosaurus torosus
based on several diagnostic characters including the position of the collateral ligament pit, form of
the distal articulation surface, and the thickness of the shaft. These specimens are signicant because
they represent one of the rst discoveries of these tyrannosaurids in Alberta, Canada. Lastly, several
signicant dierences between metatarsals of Gorgosaurus and Daspletosaurus, such as the form of
the distalmost margin of the third metatarsal as well as the shape of the distal articulation surface of
the fourth metatarsal, are recognized.
INTRODUCTION
The Tyrannosauridae is a successful group of large-bodied
theropods that dominated terrestrial ecosystems in Asia and
North America for most of the Late Cretaceous. These theropods
are recognized by the presence of large skulls with robust teeth,
short forelimbs and elongate hind limbs (Russell, 1970; Currie,
2003; Holtz, 2004). Currently, two subfamily-level subclades
within Tyrannosauridae are recognized: Albertosaurinae, which
includes Albertosaurus and Gorgosaurus, and Tyrannosaurinae,
which includes multiple taxa such as Daspletosaurus, Lythronax,
Tarbosaurus, Teratophoneus, and Tyrannosaurus (Carr et
al., 2017). Recent studies have suggested that two tribe-level
subclades are present within Tyrannosaurinae, Alioramini, which
includes Alioramus and Qianzhousaurus, and Daspletosaurini,
which consists of “Thanatotheristes” and Daspletosaurus (Lü
et al., 2014; Voris et al., 2020). Among these, it is known that
Daspletosaurus torosus and Gorgosaurus libratus coexisted
within the same paleofauna in the Dinosaur Park Formation
(e.g., Russell, 1970; Dalman and Lucas, 2015; Carr et al., 2017;
Yun, 2020a, b).
The Dinosaur Park Formation in southern Alberta, Canada,
preserves one of the best skeletal fossil records of Late Cretaceous
dinosaurs. Paleontological collecting and study of dinosaur
fossils from the Dinosaur Park Formation began in the late
1800s and continues today (Russell, 1970; Currie, 2005; Tanke,
2005). Charles H. Sternberg and his three sons were among the
first fossil prospectors who collected dinosaur skeletal remains
in the Dinosaur Park Formation (Currie, 2005). Today, many of
the dinosaur specimens that Sternberg and his sons collected are
in the collections of several renowned museums (Currie, 2005).
Among the well-preserved and complete dinosaur specimens
that Sternberg and his sons collected were also fragmentary
specimens, which consisted of isolated bones. These fragmentary
bones have been overlooked despite their historical significance.
The collections of the San Diego Natural History Museum in
San Diego, California, houses several fragmentary dinosaur
fossils collected by Sternberg and his sons in 1917. This paper
documents some previously undescribed, isolated appendicular
skeletal elements pertaining to tyrannosaurid dinosaurs from
the Dinosaur Park Formation that were collected by Charles H.
Sternberg, which are now currently deposited in the San Diego
Natural History Museum. The discovery of these specimens has
historical and paleontological significance, as they represent one
of the earliest discoveries of tyrannosaurid theropods from the
Dinosaur Provincial Park.
Institutional Abbreviations: AMNH, American Museum
of Natural History; CMN, Canadian Museum of Nature;
SDNHM, San Diego Natural History Museum; TMP, Royal
Tyrrell Museum of Palaeontology; UALVP, University of
Alberta Laboratory for Vertebrate Paleontology.
MATERIALS AND METHODS
Due to the lack of funding at the beginning of this project,
this study was conducted using high-quality casts of the original
specimens housed at the San Diego Natural History Museum,
which were made through 3D scans. Comparisons were made
through extensive literature review and direct observations of
various casts, 3D scanned images, and high-resolution images.
GEOLOGICAL SETTING
The specimens described here were recovered from the
deposits of the Dinosaur Park Formation that are exposed along
the areas that are farther south than Red Deer River in Dinosaur
Provincial Park, southern Alberta, like other dinosaur specimens
in the San Diego Natural History Museum (e.g., Yun, 2020a,
b). The tyrannosaurid specimens described here were collected
in the Dinosaur Park Formation in southern Alberta, Canada.
