Copyright © American Museum of Natural History 2011 ISSN 0003-0082
AMERICAN MUSEUM NOVITATES
Number 3717, 53 pp. May 20, 2011
e Anatomy of Dryptosaurus aquilunguis
(Dinosauria: eropoda) and a Review of
Its Tyrannosauroid Anities
STEPHEN L. BRUSATTE,1,2 ROGER B. J. BENSON,3
AND MARK A. NORELL2,4
Although among the rst theropod dinosaurs known to science, and an iconic taxon in the
history of dinosaur paleontology, the large carnivore Dryptosaurus aquilunguis from the Late
Cretaceous of New Jersey remains poorly understood. Its anatomy has been described only in
brief and its phylogenetic relationships have long been the subject of debate, although recent
work proposes Dryptosaurus as a member of the tyrannosauroid clade. Here we present a thor-
ough osteological description of the holotype of Dryptosaurus aquilunguis, supplemented with
photographs of all the material, and provide extensive comparisons with other theropods, espe-
cially tyrannosauroids. In concert with recent phylogenetic analyses, our description conrms
the tyrannosauroid anities of Dryptosaurus and supports its placement as an “intermediate”
taxon bracketed between small, basal forms (e.g., Guanlong, Dilong) and the derived, Late Cre-
taceous tyrannosaurids (e.g., Albertosaurus, Tyrannosaurus). We identify several autapomor-
phies of Dryptosaurus, including the combination of a reduced humerus and an enlarged hand.
ese forelimb proportions, which dier from the uniformly large arms of basal tyrannosau-
roids and uniformly atrophied arms of tyrannosaurids, suggest that forelimb reduction in tyran-
nosauroids may not have proceeded in a uniform fashion. Functionally, Dryptosaurus may have
used both its skull and arms as weapons for prey acquisition and processing.
1 Division of Paleontology, American Museum of Natural History.
2 Department of Earth and Environmental Sciences, Columbia University, New York, NY.
3 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.
4 Division of Paleontology, American Museum of Natural History.
2 AMERICAN MUSEUM NOVITATES NO. 3717
e Late Cretaceous theropod Dryptosaurus aquilunguis is an icon in the history of dino-
saur paleontology. When it was rst discovered in the Maastrichtian greensands of New Jersey
in 1866 little was known about carnivorous dinosaurs, especially those from North America.
Up until this time, New World theropods were represented only by isolated teeth from Mon-
tana (Leidy, 1856), but the discovery of Dryptosaurus oered paleontologists, and the general
public alike, their rst glimpse of an articulated theropod skeleton. Cope (1866) published on
the specimen within a week of its discovery, christening it Laelaps aquilunguis at a meeting of
the Academy of Natural Sciences in Philadelphia. Laelaps quickly gained popularity as both a
poetic, evocative name (Greek for “storm wind” or “hurricane”) and an attractive wastebasket
taxon for the referral of isolated theropod elements from across North America. Unfortunately,
the generic name was preoccupied by a mite, and was replaced with Dryptosaurus in 1877 by
Cope’s bitter rival, O.C. Marsh. is story long ago entered the lore of paleontological history,
and has been recounted countless times in popular works and scientic papers (see review in
Carpenter et al., 1997).
Despite the historical importance of Dryptosaurus, this taxon has remained mysterious and
confusing since its initial description by Cope (1866). Early workers noted obvious similarities
between Dryptosaurus and the handful of other large theropods known at the time, including
Megalosaurus (e.g., Cope, 1866; Leidy, 1868; Lydekker, 1888). As more theropod fossils became
known, some scientists identied features linking Dryptosaurus and Late Cretaceous tyranno-
saurids, such as Albertosaurus and Tyrannosaurus (e.g., Gilmore, 1946; Baird and Horner, 1979).
ese observations were heavily challenged, however, and until recently many authors retained
Dryptosaurus in its own monotypic family, arguing that there was no clear evidence linking it
to any other theropod clade (e.g., Russell, 1970; Molnar, 1990; Carpenter et al., 1997). e dis-
covery of basal tyrannosauroids similar in body size and morphology to Dryptosaurus, most
importantly Appalachiosaurus, has recently provided strong evidence for tyrannosauroid anity
(e.g., Holtz, 2004; Carr et al., 2005; Brusatte et al., 2009, 2010; Carr and Williamson, 2010).
However, although it has become clear that Dryptosaurus falls within the tyrannosauroid
clade, its anatomy and systematic position have received cursory attention. Although several
authors have presented short papers on the anatomy of Dryptosaurus (e.g., Cope, 1866, 1867,
1868a, 1868b; Huene, 1932; Carpenter et al., 1997), a comprehensive osteology of this taxon
and photographs of the holotype have yet to be published. Furthermore, previous authors have
listed only a handful of anatomical features supporting tyrannosauroid anity, and Dryptosau-
rus has yet to be described in a comparative framework with other tyrannosauroids. is is
becoming increasingly necessary, as new basal tyrannosauroids are being discovered at an
increasing pace and the phylogeny of the tyrannosauroid clade is being eshed out in great
detail (see Brusatte et al., 2010). Indeed, the most recent description of the holotype, that of
Carpenter et al. (1997), presented only a brief description of each bone, made only broad com-
parisons with other theropods, and was unable to determine the phylogenetic placement of
Dryptosaurus. Even a comprehensive diagnosis of Dryptosaurus was not possible at the time,
since it was unclear exactly which theropods should serve as a basis for comparison.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 3
Adding to the interest in Dryptosaurus is that it was the rst large theropod, to our knowledge,
to be reconstructed in a modern light. In a watercolor titled “Fighting Laelaps” and dated 1896,
the preeminient natural history artist Charles R. Knight depicted two Dryptosaurus, one pouncing
on the other. Prepared under the direction of E.D. Cope, this painting is unusual, and prescient,
in that it portrays these animals, giant as they were, as highly active and surely reects Cope,
Knight, and Osborn’s ideas about the agility of carnivorous dinosaurs. is is in contrast to other
contemporary illustrations of large carnivorous dinosaurs (predominantly Megalosaurus), which
depict large theropods as tail-dragging behemoths. Both the original painting (g. 1A) as well as
the plaster models Knight used as reference (g. 1B) are preserved in AMNH collections.
We present a thorough osteology of the holotype specimen of Dryptosaurus (ANSP 9995;
AMNH 2438), with photographs of every bone and detailed comparisons with other tyranno-
sauroids. Our focus is on the holotype only, and we do not address the profuse material that
has been referred to Dryptosaurus over the years, most of which cannot be condently assigned
to the taxon. We provide a diagnosis of the taxon, based on several autapomorphies, and a
focused discussion of the various features that support tyrannosauroid anities, using the
recent cladistic analysis of Brusatte et al. (2010) as a guide.
Redescription of Dryptosaurus is timely for two reasons. First, the recent discovery of sev-
eral basal tyrannosauroids allows for a more rened understanding of the anatomy and phylo-
genetic position of Dryptosaurus (e.g., Hutt et al., 2001; Xu et al., 2004, 2006; Carr et al., 2005;
Benson, 2008; Brusatte et al., 2009; Ji et al., 2009; Averianov et al., 2010; Carr and Williamson,
2010; Li et al., 2010; Rauhut et al., 2010). Second, the holotype material is in poor shape, as it
has suered many generations of breaks and repairs and has been seriously degraded by pyrite
disease (Spamer et al., 1995). It is critical to publish a full description and photographs before
further deterioration occurs.
e following acronyms are used throughout this work:
AMNH FARB American Museum of Natural History, New York
ANSP Academy of Natural Sciences, Philadelphia, PA
FDRC Fossil Research and Development Center, Gansu Bureau of Geology and Mineral
Resources Exploration, Lanzhou, People’s Republic of China
IGM Institute of Geology, Ulaan Baatar, Mongolia
IVPP Institute of Vertebrate Paleontology and Paleoanthropology, Beijing,
People’s Republic of China
LH Long Hao Institute of Geology and Paleontology, Hohhot, People’s Republic
MIWG Museum of Isle of Wight Geology (Dinosaur Isle, Isle of Wight Museum Services,
IWCMS), Sandown, England, UK
NHM Natural History Museum, London, England, UK
OUMNH Oxford University Museum of Natural History, Oxford, England, UK
ZPAL Instytut Paleobiologii PAN, Warsaw, Poland
4 AMERICAN MUSEUM NOVITATES NO. 3717
FIG. 1. e painting “Fighting Laelaps” by Charles R. Knight (1896), depicting one Drypto-
saurus individual leaping onto another (A). A plaster model of Dryptosaurus that Knight used
as a reference while painting his watercolor (B). Both the painting and the model are cata-
loged in the Division of Paleontology, American Museum of Natural History.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 5
eropoda Marsh, 1881
Tetanurae Gauthier, 1986
Coelurosauria Huene, 1914
Tyrannosauroidea Osborn, 1905
Dryptosaurus Marsh, 1877
Dryptosaurus aquilunguis Cope, 1866
H: ANSP 9995. Fragmentary skeleton found associated and belonging to a single
individual, including a fragment of the right maxilla, a fragment of the right dentary, a frag-
ment of the right surangular, lateral teeth, 11 middle-distal caudal vertebrae, le and right
humeri, three manual phalanges from the le hand (I-1, II-2, and an ungual), shas of the le
and right pubes, a fragment of the right ischium, le femur, le tibia, le bula, le astragalus,
midsha fragment of metatarsal III. e neurocentral sutures are closed in all caudal vertebrae,
suggesting that the holotype individual was mature or nearing maturity.
A M: AMNH FARB 2438. Le metatarsal IV, likely from the same indi-
vidual as the holotype (see Carpenter et al., 1997).
C M: Well-preserved, historic casts of most of the holotype, including AMNH
FARB 2438, are present in the collections of the Natural History Museum in London (NHM
OR50100). In some cases the casts show detail that is no longer preserved on the original
T L: West Jersey Marl Company Pit, near Barnsboro, Gloucester County, New
Jersey. e Dryptosaurus holotype was discovered in 1866 in a unit comprised of marl and sand-
stone, which was dened nearly a century later as the New Egypt Formation, regarded as Maas-
trichtian in age (Olsson, 1960). e New Egypt Formation is a marine unit, comprised of claystone,
sandstone, and marls, with frequent glauconitic and sideritic nodules (Olsson, 1963). It is up to
10.5 m thick in its type area, along Crosswicks Creek, but thins to the south, such that it is con-
siderably thinner at the Dryptosaurus holotype locality (Landman et al., 2004). e New Egypt
Formation conformably overlies the Navesink Formation, from which potential Dryptosaurus
referred material has been reported, and is considered to be a deeper-water equivalent of the
Tinton and Red Bank formations (Olsson, 1987). See Gallagher (1993) and Landman et al. (2004)
for a more detailed description of the stratigraphy of Late Cretaceous units in New Jersey.
D: A tyrannosauroid theropod possessing the following autapomorphies: combi-
nation of a reduced humerus (humerus: femur ratio = 0.375) and a large hand (phalanx I-1:
femur ratio = 0.200) (see text for numerical comparisons to other tyrannosauroids); strong
mediolateral expansion of the ischial tubercle, which is approximately 1.7 times as wide as the
sha immediately distally; an ovoid fossa on the medial surface of the femoral sha immedi-
ately proximal to the medial condyle, demarcated anteriorly by the mesiodistal crest and medi-
ally by a novel crest; proximomedially trending ridge on the anterior surface of the bula
immediately proximal to the iliobularis tubercle; lip on lateral surface of lateral condyle of
astragalus prominent and overlapping the proximal surface of the calcaneum; metatarsal IV
6 AMERICAN MUSEUM NOVITATES NO. 3717
with a at sha proximally, resulting in a semiovoid cross section that is much wider medio-
laterally than long anteroposteriorly.
N N: In this paper we employ the clade names Tyrannosauroidea and
Tyrannosauridae following the denitions of Sereno et al. (2005). Tyrannosauroidea is the most
inclusive clade that contains Tyrannosaurus rex but not other coelurosaurs (Ornithomimus
edmontonicus, Troodon formosus, Velociraptor mongoliensis). Tyrannosauridae is a less inclusive
clade, dened as the least inclusive clade containing Tyrannosaurus rex and Gorgosaurus libratus.
erefore, Tyrannosauroidea is a larger group that includes Tyrannosauridae, as well as an array
of taxa on the stem toward Tyrannosauridae, including Guanlong, Proceratosaurus, Sinotyran-
nus, Dilong, Eotyrannus, Stokesosaurus, Xiongguanlong, Raptorex, Bistahieversor, Appalachio-
saurus, Dryptosaurus (following the phylogeny of Brusatte et al., 2009, 2010). Tyrannosauridae,
on the other hand, is a less inclusive group of derived tyrannosauroids that includes Alberto-
saurus, Alioramus, Daspletosaurus, Gorgosaurus, Tarbosaurus, and Tyrannosaurus.
One further matter deserves comment. Dryptosaurus is included with Tyrannosauroidea,
and is also the type genus for a family-level taxon, Dryptosauridae (Marsh, 1890). Dryptosau-
ridae, in fact, was named 15 years prior to Osborn’s (1905) establishment of Tyrannosauridae,
and therefore has priority at all family levels by the International Code of Zoological Nomen-
clature Principle of Coordination, which deems all family group taxa (superfamilies, families,
subfamilies, etc.) to be established upon the rst use of any family group name. us, it may
be expected that Tyrannosauroidea should be renamed using Dryptosaurus as an eponym.
However, Article 35.5 of the Code allows for names in prevailing usage at higher rank, such as
Tyrannosauroidea, to be retained in cases such as these, because a superfamily name based on
Dryptosaurus has never (to our knowledge) been used in the literature, whereas Tyrannosau-
roidea is commonly used and has been for many years. A similar nomenclatural situation was
recently confronted by Kammerer and Angielczyk (2009), who also invoked Article 35.5 to
retain a commonly used superfamily name despite an older name being available under the
Principle of Coordination.
M: Only the anteroventral corner of the right maxilla is known (g. 2), and this
fragment is poorly preserved and has clearly been damaged since the original descriptions of
Cope (1866, 1869). e fragment is 89 mm long anteroposteriorly by 77 mm deep dorsoven-
trally, and preserves most of the rst four alveoli. It is broken ventral to the antorbital fossa,
and therefore no details of the antorbital region are apparent.
e lateral surface is much better preserved than the medial surface and exhibits a unique
feature (g. 2A): a conspicuous row of neurovascular foramina situated approximately 35 mm
above the alveolar margin, which is preserved as original bone surface at the anteroposterior
midpoint of the fragment. Because the entire alveolar margin is not preserved, it is not possible
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 7
FIG. 2. Right maxilla of the holotype (original material and casts) of Dryptosaurus aquilunguis (ANSP
9995) in lateral (A), anterior (B), medial (C, D), and ventral (E, F) views. Bones A, C, and E are original
material, and B, D, and F are casts. Abbreviations: amp, anteromedial process; idp, interdental plates; pnr,
primary neurovascular foramina row. Scale bar = 50 mm.
8 AMERICAN MUSEUM NOVITATES NO. 3717
to determine whether this neurovascular row is the primary alveolar row or a secondary set of
foramina occupying the midpoint of the lateral surface of the maxilla. e primary row is
positioned far ventrally, usually immediately above the alveolar margin, in other tyrannosau-
roids, including Albertosaurus (Currie, 2003a: g. 6), Alioramus (Brusatte et al., 2009: g. 1),
Appalachiosaurus (Carr et al., 2005: g. 5), Eotyrannus (Hutt et al., 2001: g. 3), Gorgosaurus
(Currie, 2003a: g. 2), Guanlong (Xu et al., 2006: g. 2), Proceratosaurus (Rauhut et al., 2010:
g. 3), Raptorex (Sereno et al., 2009: g. 1), Tarbosaurus (Hurum and Sabath, 2003: g. 1), and
Tyrannosaurus (Brochu, 2003: g. 2). erefore, if this row of foramina is the primary row in
Dryptosaurus, then its dorsal displacement is an autapomorphy of the genus. We do not, how-
ever, recognize this as a formal autapomorphy in the diagnosis due to the uncertainty over the
identication of the foramina. ese foramina are deep, circular, and large, approximately 4–5
mm in diameter, and open ventrally in deep grooves that extend to the alveolar margin. e
row is straight horizontally, and there are approximately two foramina per alveolus. e remain-
der of the lateral surface is generally smooth, without the deep grooves and ridges that are oen
seen on large specimens of derived tyrannosaurids (e.g., Brochu, 2003; Currie, 2003a).
e medial surface is heavily damaged and extensively covered by layers of consolidant,
rendering many details apparent in the original publications of Cope (1866, 1869) impossible
to ascertain (g. 2C). Fortunately, the NHM cast preserves some important morphology (g.