The exact locality of the specimens is unknown; however, the
preservation of the specimens suggests that the specimens are
indeed from the Dinosaur Park Formation and not from the
stratigraphically older Oldman Formation. The gray color of
these specimens indicates that these came from the lower or
middle part of the formation, as the rocks of the lower half of
the Dinosaur Park Formation are more distinctly grayer than
the upper parts (Eberth, 2005). The Dinosaur Park Formation
is approximately 75 m thick, and its sediments were deposited
during the Campanian stage between 76.9 and 75.8 Ma (Fowler,
2017). The sediments that constitute the formation are interpreted
to have been derived from the erosion of western mountains, and
those sediments were deposited during the initial stages of the
last major transgression of the Western Interior Seaway (Eberth,
2005).

570
SYSTEMATIC PALEONTOLOGY
THEROPODA Marsh, 1881
COELUROSAURIA Huene, 1914
TYRANNOSAURIDAE Osborn, 1906
ALBERTOSAURINAE Currie et al., 2003
Gorgosaurus Lambe, 1914
Gorgosaurus libratus Lambe, 1914
Figs. 1-5
Material: Distal metatarsals (SDNHM 35451) (right II),
(SDNHM 35452) (left II), (SDNHM 35454) (right III) and
(SDNHM 35455) (right IV), and an isolated pedal phalanx
(SDNHM 35453) (II-1)
Locality and age: Dinosaur Provincial Park, southern
Alberta, Canada. Upper Cretaceous (Campanian) Dinosaur Park
Formation.
COMPARATIVE DESCRIPTION
SDNHM 35451 (Fig. 1): Specimen SDNHM 35451
corresponds to the distal end of the right metatarsal two (Fig. 1).
The proximodistal length of the lateral surface of the metatarsal
is 120 mm; the mediolateral width is 58 mm, and the height of
the preserved metatarsal is 61 mm. The circumference of the
preserved shaft is 173 mm. The ratio between dorsoventral length
and transverse width of the shaft is 1.22, which matches the ratio
in the specimen of Gorgosaurus libratus (TMP 1994.12.602).
The lateral surface of the preserved shaft bears the distal part
of a large, at, teardrop-shaped buttressing surface for the
articulation with metatarsal III, indicating the arctometatarsalian
condition that is present in derived tyrannosauroids (e.g.,
Holtz, 2004; Carr et al., 2017; Fig. 1A). The ventral heel that
comprises the ventral portion of the articulation surface is
moderately developed and rough. Distal to this heel, there is
a low rugosity proximal to the distal hemicondyles. In dorsal
view, the distal end strongly diverges medially (Fig. 1C), as
in derived tyrannosauroids with arctometatarsalian feet (e.g.,
Holtz, 2004; Brusatte and Carr, 2016; Carr et al., 2017). A large
and deep hyperextensor pit is present directly proximal to the
distal surface; also, a prominent tuberosity proximal to this
pit and medial to the articulation surface for metatarsal III is
present (Fig. 1C), which is typical of tyrannosaurids (Brochu,
2003). The distal end is condylar and nearly square-shaped, as in
most tyrannosaurids (Brochu, 2003; Holtz, 2004; Fig. 1F). The
lateral hemicondyle is mediolaterally broader, and more distally
extended than the medial hemicondyle. A deep and sharp sulcus
separates these condyles ventrally. The collateral ligament pit
FIGURE 1. Gorgosaurus libratus right metatarsal II (SDNHM 35451). A, lateral view; B, medial view; C, dorsal view; D, ventral
view; E, proximal view; F, distal view. Abbreviations: hep, hyperextensor pit; lc, lateral hemicondyle; lcp, lateral collateral ligament
pit; lvh, lateroventral heel; mc, medial hemicondyle; mcd, medial collateral ligament depression; MTIIIar, articulation for metatarsal
III; tb, tuberosity.