2D). First, although Carpenter et al. (1997: 562) reported “no trace of the interdental plates,”
the cast shows that the interdental plates were in fact present, increased in dorsoventral depth
posteriorly, and were unfused and subpentagonal (rectangular, but tapering to a ventral point)
as in other tyrannosauroids (e.g., Currie, 2003a: g. 6B). An eroded anteromedial process
(= palatal process), for articulation with the vomer, premaxilla, and opposing maxilla, is pres-
ent. Only the anterior region of the process is well preserved, but thick, broken bone surfaces
posteriorly indicate that the process was anteroposteriorly elongate, creating an extensive pal-
ate, as is characteristic of tyrannosauroids (this is sometimes referred to as a “secondary palate,”
but is not similar in morphology, composition, or position to the true secondary palate of
mammals). is process is arched: it is located only slightly dorsal to the interdental plate above
the rst alveolus, but posteriorly it sweeps dorsally, as is characteristic of most tyrannosauroids
(e.g., Carr, 1999: g. 2K; Hurum and Sabath, 2003: gs. 4, 11). e region between the antero-
medial process and the interdental plates is smooth where it is well preserved, albeit heavily
eroded in places.
Few details of the dentition are apparent. e rst three alveoli are preserved in full,
whereas only the anterior margin of the fourth remains (g. 2E–F). An erupted tooth is present
in the second alveolus of the cast, and so much of the medial maxilla is missing above the third
alveolus that a replacement tooth is exposed. e cast preserves few details, but it is clear that
these teeth are ziphodont and are much thinner labiolingually than the incrassate maxillary
teeth of adult tyrannosaurids, which have a width that is greater than 60% of their mesiodistal
length (Brochu, 2003; Currie, 2003a; Hurum and Sabath, 2003). e labial parapet of the max-
illa, formed by the lateral surface of the maxilla, extends further ventrally than the lingual
parapet, formed by the interdental plates, exposing the alveoli quite widely in medial view. In
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 9
ventral view, it is clear that the rst alveolus is smaller and more circular than the remaining
alveoli, as originally noted by Cope (1869). is condition, in which the rst maxillary tooth
is small (“incisiform”) is common in tyrannosauroids, and is especially pronounced in Aliora-
mus (Brusatte et al., 2009).
D: A fragment of the right dentary, broken both anteriorly and posteriorly, is pres-
ent (gs. 3–4). Carpenter et al. (1997) suggested that this piece is from the anterior region of
the dentary, but it is more likely that it represents the midportion, since the primary neurovas-
cular groove is not positioned immediately ventral to the alveolar margin as occurs anteriorly
in other tyrannosauroids (e.g., Alioramus: Brusatte et al., 2009). Five complete alveoli are pre-
served in the NHM cast (g. 4), as well as portions of two others, one anterior and one posterior.
As noted by Carpenter et al. (1997), the holotype specimen has been damaged since the origi-
nal descriptions of Cope (1866, 1869), and much of the dorsal and posterior margins have been
lost. As a result, only four complete alveoli and one partial alveolus remain in the holotype.
e lateral surface of the dentary is convex and the medial surface subtly concave, resulting
in an approximately D-shaped cross section. e lateral surface is generally smooth and lacks
the deep grooves, ridges, and other rugosities of large tyrannosaurids (e.g., Molnar, 1991).
ere are two primary rows of foramina: a primary (alveolar) and a ventral row. e primary
row is located approximately 22 mm ventral to the dorsal margin of the bone, and because this
margin is eroded, would have been even further separated from the true alveolar margin. e
foramina are large, 5–6 mm in diameter, circular, and lie in a shallow groove. is groove is
distinct, but is not sharp and deep as in Guanlong (Xu et al., 2006), Proceratosaurus (Rauhut
et al., 2010), and Sinotyrannus (Ji et al., 2009). Each individual foramen opens anteriorly into
the groove, and together they comprise an approximately linear row. e most anterior fora-
men is positioned approximately ve mm dorsal to the remainder of the row, and probably
represents part of the transition as the row sweeps from a dorsal position anteriorly to become
situated closer to the center of the dentary posteriorly. is prole, with the row located close
to the alveolar margin anteriorly and deecting ventrally as it continues posteriorly, is present
in all other tyrannosauroids (e.g., Brochu, 2003; Currie, 2003a; Hurum and Sabath, 2003; Xu
et al., 2004, 2006; Brusatte et al., 2009; Rauhut et al., 2010).
e ventral row of foramina is present along the ventral margin of the lateral surface, and
occupies the corner where the lateral and ventral surfaces meet. is row occupies the entire
preserved length of the bone, and therefore was anteroposteriorly elongate as in all other tyran-
nosauroids (e.g., Brochu, 2003; Currie, 2003a; Brusatte et al., 2009; Rauhut et al., 2010). e
individual foramina face laterally and ventrally, and vary more widely in morphology than the
more uniform foramina of the primary row. e anterior foramina are larger, deeper, and more
anteroposteriorly elongate, but posteriorly they become smaller, rounded, and less deeply inset.
ey are not set into a groove. e more circular posterior foramina are unusual, because in
other tyrannosauroids the posterior foramina become progressively more anteroposteriorly
elongate (e.g., Brochu, 2003; Currie, 2003a; Hurum and Sabath, 2003; Brusatte et al., 2009;
Rauhut et al., 2010). More circular posterior foramina are also gured in the holotype of Appa-
lachiosaurus (Carr et al., 2005: g. 5), but this specimen is poorly preserved in this region. It
10 AMERICAN MUSEUM NOVITATES NO. 3717
FIG. 3. Right dentary of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in lateral (A), medial
(B), and dorsal (C) views. Abbreviations: idp, interdental plates; mg, Meckelian groove; pnr, primary
neurovascular foramina row; snr, secondary neurovascular foramina row. Scale bar = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 11
FIG. 4. Casts of the right dentary of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in
lateral (A) and medial (B) views. Scale bar = 50 mm.
12 AMERICAN MUSEUM NOVITATES NO. 3717
is possible that circular posterior foramina are either an autapomorphy of Dryptosaurus or a
synapomorphy of Dryptosaurus and Appalachiosaurus, but because the ventral row is easily
aected by preservation, and perhaps individual variation, we hesitate to place too much
emphasis on this feature.
On the medial surface, the Meckelian groove is shallow, similar to the condition in Eotyran-
nus (MIWG 1997.550) and basal coelurosaur outgroups (e.g., Peyer, 2006), but unlike the con-
dition in more derived tyrannosauroids, including Raptorex (LH PV18), in which the groove is
deeply inset into the bone. e groove is essentially nonexistent anteriorly, where it is dorso-
ventrally shallow and not inset into the bone, such that it is medially at. Posteriorly it funnels
out in depth and becomes slightly more inset. e groove is positioned closer to the ventral
margin and, as a result, the lingual bar above the groove is dorsoventrally deeper than the region
below the groove. However, the groove is not located immediately above the ventral margin as
in most other coelurosaurs (e.g., Ostrom, 1969; Kobayashi and Barsbold, 2005; Peyer, 2006;
Norell et al., 2009). is more centered position of the groove is also present in other tyranno-
sauroids (e.g., Alioramus: IGM 100/1844; Daspletosaurus: Currie, 2003a: g. 33; Eotyrannus:
MIWG 1997.550; Raptorex: LH PV18; Tarbosaurus: Hurum and Sabath, 2003: g. 19; Tyranno-
saurus: Brochu, 2003: gs. 40–41), as well as the closest coelurosaur outgroups (e.g., Madsen,
1976; Brusatte et al., 2008).
Small remnants of the interdental plates are preserved. e plates are clearly unfused, short
dorsoventrally, and somewhat triangular in shape, as is characteristic of the posterior interdental
plates of tyrannosauroids (e.g., Molnar, 1991; Brochu, 2003; Currie, 2003a; Hurum and Sabath,
2003) and many other theropods (e.g., Madsen, 1976; Bever and Norell, 2009). ere is a shal-
low groove between the interdental plates and the lingual bar; the groove is sharp and thin, and
demarcates a pronounced step between the two regions. It is likely that an ossied supradentary
element t against the interdental plates medially and lled the pronounced step, as in other
tyrannosauroids (e.g., Brochu, 2003; Currie, 2003a).
e alveoli are dicult to interpret in the holotype (g. 3C), but comparison with the
NHM cast reveals some anatomical features. e best preserved alveoli are ovoid, longer mesio-
distally than wide labiolingually. ey are not gure-eight shaped, due to a labiolingual con-
striction at midlength, as in some tyrannosauroids (most prominent in Alioramus: IGM
100/1844). Erupted teeth are present in the rst and fourth preserved alveoli in the cast, but
these have broken o in the holotype; these teeth are similar to the maxillary teeth in morphol-
ogy, and are not labiolingually thickened as is typical of adult tyrannosaurids. Two replacement
teeth are still visible in the rst alveolus of the holotype. e rst is erupting medial to the
erupted tooth, whereas the second, smaller tooth is located ventromedial to the rst replace-
ment tooth, as described by Carpenter et al. (1997).
S: e posterodorsal region of the right surangular is present (g. 5). is bone
was rst identied by Cope (1869) as a “malar,” or zygomatic arch, a bone that is not present
in dinosaurs. Carpenter et al. (1997) pointed out Cope’s (1869) misinterpretation but could not
identify the fragment with any certainty, and tentatively suggested that it was a prearticular or
an angular. As a result, they only provided a measurement and an unlabelled gure of the bone,
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 13
FIG. 5. Right surangular of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in lateral
(A), medial (B), dorsal (C), and ventral (D) views. Abbreviations: add, adductor muscle attach-
ment; gle, glenoid; lgfossa, lateral fossa below the glenoid; lss, lateral surangular shelf; mpr,
medial process; psfor, posterior surangular foramen. Scale bar = 50 mm.
14 AMERICAN MUSEUM NOVITATES NO. 3717
and did not describe or compare it in detail. Carr et al. (2005) correctly identied this specimen
as a right surangular, and noted that it possesses a characteristic tyrannosauroid feature, a large
posterior surangular foramen. However, this bone has yet to be fully described and compared
to the surangulars of other tyrannosauroids.
e surangular fragment is heavily eroded and 159 mm long anteroposteriorly as preserved.
As reported by Carpenter et al. (1997), it has been slightly broken since Cope’s (1869) original
description, such that it is now shorter. On the lateral surface, the surangular shelf is robust: it
is thick dorsoventrally (11 mm) and laterally overhangs the remainder of the bone by 24 mm
at its midpoint (g. 5A). Shelves of similar prominence are seen in other tyrannosauroids more
derived than Dilong (e.g., Molnar, 1991; Brochu, 2003; Currie, 2003a). e shelf is most promi-
nent immediately dorsal to the posterior surangular foramen and becomes progressively less
oset anteriorly. It projects straight laterally and does not project ventrolaterally to overhang
the posterior foramen. is condition is shared with most other tyrannosauroids with a promi-
nent shelf, including Albertosaurus (Currie, 2003a), Alioramus (Brusatte et al., 2009), Eotyran-
nus (MIWG 1997.550), and Raptorex (Sereno et al., 2009), whereas the derived tyrannosaurines
Daspletosaurus, Tarbosaurus, and Tyrannosaurus possess the ventrolaterally overhanging mor-
phology (Brochu, 2003; Currie, 2003a; Hurum and Sabath, 2003). Furthermore, the shelf is
oriented nearly horizontally in lateral view, as in tyrannosauroids more derived than Eotyran-
nus (e.g., Molnar, 1991; Brochu, 2003; Currie, 2003a; Sereno et al., 2009).
Above the surangular shelf, the attachment surface for the jaw adductor muscles is promi-
nent (g. 5A). is scar is mediolaterally broad (32 mm immediately in front of the glenoid)
and has a smooth and deeply concave surface that becomes progressively atter anteriorly. is
attachment site faces almost equally dorsally and laterally, as in many tyrannosauroids, includ-
ing Albertosaurus (Currie, 2003a), Alioramus (Brusatte et al., 2009), Daspletosaurus (Currie,
2003a), Eotyrannus (MIWG 1997.550), Gorgosaurus (Currie, 2003a), and Raptorex (Sereno et
al., 2009). In contrast, the scars of Tarbosaurus and Tyrannosaurus face nearly entirely laterally,
resulting in a dorsoventrally deep surangular (Molnar, 1991; Brochu, 2003; Hurum and Sabath,
2003), whereas those of more basal taxa such as Guanlong (Xu et al., 2006) and Proceratosaurus
(Rauhut et al., 2010) face almost entirely dorsally.
Immediately below the surangular shelf, the dorsal margin of the posterior surangular
foramen is present (g. 5A). e foramen is enormous: its broken ventral edge is 25 mm long
anteroposteriorly, but the diameter of the foramen would have been much larger in life. e
enlarged foramen has long been recognized as a tyrannosauroid hallmark, and it is present in
all taxa more derived than Eotyrannus (e.g., Molnar, 1991; Brochu, 2003; Currie, 2003a; Hurum
and Sabath, 2003; Brusatte et al., 2009; Sereno et al., 2009). Unfortunately, erosion posterior to
the posterior surangular foramen makes it dicult to assess the presence or absence of a pneu-
matic pocket, which is present in all tyrannosauroids more derived than Eotyrannus and espe-
cially deep and invaginated in some derived taxa such as Alioramus (Brusatte et al., 2009).
Anterior to the posterior surangular foramen, and ventral to the prominent shelf, there is a
small preserved region of the ventral ramus of the surangular. is region is sheetlike and
measures only 6 mm in mediolateral thickness. However, it is still more robust than in the
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 15
holotype of Alioramus altai, which measures
1.5 mm thick (IGM 100/1844).
e glenoid and retroarticular region is
heavily eroded and few comparisons can be
made with other tyrannosauroids. However,
one important tyrannosauroid feature is
apparent: there is a smooth fossa on the lat-
eral surface of the surangular immediately
ventral to the glenoid, as in all tyrannosau-
roids more derived than Eotyrannus (Bru-
satte et al., 2010).
D: Several disarticulated lateral
teeth are preserved (g. 6), as is a complete
replacement tooth in the dentary (g. 7).
ese are transversely narrow, recurved, and
serrated, similar to those of most other thero-
pods, but unlike the laterally incrassate lateral
teeth of tyrannosaurids (see description of
maxilla above; Brochu, 2003; Currie, 2003a;
Hurum and Sabath, 2003). e distal carina
is well preserved on two disarticulated teeth
and bears 17 denticles/cm, measured at the
middle of the carina. e denticles diminish
in size adjacent to the apex and base of the
tooth. e mesial carina is preserved on one
disarticulated tooth. It extends to the base of
the tooth and bears 18 denticles/cm. Subtle,
bandlike enamel wrinkles are present on the
labial and lingual tooth surfaces, extending
between the mesial and distal carinae as apically concave undulations of consistent relief over
their mesiodistal length. ese are widespread among tetanuran theropods, including tyran-
nosaurids (Brusatte et al., 2007). Interdenticular sulci (sensu Smith, 2007; “blood grooves” of
Currie et al., 1990) extend a short distance onto the tooth from between the denticles of the
distal carina. ese are approximately as long as the denticles are high and are horizontal, not
inclined basally. Interdenticular sulci are widespread among theropods including tyrannosau-
rids (Currie et al., 1990), ceratosaurs (Smith, 2007) and basal tetanurans (Benson, 2009).
C V: Eleven caudal vertebrae are preserved (gs. 8–10). e neurocentral
sutures are rmly joined, but still visible, in all elements, indicating likely adult ontogenetic
FIG. 6. Lateral tooth of the holotype of Dryptosau-
rus aquilunguis (ANSP 9995) in labial/lingual (A, D),
and distal (B, C) views. Scale bars = 10 mm.
FIG. 7. Replacement tooth in the dentary of the
holotype of Dryptosaurus aquilunguis (ANSP 9995)
in lingual view. Scale bar = 10 mm.