571
of the lateral hemicondyle is deep, ovoid-shaped and largely
separated from the ventral margin of the bone (Fig. 1A). In
contrast, the medial collateral ligament pit forms a large, subtle
circular depression (Fig. 1B).
The combination of a lateral collateral ligament pit that
is largely separated from the caudal margin of the bone, the
presence of a low rugosity on its caudolateral edge, and a distal
articulation surface that is closely spaced with the lateral margin
of the shaft in dorsal view, suggests that SDNHM 35451 is
assignable to Gorgosaurus libratus (Carr, 2005; Yun, 2020a;
Fig. 1A, C).
SDNHM 35452 (Fig. 2): This specimen corresponds to
the distal part of a left metatarsal two from a tyrannosaurid
theropod. The specimen has a length of 130 mm at lateral surface,
measures 58 mm dorsoventrally and 56 mm mediolaterally
at the distal articulation surface, and the circumference of the
preserved shaft is 168 mm. The specimen is similar to SDNHM
35451 in overall morphology and size, except for the lack of
a mediolateral pinching on the distal articulation surface and
a smaller, bean-shaped morphology of the medial collateral
ligament depression (Fig. 2A, F). The overall shape of the
specimen is consistent with that of a tyrannosaurid metatarsal
two, and the specimen can be assigned to Gorgosaurus libratus
for the same reasons as SDNHM 35451 because it presents
almost the same characteristics as the specimen, except for
lacking the low rugosity on the caudolateral margin (although
there are other individuals of G. libratus that lack this feature:
Carr, 2005).
SDNHM 35453 (Fig. 3): The specimen is identied as the
distal part of right pedal phalanx II-1 of a large tyrannosaurid
dinosaur. The preserved length of the phalanx is 92 mm at the
lateral surface, and the circumference of the shaft is 140 mm.
The specimen compares well with pedal phalanx II-1 of other
tyrannosaurid specimens in having a large size, an elongated,
FIGURE 2. Gorgosaurus libratus left metatarsal II (SDNHM 35452). A, medial view; B, lateral view; C, dorsal view; D, ventral
view; E, proximal view; F, distal view. Abbreviations: hep, hyperextensor pit; lc, lateral hemicondyle; lcp, lateral collateral ligament
pit; lvh, lateroventral heel; mc, medial hemicondyle; mcd, medial collateral ligament depression; MTIIIar, articulation for metatarsal
III; tb, tuberosity.

572
constricted and subcylindrical shaft that is wider than tall, an
asymmetrical and ginglymoid distal articulation surface, merged
condyles and very deep, ovoid collateral ligament pits (Brochu,
2003; Holtz, 2004; Brownstein, 2018). Although the proximal
part of the bone is missing, the morphology of the preserved
shaft is slender, indicating that the ratio between the total length
of the bone and the width of the shaft was greater than 3.0. This
characteristic argues against it being a derived tyrannosaurine
such as Daspletosaurus and Tyrannosaurus (Brusatte and Carr,
2016; Carr et al., 2017). The hyperextensor pit on the dorsal
surface of SDNHM 35453 is deep and has an oval-shaped.
The distal articulation surface is expanded and trapezoidal, as
in other tyrannosaurids (Brochu, 2003). The medial condyle is
deeper and wider than the lateral condyle.
The slender morphology of the shaft suggests that SDNHM
35453 belongs to a specimen of Gorgosaurus libratus, and not
to the contemporaneous Daspletosaurus torosus (Brusatte and
Carr, 2016; Carr et al., 2017).
SDNHM 35454 (Fig. 4): This isolated distal metatarsal
is referred to as right metatarsal III of tyrannosaurid theropod.
The specimen measures 70 mm at the medial surface, 55 mm
dorsoventrally and 60 mm mediolaterally at the distal surface.