16 AMERICAN MUSEUM NOVITATES NO. 3717
status (see Irmis, 2007, for a discussion of the utility of this feature in assessing ontogenetic
stage in archosaurs). ree of the vertebrae have substantially larger articular surfaces than the
others (g. 8). One of these preserves an incomplete neural arch, which bears the broken base
of a dorsolaterally inclined transverse process (g. 8H–K). e anteroposteriorly elongate pro-
portions (table 1) and the presence of a transverse process in these three larger vertebrae indi-
cates that they are middle caudal vertebrae. e remaining vertebrae are smaller and lack
transverse processes, indicating that they are distal caudal vertebrae. Six of these represent
anterior or middle distal caudal vertebrae (g. 9; note that one is very poorly preserved and
not gured). e two smallest vertebrae are the most distal and are much smaller than the
others (g. 10). One is poorly preserved, but the other is represented by a centrum and partial
All the caudal vertebrae of Dryptosaurus are proportionally longer relative to their height
than the equivalent bones in Tyrannosaurus (table 1; Brochu, 2003). All vertebrae are amphi-
coelous. e ventral rim of the posterior surface is thickened, so that the ventral part of the
articular surface is convex and forms a poorly dened, posteroventrally facing chevron facet.
A broad, but shallow, longitudinal groove is present on the ventral surface of all preserved
centra. is is more pronounced posteriorly than anteriorly in the middle caudal elements (g.
8), but is developed both anteriorly and posteriorly on the distal caudal centra, in which the
middle section of the “groove” forms a attened strip (g. 9). e proximalmost preserved
FIG. 8. Middle caudal vertebrae the holotype of Dryptosaurus aquilunguis (ANSP 9995) in anterior (A, H),
le lateral (B, E, I), ventral (C, F, J), posterior (D), right lateral (G), and dorsal (K) views. Abbreviations: path
for, pathological foramen; tp, transverse process. Scale bar = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 17
TABLE 1. Measurements of vertebrae (in millimeters) of the holotype of Dryptosaurus aquilunguis (ANSP
9995). Abbreviations: i, measurement of incomplete specimen; *, measurement estimated from slightly incom-
plete specimen. Two additional vertebrae were too incomplete to provide measurements.
Caudal vertebra Length
Middle (g. 8A–D) 110i 56 82i 76i 77i 69i
Middle (g. 8E–G) 115 51 86i 65i 85i 65i
Middle (g. 8H–K) 115 54 85*80*81i 63i
Distal (g. 9A–E) 118 51 68 74 68 65i
Distal (g. 9F–J) 118 48 63*70*65*67i
Distal (g. 9K–O) 113 42 60*65*61*64i
Distal (g. 9P–R) 108 39 ? ? 57i 55i
Distal (g. 9S–W) 104 36 48 53 44i 51i
Distal (g. 10A–F) 72 22 23*30*24*28i
FIG. 9. Anterior-middle distal caudal vertebrae of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in
anterior (A, F, K, S), le lateral (B, G, L, P, T), posterior (C, H, M, Q, U), ventral (D, I, N, R, V), and dorsal
(E, J, O, W) views. Abbreviations: gr, groove; r, ridge. Scale bar = 50 mm.
18 AMERICAN MUSEUM NOVITATES NO. 3717
centrum bears a large foramen on the ventral surface posteriorly and to the le side (g. 8A–
D). is slight asymmetry in location suggests that this foramen is pathological. e anterior
surface of the centrum is oset dorsally relative to the posterior surface of the centrum in the
middle caudal vertebrae, a characteristic of more proximal caudal vertebrae in theropods. e
distal caudal centra have articular surfaces that are not oset from each other and are wider
transversely than they are tall dorsoventrally (gs. 9–10).
Although the distal caudals lack transverse processes, in the proximalmost preserved distal
caudal an angular ridge is present on the lateral surface of the neural arch in the position of the
transverse process of more proximal vertebrae (g. 9B). In the succeeding vertebra, this ridge
is modied into a longitudinal groove (g. 9G, J) that may be autapomorphic as it is absent in
Tyrannosaurus (Brochu, 2003). However, as the distal caudal vertebrae of most tyrannosauroids
are poorly documented, we do not formally designate this feature as an autapomorphy of Dryp-
tosaurus. A second angular ridge, which is longer anteroposteriorly, is located more ventrally on
the lateral surface of the neural arch, just dorsal to the neurocentral suture. is is pronounced
in the more proximal distal caudal vertebra, but is weak or absent in succeeding elements.
e prezygapophyses of the distal caudal vertebrae project anterolaterally from a common
origin immediately dorsal to the neural canal. ey extend only slightly further anteriorly than
the centrum and have dorsomedially facing facets. is diers from the situation in Appalachio-
saurus and Tyrannosaurus (Brochu, 2003; Carr et al., 2005), in which the distal caudal vertebrae
have much longer prezygapophyses that project anteriorly, approximately half the length of the
centrum. Postzygapophyses are not preserved in any of the caudal vertebrae of Dryptosaurus.
Although broken in all specimens, the neural spine is approximately half the anteroposterior
length of the centrum and is located posteriorly. It is transversely narrow and sheetlike.
H: Both humeri are known, but each is heavily abraded and missing substantial
portions (g. 11; table 2). e le humerus is more complete and better preserved, but its
proximal and distal ends are abraded (g. 11A–E). In overall shape, the humerus is subcylindri-
FIG. 10. Posterior distal caudal vertebrae of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in anterior
(A, G), le lateral (B, H), posterior (C), right lateral (D), dorsal (E), and ventral (F) views. Scale bar = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 19
cal. e proximal half is inclined proximomedially and the distal condyles are inclined antero-
distally, contributing to the appearance of a sigmoidal curvature to the otherwise straight sha.
e distal condyles are rotated by approximately 50° relative to the plane of the proximal end
so that the medial (ulnar) condyle is located anteromedially. Such rotation is present in basal
tyrannosauroids such as Dilong (IVPP V14242) and Guanlong (IVPP V14531), but the long
axes of the proximal and distal ends are essentially parallel, due to a lack of rotation, in Eotyran-
nus (MIWG 1997.550), Raptorex (LH PV18), and tyrannosaurids (e.g., Lambe, 1917; Parks,
1928; Brochu, 2003).
e proximal end ot the humerus is damaged, but several details are visible (g. 11E). e
humeral head and medial epicondyle are broken o and, thus, only part of the lateral epicondyle
is well preserved. Although the head is broken o, the prole of the break and the morphology
of the remainder of the proximal end suggest that a hemispherical humeral head was not pres-
FIG. 11. Le (A–E) and right (F–I) humeri of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in
anterior (A, F), lateral (B, G), posterior (C, H), medial (D, I), and proximal (E) views. Abbreviations: dpc,
deltopectoral crest; epi, lateral epicondyle; scar, muscle attachment scar. Scale bar = 100 mm.
20 AMERICAN MUSEUM NOVITATES NO. 3717
ent. e hemispherical condition—in which the head is bulbous, approximately as broad
anteroposteri orly as mediolaterally, and overhangs both the anterior and posterior surfaces of
the humerus—is present in Raptorex (Sereno et al., 2009) and tyrannosaurids (e.g., Lambe, 1917;
Parks, 1928; Brochu, 2003). In Dryptosaurus, on the contrary, it appears as if the head was longer
transversely than anteroposteriorly, as in most other theropods, including basal tyrannosauroids
such as Dilong (IVPP V14243), Guanlong (Xu et al., 2006), and Eotyrannus (Hutt et al., 2001).
We emphasize, however, that this interpretation is based on the shape of broken surfaces, so it
cannot be absolutely ruled out that Dryptosaurus possessed a hemispherical head.
e deltopectoral crest is low and emerges anterolaterally from the sha. e most promi-
nent portion of the crest terminates 102 mm from the proximal surface, just over one-third the
length of the humerus. e extent of the deltopectoral crest exhibits phylogenetically informa-
tive variation in tyrannosauroids, and in general more derived taxa (i.e, those progressively
more closely related to tyrannosaurids) possess a progressively shorter crest. Basal taxa such
as Dilong (IVPP 14243) and Guanlong (IVPP V14531) possess a crest that extends 40%–50%
of the length of the humerus, Eotyrannus (MIWG 1997.550) and Dryptosaurus exhibit a crest
that is approximately 30% of the length of the humerus, and Raptorex (Sereno et al., 2009) and
TABLE 2. Measurements of appendicular bones (in millimeters) of the holotype of Dryptosaurus aquilunguis
(ANSP 9995 and AMNH FARB 2438). Asterisk indicates incomplete measurement due to damage.
Humerus (le element)
Proximodistal length: 282* (300 estimated)
Proximal end, anteroposterior: 27*
Proximal end, mediolateral: 85 estimated
Minimum sha circumference: 143
Manual Phalanx I-1
Proximodistal length: 148* (160 estimated)
Sha, minimum anteroposterior: 32
Sha, minimum mediolateral: 31
Minimum sha circumference: 111
Manual Phalanx II-2
Proximodistal length: 126
Proximal end, anteroposterior: 45
Proximal end, mediolateral: 43
Distal end, anteroposterior: 41
Distal end, mediolateral: 46
Minimum sha circumference: 114
Proximodistal length: 176
Proximal end, anteroposterior: 73
Proximal end, mediolateral: 40
Femur (le element)
Proximodistal length: 781
Distal end, anteroposterior: 90
Distal end, mediolateral: 160
Minimum sha circumference: 275
Tibia (le element)
Proximodistal length: 759
Proximal end, anteroposterior: 184*
Proximal end, mediolateral: 125*
Distal end, anteroposterior: 191
Distal end, mediolateral: 65
Minimum sha circumference: 260
Fibula (le element)
Proximal end, anteroposterior: 138*
Proximal end, mediolateral: 48*
Astragalus (le element)
Condyle region, mediolateral width: 161
Condyle region, anteroposterior length at midpoint: 63
Metatarsal IV (le element)
Proximodistal length: 396
Proximal end, anteroposterior: 66
Proximal end, mediolateral: 78
Distal end, anteroposterior: 77
Distal end, mediolateral: 54
Minimum sha circumference: 138
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 21
tyrannosaurids (e.g., Brochu, 2003) possess a short crest that is less than 25% of the humeral
length. e posterolaterally facing surface of the deltopectoral crest and the adjacent part of
the sha are weakly concave. A suboval depression, marked by rugose surface texture, is located
on the lateral surface of the sha at the distal end of the deltopectoral crest. is may corre-
spond to both the rst and second humeral tuberosity of Brochu (2003, g. 85E), as it is located
at the same position as portions of both structures in Tyrannosaurus. However, the scars of
Tyrannosaurus and close relatives are larger and more rugose than the potentially correspond-
ing structure in Dryptosaurus.
Much of the sha is present, but heavily damaged in places. Unfortunately, the distal end
is only partially preserved on the le humerus and completely missing on the right bone. e
posterior surface of the distal humerus is at up to the point at which it is broken. By contrast,
the anterior surface of the distal humerus is concave.
M: Parts of the le hand are preserved, including two nonungual phalanges and one
large ungual (g. 12; table 2). e two elongate nonungual phalanges likely represent phalanges
I-1 and II-2 (Parks, 1928; Russell, 1970). e least well preserved of these (g. 12A–D) is identi-
ed as phalanx I-1 due to its marked lateral curvature, as seen in phalanx I-1 of Tyrannosaurus
and closely related taxa (e.g., Lambe, 1917; Parks, 1928; Brochu, 2003). In tyrannosaurids, this
curvature is related to asymmetry in the distal articular surface of metacarpal I, which has
paired dorsoventrally curving distal condyles. Characteristically, the lateral condyle projects
further distally than the medial condyle (Brochu, 2003). is also indicates that the le side of
the Dryptosaurus phalanx I-1 is the lateral side, arming that these phalanges represent the
le hand. Our identications therefore dier from those of Carpenter et al. (1997) who identi-
ed our phalanx I-1 as phalanx II-2 and vice versa.
Phalanx II-2 (g. 12E–J) is much better preserved than phalanx I-1, which is heavily
abraded and missing much of the distal articular surface. Phalanx II-2 is slightly more robust
than I-1, and the ventral surfaces of both phalanges are at. e distal articular surface of II-2
consists of paired, dorsoventrally oriented condyles divided by a dorsoventral trough. e
medial condyle is slightly taller dorsoventrally than the lateral condyle. Suboval ligament pits
are present on the lateral and medial surfaces of the bone adjacent to the distal end. e proxi-
mal articular surface consists of paired depressions separated by a dorsoventrally oriented
angular ridge. e le (lateral) depression is deeper and slightly broader.
An ungual phalanx may represent either phalanx I-2 or II-3 of the le hand (g. 12K–N).
As in many theropod ungual phalanges, it has pronounced posterodistal curvature and tapers
to a point at its distal end. Expansive longitudinal grooves curve anteromedially along the lat-
eral and medial surfaces of the phalanx. e exor tubercle is comparable in relative size to
those of Raptorex (Sereno et al., 2009) and tyrannosaurids (e.g., Parks, 1928; Brochu, 2003),
but less prominent than the discrete, rugose tubercles of Guanlong (IVPP V14531), Dilong
(IVPP V14243), and Eotyrannus (MIWG 1997.550). e proximal articular surface bears a
dorsoventrally oriented angular ridge that is slightly oset medially, similar to the asymmetry
seen in the articular surfaces of the well-preserved phalanx II-2. Although the identity of the
ungual phalanx is uncertain, the ungual phalanges of the two digits of derived tyrannosauroids
22 AMERICAN MUSEUM NOVITATES NO. 3717
are of approximately equal length. An estimate of the length of digit I can therefore be obtained
by adding the length of the ungual phalanx (176 mm) to the length of rst phalanx (160 mm)
and an estimate of the length of metacarpal I based on comparison with other derived tyran-
nosauroids (~96 mm: Parks, 1928; Brochu, 2003).
P: Much of the shas of both pubes are present (g. 13). e preserved region of the
le sha is approximately 450 mm long, whereas that of the right is 465 mm. Unfortunately,
the distal region, which is expanded into a boot in other tyrannosauroids, is not preserved in
e proximal obturator region is completely missing on the le pubis, but eroded remnants
of both peduncles are present on the right specimen. ese are not sucient to infer the shape
and size of the peduncles, but it is clear that there was no enclosed obturator foramen or a dis-
crete obturator notch, as the widely open region where the ischial peduncle and sha diverge
is preserved as original bone surface. is condition is shared with most other tyrannosauroids,
whereas the basal taxa Guanlong (Xu et al., 2006) and Stokesosaurus (Benson, 2008) possess
FIG. 12. Le manual elements of the holotype of Dryptosaurus aquilunguis (ANSP 9995). Phalanges I-1
(A–D) and II-2 (E–J), and an ungual phalanx (K–N) in anterior (A, E), lateral (B, F, K), ventral (C, G, M),
medial (D, H, L), distal (I), and proximal (J, N) views. Scale bar = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 23
FIG. 13. Pubes of the holotype of Dryptosaurus aquilunguis (ANSP 9995). Right pubis in anterior
(A), posterior (B), lateral (C), and medial (D) views; le pubis in anterior (E), posterior (F), lateral
(G), and medial (H) views. Scale bar = 50 mm.
24 AMERICAN MUSEUM NOVITATES NO. 3717
discrete, inset notches that are demarcated ventrally by extensive obturator anges. is ventral
ange is clearly absent in Dryptosaurus, a strong indication that a notch is absent.
e pubic shas are heavily damaged and most of the external surface is poorly preserved,
making it dicult to identify muscle attachment sites and other subtle surface details. e
shas are concave anteriorly and convex posteriorly, the “bowed posteriorly” condition identi-
ed by Carr and Williamson (2010) and Brusatte et al., (2010) as a synapomorphy of derived
tyrannosauroids including Dryptosaurus, Appalachiosaurus, and tyrannosaurids. Interestingly,
this feature is not present in Raptorex (LH PV18; Sereno et al., 2009) or a putative tyrannosau-
roid pubis from the Aptian or Albian of Australia (Benson et al., 2010a), which have more of
a straight pubic sha as in more basal taxa such as Guanlong (Xu et al., 2006) and Stokesosaurus
(Benson, 2008). All tyrannosauroids with an anteriorly concave pubic sha possess an enlarged
pubic boot that is at least 60% of the sha length, with a prominent anterior process. us, it
is reasonable to hypothesize that Dryptosaurus would have also possessed this feature and the
concomitant enlarged femoral extensors that attached here (Carrano and Hutchinson, 2002).