The circumference of the shaft is 130 mm. The distal articulation
surface is nearly rectangular (Fig. 4F), which is characteristic of
the metatarsal III of tyrannosaurids. The medial portion of the
surface is wider than the lateral portion. The collateral ligament
pits are large and circular, although the medial pit is slightly larger
(Fig. 4A, B). In ventral view, a raised subtriangular platform
proximal to the distal condyles is present (Fig. 4D), a feature
that is diagnostic of derived tyrannosauroids (Brusatte and Carr,
2016; Carr et al., 2017). The dorsal surface of the shaft bears a
deepened hyperextensor pit. The distalmost margin of the distal
surface is indented in dorsal view (Fig. 4C) as in Gorgosaurus
libratus (Carr, 2005). In Daspletosaurus torosus (CMN 350),
the margin is straight, similar to subadult Tyrannosaurus (Carr,
2005). Thus, despite its fragmentary nature, the specimen can be
identied as G. libratus.
SDNHM 35455 (Fig. 5): This specimen is a well-preserved
distal end of the right metatarsal IV of Gorgosaurus libratus.
The proximodistal length of the preserved metatarsal fragment is
83 mm at the medial surface. The distal surface measures 50 mm
dorsoventrally and 45 mm mediolaterally, a ratio of 1.11. The
circumference of the shaft is 145 mm. Several synapomorphies
of derived tyrannosauroids can be observed in this specimen,
including the “D”-shaped cross section of the shaft, the laterally
diverging distal end, a lateral groove that delineates the
dorsolateral margin of the lateral hemicondyle, and the absence
of an isolated collateral ligament pit (Zanno et al., 2018).
On the medial surface, a moderately developed collateral
ligament pit is present. Proximally, a broken shaft bears
D-shaped cross section, as well as the hollow core of the bone
typical of theropods. In ventral view, both distal hemicondyles
are on the same height, although the medial one is broader. The
distal articulation surface is nearly square in morphology, as the
ratio between length and width of the surface is less than 1.2,
as in some tyrannosaurids such as Albertosaurus, Gorgosaurus,
Tarbosaurus and Tyrannosaurus (Carr et al., 2017). The lateral
hemicondyle is narrow, and strongly deected laterally, whereas
the medial hemicondyle is wide and points ventrally. A sulcus
separating the two hemicondyles is sharp and deep.
The combination of a square distal surface, a laterally
deected lateral hemicondyle and a dorsoventrally deep sulcus
between the hemicondyles suggests that SDNHM 35455
corresponds to Gorgosaurus libratus (Carr et al., 2017). In
Daspletosaurus torosus (UALVP 11, TMP 2001.36.1), the cleft
between the distal condyles extends onto the distal surface, the
lateral hemicondyle is slightly curved laterally, and the sulcus
between the two hemicondyles is subtle.
FIGURE 3. Gorgosaurus libratus right pedal phalanx II-1 (SDNHM 35453). A, dorsal view; B, medial view; C, lateral view; D,
ventral view; E, distal view; F, proximal view. Abbreviations: hep, hyperextensor pit; lc, lateral hemicondyle; lcp, lateral collateral
ligament pit; mc, medial hemicondyle; mcp, medial collateral ligament pit.

573
FIGURE 4. Gorgosaurus libratus right metatarsal III (SDNHM 35454). A, lateral view; B, medial view; C, dorsal view; D, ventral
view; E, proximal view; F, distal view. Abbreviations: hep, hyperextensor pit; lc, lateral hemicondyle; lcp, lateral collateral ligament
pit; mc, medial hemicondyle; mcp, medial collateral ligament pit; stp, subtriangular platform.

574
FIGURE 5. Gorgosaurus libratus right metatarsal IV (SDNHM 35455). A, lateral view; B, medial view; C, dorsal view; D, ventral
view; E, proximal view; F, distal view. Abbreviations: lc, lateral hemicondyle; mc, medial hemicondyle; mcp, medial collateral
ligament pit.

575
THEROPODA Marsh, 1881
COELUROSAURIA Huene, 1914
TYRANNOSAURIDAE Osborn, 1906
TYRANNOSAURINAE Osborn, 1906
DASPLETOSAURINI Voris et al., 2020
Daspletosaurus Russell, 1970
Daspletosaurus torosus Russell, 1970
Fig. 6
Material: An isolated pedal phalanx (SDNHM 35457) (IV-
1).