Carpenter et al. (1997: g. 7D) reconstructed the medial contact between the pubic shas
as lacking the angelike medial apron that is common among theropods, and instead showed
the shas making contact by simply lying against each other. However, the medial surfaces of
both pubes, especially the le, are heavily eroded, so it is unclear whether a thin apron arose
from the sha. It is possible, however, to trace a helical broken surface, which arises from the
open obturator notch and then curves onto the medial surface of the pubic sha. It is likely that
this surface represents the apron. If so, the pubic apron is proximodistally elongate, and the
contact between the pubes was extensive. Extensive contact is seen in basal tyrannosauroids such
as Guanlong (IVPP V14531), Dilong (IVPP V14243), Stokesosaurus (Benson, 2008), and a tyran-
nosauroid from the Aptian-Albian of Australia (Benson et al., 2010a), but not more derived
taxa such as Tyrannosaurus (Brochu, 2003), in which contact begins at approximately the mid-
sha of the pubis. e shas begin to diverge slightly immediately above the distal broken
surface, which probably represents the pubic fenestra between the two pubes within the distal
part of the apron. If correct, the fenestra faces anteroposteriorly (i.e., opens straight anteriorly
and posteriorly), as is normal for tetanurans (e.g., Rauhut, 2003; Benson et al., 2009).
I: A heavily eroded fragment of the proximal right ischium is present (g. 14).
is piece was correctly identied by Carpenter et al. (1997), who recognized it as an ischium
based on the presence of an ovoid, rugose scar on the posterior margin: the ischial tubercle for
attachment of the exor tibialis musculature (g. 14F; Hutchinson, 2001; Carrano and Hutchin-
son, 2002). However, Carpenter et al. (1997: 565) provided only measurements and a small
illustration of this bone, feeling that “too little of it remains for meaningful comparison with
other theropods.” e identity of Dryptosaurus as a tyrannosauroid, as well as the discovery of
several new tyrannosauroid taxa in recent years, now allow for a more focused comparison
with other taxa.
e lateral surface of the ischium is highly convex and the medial surface at. Only the
proximal portion of the sha is preserved, but it clearly tapered distally. Unfortunately, the
pubic and iliac peduncles, the obturator process, and the acetabular margin are not preserved
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 25
(g. 14E). However, the base of the acetabulum is partially preserved proximally, and the
ischium is swollen both laterally and medially in this region. e most conspicuous feature of
the bone is the rugose ischial tubercle identied by Carpenter et al. (1997). e scar is present
as an expanded ovoid eminence, 64 mm proximodistally by 40 mm mediolaterally, on the
corner where the posterior and lateral surfaces meet (g. 14F). It is smoothly concave at its
center and is surrounded by a swollen and rugose rim. e anterior rim is much more promi-
nent than the posterior rim, and forms a sharp lip that separates the scar from the lateral
surface of the bone.
FIG. 14. Right ischium of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in lateral (A), medial (B),
anterior (C), posterior (D), and proximal (E) views, with a closeup of the ischial tubercle in posterior view
(F). Abbreviations: it, ischial tuberosity. Scale bars = 50 mm.
26 AMERICAN MUSEUM NOVITATES NO. 3717
Carpenter et al. (1997: 565) stated that the scar is “not as prominent as in tyrannosauroids.”
Although the ischial tubercle does become more swollen and rugose in extremely large indi-
viduals of derived tyrannosaurids (e.g., Brochu, 2003), the morphology in Dryptosaurus is
broadly similar to that of other tyrannosauroids more derived than Stokesosaurus. Whereas the
tubercle is expressed as a subtle groove in Guanlong (Xu et al., 2006) and a convex bulge on
the posterior surface of the ischium in Stokesosaurus (Benson, 2008), in Dryptosaurus, Appa-
lachiosaurus, Raptorex, and tyrannosaurids it takes the form of a rugose ange whose central
surface is depressed relative to the remainder of the bone. In Dryptosaurus the tubercle does
not project far posteriorly as a discrete ange in lateral view, but is more expanded in the
mediolateral direction (and thus most visible in posterior view as a swollen, enlarged struc-
ture). A similar morphology appears to be present in Appalachiosaurus (Carr et al., 2005) and
may be a synapomorphy of these two taxa. However, the strong mediolateral expansion of the
tubercle, which is approximately 1.7 times as wide as the sha immediately distally, is here
regarded as an autapomorphy of Dryptosaurus, pending further study of Appalachiosaurus. In
contrast, other tyrannosauroids with a prominent tubercle exhibit a scar that projects posteri-
orly as a discrete ange, oen triangular, that is only slightly mediolaterally expanded relative
to the remainder of the ischium (e.g., Brochu, 2003).
F: e le femur is nearly complete but heavily damaged in places, and the surface
texture is poorly preserved across the entire bone (g. 15; table 2). Much of the head, greater
trochanter, and lesser trochanter are eroded, precluding accurate measurements and descrip-
tions of these features. e same is true of portions of the distal end, including the crista tibio-
bularis. erefore, we are conservative in our description, and are unable to verify some of
the features of the proximal and distal ends identied by Carpenter et al. (1997).
e sha is bowed anteriorly, which appears to be a genuine feature and not a product of
deformation. e anterior surface of the sha is highly convex and the lateral and medial sur-
faces more subtly so, whereas the posterior surface is nearly at. Surface texture is poorly pre-
served, and in many places the surface is caked with preservative and glue. is makes it
impossible to identify most of the muscle attachment sites identied by Hutchinson (2001),
Carrano and Hutchinson (2002), and Brochu (2003), including the anterior intramuscular line,
which is usually a prominent feature in well-preserved theropod femora of this size.
e proximal end is heavily weathered, but it is clear that the head projects medially and
not at all anteriorly (g. 15E). Unfortunately, it is not possible to determine whether the head
was also projected proximally, the “elevated” condition present in most tyrannosauroids (e.g.,
Brochu, 2003). Carpenter et al. (1997: 565) stated that a “sucient” amount of the head was
FIG. 15. Le femur of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in anterior (A), posterior (B),
lateral (C), medial (D), proximal (E), and distal (F) views. Abbreviations: autfos, autapomorphic fossa on
medial surface of distal end; ext, extensor groove; e, exor groove; r, fourth trochanter; gtr, greater trochan-
ter; h, head; lc, lateral condyle; lcb, bulge on anterior surface of lateral condyle; ltr, lesser trochanter; mc,
medial condyle; mdc, mesiodistal crest; mdcps, mesiodistal crest posterior surface; ts, trochanteric shelf. Scale
bar for A–D = 100 mm, for E–F = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 27
28 AMERICAN MUSEUM NOVITATES NO. 3717
present to “show that it is of the elevated type,”
but this cannot be determined with certainty
because the entire proximal surface of the head
is eroded. Additionally, it is dicult to describe
the morphology of the greater trochanter,
including systematically informative characters
relating to its separation from the head and
lesser trochanter (Brusatte et al., 2010).
e lesser trochanter is prominent, and
in proximal view is seen to extend far anterior
relative to the head, curving medially as it con-
tinues anteriorly (g. 15E). It is thick medio-
laterally, and its rugose lateral surface is one of
the better-preserved regions of original bone
texture on the femur. is surface is highly
con vex anteriorly, where it is covered with a
series of ne striations for muscle attachment,
and is concave posteriorly. is concavity, which
forms a proximodistally elongate trough, is
also covered by ne, rugose striations. Medially,
the tro chanter is separated from the head by a
deep, smooth fossa. Unfortunately, the broken
anterior margin makes it difficult to identify
and describe the accessory trochanter, but the
NHM cast (g. 16) suggests that this structure
was reduced to a subtle convexity as in Appa-
lachiosaurus (Carr et al., 2005), Raptorex (LH
PV18), and all other tyrannosauroids more
derived than Xiongguanlong (Li et al., 2010).
e trochanteric shelf is well preserved on the
lateral surface of the sha, and takes the form
of a highly convex bulge, beginning approxi-
mately 105 mm distal to the proximalmost
preserved margin of the head (gs. 15–16).
e bulge is ovoid, 60 mm proximodistally by
40 mm anteroposteriorly, and is separated from
the lesser trochanter by a smooth, concave
e fourth trochanter is heavily eroded but visible as a mediolaterally thick, proximodis-
tally elongate ridge on the posterior surface of the femur, paralleling the medial margin of the
sha and beginning immediately distal to the distal end of the trochanteric shelf (g. 15D).