Locality and age: Dinosaur Provincial Park, southern
Alberta, Canada. Upper Cretaceous (Campanian) Dinosaur Park
Formation.
COMPARATIVE DESCRIPTION
SDNHM 35457 (Fig. 6): The isolated pedal phalanx
is not well-preserved, as it lacks its proximal portion and the
medioventral section of the distal end. The maximum length of
the specimen is 68 mm at the lateral surface, and the width of the
shaft is 47 mm. The preserved circumference of the shaft is 125
mm; if complete it would probably be 160 mm. The preserved
distal surface is trapezoidal, concave but not ginglymoid, and
the medial condyle is taller. In dorsal view, the proximomedial
margin of the distal surface strongly extends proximally and
close to the medial margin of the bone. A deep hyperextensor
pit is proximal to the distal surface of the bone. A deep, bean-
shaped, collateral ligament pit is laterally present; whereas
only the deepest portion of the medial collateral ligament
pit is preserved, due to the breakage of the specimen. These
morphological features suggest that this is the rst phalanx from
digit IV of the left pes.
Despite its fragmentary nature, the preserved shaft is quite
robust and wide, indicating the ratio of the phalanx length to
midshaft width is less than 3.0, as in derived tyrannosaurines
such as Daspletosaurus, Tarbosaurus and Tyrannosaurus
(Brusatte and Carr, 2016; Carr et al., 2017). Based on this
morphological character, this specimen can be referred to
Daspletosaurus torosus.
DISCUSSION
Historical Signicance and Implications
Although largely fragmentary, these specimens bear
historical signicances in representing previously undocumented
discoveries made by Charles H. Sternberg in 1917, which
are poorly known (Currie, 2005; Tanke, 2005). In particular,
SDNHM 35457 likely represents one of the rst non-dental
materials of Daspletosaurus torosus ever collected, since it
predates the discovery of the holotype in 1921 by about four
years (Russell, 1970). The discovery of this specimen in the
San Diego Natural History Museum’s collections conrms that
skeletal remains of Daspletosaurus torosus had been collected
more than ve decades before the taxon’s 1970 description,
thereby raising the possibility that other underscribed remains
of this taxon collected in the early 20th century may be present
in various institutions or universities.
It is possible that such specimens were collected under
the presupposition of being material of the more common
tyrannosaurid theropod Gorgosaurus libratus (Russell, 1970),
or of being fragmentary, and thus only designated as Theropoda
indet., without any further taxonomic resolution. Indeed, three
of the specimens (SDNHM 35453, 35455, 35457) have been
classied as Theropoda indet. in the museum records, before
this study was conducted. The present case exposes the risks
of disregarding fragmentary material despite their potential
to be correctly identied. Furthermore, while these isolated
bones provide little new information on the anatomy of the
tyrannosaurid species to which they pertain, they are nevertheless
important in being a sample of the exceptional variability of the
dinosaur fauna of the Dinosaur Park Formation (Maganuco,
2004), increasing the value of the dinosaur collection of the
San Diego Natural History Museum. Such materials are also
important in identifying or relocating dinosaur fossils collected
in the Dinosaur Provincial Park in the past centuries, contributing
minimally but certainly not negligible (Maganuco, 2004).
The Gorgosaurus metatarsals described in this study likely
pertain to juvenile individuals, as they are substantially smaller
than the adult examples of this taxon (Lambe, 1914). Although
many examples of juveniles belong to this taxon have been
recognized, their postcranial ontogenetic dimorphism has been
poorly described in the literature (Holtz, 2004; Carr, 2005). Yun
(2020a) found that a very small juvenile Gorgosaurus libratus
metatarsal two (SDNHM 35456), which was also collected
by Charles H. Sternberg in 1917, exhibits a morphology that
is quite similar to large examples of this taxon, suggesting that
at least the postcranial anatomy of this taxon could be more
conservative throughout the ontogeny when compared to the
cranial. The specimens described in this paper further support
this hypothesis, demonstrating that such old, fragmentary
material could be more informative than rampant assumptions
and should not be ignored.
Dierences Between Gorgosaurus and Daspletosaurus
Metatarsals
In this study, four distal ends of metatarsals (SDNHM
35451, 35452, 35454, 35455) and one pedal phalanx II-1
(SDNHM 35453) are referred to Gorgosaurus libratus; whereas,
an isolated pedal phalanx IV-1 (SDNHM 35457) is assigned to
Daspletosaurus torosus, based on distinct diagnostic characters
that are present in previously described specimens of these taxa
(e.g., Carr, 2005; Brusatte and Carr, 2016; Carr et al., 2017; Yun,
2020a). Among these, the metatarsals of G. libratus described
in this study are similar in size and degree of preservation,
suggesting that they could belong to the same individual.
Daspletosaurus torosus and Gorgosaurus libratus represent
the only two contemporaneous tyrannosaurids in the paleofauna
of the Dinosaur Park Formation (Russell, 1970). Although these
two tyrannosaurids can be easily distinguished by the numerous
dierences in cranial anatomy, the postcranial anatomy of these
taxa is fairly uniform, and, therefore, correctly identifying
isolated tyrannosaurid postcranial elements from the Dinosaur
Park Formation can be dicult (Holtz, 2004; Carr, 2005).
However, during this study, previously unrecognized dierences
were found in the metatarsals of these taxa. For example, the
distalmost margin of metatarsal III in G. libratus, including
SDNHM 35454, is indented when observed in dorsal view
(Carr, 2005); while in D. torosus (CMN 350), the margin is
straight. As evidenced by SDNHM 35455 and other examples
of Gorgosaurus metatarsal IV (e.g., TMP 1994.12.602), the
lateral hemicondyle is strongly deected laterally, and the
sulcus separating two condyles is sharp. On the other hand, the
lateral condyle in D.torosus (UALVP 11, TMP 2001.36.1) is
very weakly deected, and the sulcus is only subtly developed.
In addition, the distal articular surface of metatarsal four of D.
torosus (UALVP 11, TMP 2001.36.1) is square-shaped (the
ratio between length and width of the surface is less than 1.2),
unlike its sister taxon D. horneri (Carr et al., 2017). Lastly, in
D. torosus, the cleft that separates the two distal condyles of
the metatarsal four extends over the distal surface, as in the
Alectrosaurus holotype AMNH 5664 (Carr, 2005).
Using these criteria, isolated tyrannosaurid metatarsals from
assemblages of the Dinosaur Park Formation could be identied
at the genus and species level. Therefore, they can be used for
correctly identifying the fragmentary materials that have been
merely designated as Tyrannosauridae indet. In this way, using
the above information, tyrannosaurid postcranial anatomy

576
FIGURE 6. Daspletosaurus torosus left pedal phalanx IV-1 (SDNHM 35457). A, dorsal view; B, ventral view; C, lateral view; D,
medial view; E, distal view. Abbreviations: hep, hyperextensor pit; lc, lateral hemicondyle; lcp, lateral collateral ligament pit; mc,
medial hemicondyle; mcp, medial collateral ligament pit.

577
could be used for distinguishing between taxa and potentially
be taxonomically informative. It is expected that additional
phylogenetically informative variations of the postcranial
skeleton of tyrannosaurids will be recognized in future studies.
ACKNOWLEDGMENTS
The author thanks Kesler Randall for the permission to
study about the material described in this paper. Jin-Kyeom Kim
made the casts that were used for this research, and the author
is grateful for his help. The author thanks Thomas D. Carr for
valuable comments on an early version of this manuscript, which
greatly improved the quality of this submission. Special thanks
go to Philip Currie, Sebastian Dalman, Rafael Delcourt, Peter
Bishop and Jordan Mallon for sharing images of tyrannosaurid
specimens that were used for comparisons. This paper greatly
beneted by reviews from Claudia Serrano-Brañas and Sebastian
Dalman and the editing of Spencer G. Lucas.
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IV of Gorgosaurus libratus (SDNHM 35455). Not to scale.

578
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