FIG. 16. Cast of the le femur of the holotype of
Dryptosaurus aquilunguis (ANSP 9995) in lateral
(A) and medial (B) views. Abbreviations: atr, ac-
cessory trochanter; autfos, autapomorphic fossa on
medial surface of distal end; r, fourth trochanter;
gtr, greater trochanter; h, head; ltr, lesser trochan-
ter; ts, trochanteric shelf. Scale bar = 100 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 29
e position of the distal end of the trochanter is estimated at less than 35% of the femoral
length. is condition is present in Appalachiosaurus and more basal tyrannosauroids, whereas
tyrannosaurids have a more elongate trochanter that extends to a point more than 40% of the
length of the sha (Brusatte et al., 2010). Distal to the fourth trochanter, on the posterior
surface of the bone, Dryptosaurus lacks the prominent circular scar of Tarbosaurus and Tyra n -
nosaurus (Brochu, 2003; Carr and Williamson, 2010).
e distal end is damaged: the crista tibiobularis is completely missing and ne details of
both condyles, such as which projected further ventrally, are not apparent (g. 15F). However,
in distal view, it is evident that the lateral and medial condyles are separated by a deep, antero-
posteriorly oriented groove, which is continuous with the extensor groove on the anterior sur-
face but separated from the exor groove on the posterior surface by a lip of bone. It is possible
that a lip was also present anteriorly but has been eroded. e lateral condyle is 88 mm long
anteroposteriorly by 71 mm mediolaterally, whereas the medial condyle measures 81 by 59
mm. ese are likely underestimates due to slight amounts of surface erosion, but the general
shapes of the condyles appear genuine. e lateral condyle, for instance, preserves a discrete
bulge along the midpoint of its anterior surface, a subtle feature also present in other tyranno-
sauroids more derived than Dilong (Brusatte et al., 2010).
e extensor groove is deep. It is dened by the lateral and medial condyles, both of which
have remarkably convex anterior surfaces. As a result, in distal view, the groove is expressed as
a discrete, U-shaped notch between the anterior surfaces of the condyles (g. 15F). Because
the lateral condyle is mediolaterally wider than the medial condyle, the groove is displaced
somewhat medially from the midpoint of the femur. A deep notch is seen in all tyrannosau-
roids more derived than Xiongguanlong, including Raptorex and tyrannosaurids (Brusatte et
al., 2010). In contrast, the extensor groove of Stokesosaurus is expressed as a shallower, more
broadly concave margin in distal view (Benson, 2008). In Guanlong (IVPP V14531) and Dilong
(IVPP V14243) the extensor grooves are so shallow that the anterior surface between the con-
dyles is essentially at. In addition, the condition in derived tyrannosauroids diers from that
in large-bodied basal tetanurans, such as allosauroids, in which the medial condyle is at ante-
riorly and gently slopes into the groove, giving the groove an asymmetrical, wedgelike shape in
distal view (e.g., Madsen, 1976; Brusatte et al., 2008; Benson et al., 2009; Benson, 2009, 2010).
In anterior view, the extensor groove is smoothly concave and extends dorsally and medi-
ally to abut the mesiodistal crest, which is sharp and strongly overhangs the medial surface of
the femur (g. 15A). Most of the crest is broken here, but it was clearly extensive, measuring
approximately 160 mm in proximodistal length. is is similar to the condition in Tyra nno-
saurus (Brochu, 2003) and other tyrannosauroids, in which the crest is robust and visible in
posterior view as a medially extended ange. e crest extends laterally as it continues proxi-
mally, eventually merging with the sha. Proximal to where these two regions merge the exten-
sor groove ceases to exist as a discrete depression.
e exor groove is deep, but it is dicult to measure its true depth since the crista tibio-
bularis, which forms its lateral bounding wall, is missing (g. 15F). e groove is smooth and
concave where it is well preserved on the posterior surface of the distal femur, and it extends
30 AMERICAN MUSEUM NOVITATES NO. 3717
proximally while gradually merging with the
sha, such that it is no longer present as a dis-
crete depression at the same level where the
extensor groove disappears.
e femur exhibits an autapomorphic fea-
ture on the medial surface of the distal end: an
ovoid fossa (150 mm in proximodistal extent)
on the sha immediately proximal to the
medial condyle (figs. 15–17). The fossa is
dened anteriorly by the mesiodistal crest,
which forms an enlarged ange that overhangs
the medial surface anteriorly as is normal for
tyrannosauroids and most other theropods.
However, the medial margin of the posterior
surface also thins into a much weaker crest,
which serves to dene the fossa posteriorly
(g. 17). is posterior crest is unknown in
other tyrannosauroids (e.g., Brochu, 2003),
and its presence, in conjunction with the standard mesiodistal crest, serves to dene the autapo-
morphic fossa. Unfortunately, erosion makes it impossible to discern whether the mesiodistal
crest bifurcates distally when it reaches the medial condyle, as is the case in some tyrannosau-
rines (e.g., Alioramus, Tarbosaurus, Tyrannosaurus: Brusatte et al., 2010). However, it is clear
that the medial surface of the medial condyle faces completely medially, and is not largely visible
in anterior view as in Tyrannosaurus (Brochu, 2003: g. 95). is latter condition may be an aut-
apomorphy of Tyrannosaurus, as it is absent in closely related taxa such as Alioramus (IGM
100/1844) and Tarbosaurus (ZPAL MgD/I-09), which have an identical morphology to that of
T: e le tibia is preserved in a comparable condition to the femur (g. 18; table 2).
e bone is nearly complete, but ne surface details have been obliterated over most of the
specimen, and the proximal and distal ends are slightly eroded. e proximodistal length of
the tibia is slightly less than that of the femur, a derived feature shared with large-bodied
tyrannosauroids such as Appalachiosaurus (Carr et al., 2005) and tyrannosaurids (e.g., Lambe,
1917; Parks, 1928; Brochu, 2003). In contrast, the small-bodied, more basal tyrannosauroids,
including Raptorex (Sereno et al., 2009), possess a tibia that is longer than the femur, a condi-
tion also present in outgroup coelurosaurian taxa (e.g., Kobayashi and Lu, 2003; Carpenter et
al., 2005a, 2005b; Peyer, 2006; although note that the relative length of the femur is positively
allometric compared to the tibia: Russell, 1970; Currie, 2003b). e anterior surface of the sha
is at to slightly convex in places, whereas the posterior, lateral, and medial surfaces are more
e proximal end is generally well preserved but is damaged in some regions, especially
anteriorly and on the proximal articular surface (g. 18E). e cnemial crest is large and stout,
FIG. 17. Le femur of the holotype of Dryptosaurus
aquilunguis (ANSP 9995) in medial view: original
material (A) and cast (B). Abbreviations: autfos,
autapomorphic fossa on medial surface of distal
end. Scale bar = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 31
FIG. 18. Le tibia of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in anterior (A), pos-
terior (B), lateral (C), medial (D), proximal (E), and distal (F) views. Abbreviations: af, astragalus
facet; cn, cnemial crest; fc, bular crest; it, incisura tibialis; lc, lateral condyle; lateral malleolus; mc,
medial condyle; mm, medial malleolus. Scale bar for A–D = 100 mm, for E–F = 50 mm.
32 AMERICAN MUSEUM NOVITATES NO. 3717
and curves laterally as it continues anteriorly. e crest appears somewhat triangular in proxi-
mal view and its base is thick mediolaterally, almost as wide as the posterior condyles. As a
result, there only appears to be a subtle incisura tibialis, the depression between the crest and
lateral condyle on the lateral surface, which led Carpenter et al. (1997) to consider Dryptosau-
rus as lacking the deep notch of tyrannosaurids. However, because the anterior surface of the
cnemial crest is broken, and it is likely that the crest would have extended further anteriorly in
life, a deeper and more dened incisura tibialis was almost certainly present. Because of ero-
sion, it is not possible to determine the presence or absence of an anterior process projecting
into the incisura from the anterior surface of the lateral condyle, contra Carr et al. (2005) who
described the absence of this process as a diagnostic dierence between Dryptosaurus and
Appalachiosaurus. However, if this process had been present, it is clear that it was not conuent
with the bular crest. Although the crest approaches the proximal end of the tibia, there is a
small region of smooth, concave original bone on the lateral surface of the tibia, between the
crest and the eroded region that may have housed the anterior process.
Posteriorly on the proximal end, separate lateral and medial condyles are divided by a deep,
concave notch. Unfortunately, it is not possible to determine which condyle extended further
posteriorly, as both are eroded. e medial surface of the medial condyle is gently convex, and
smoothly merges with the medial surface of the cnemial crest to form a single, sweeping medial
margin of the proximal tibial. e lateral surface of the lateral condyle, on the other hand, is
strongly convex, forming the posterior wall of the incisura tibialis. Although the proximal
articular surface is subtly eroded, it appears as if the lateral condyle was slightly convex poste-
riorly and concave anteriorly, with a slight lip on its lateral margin. e medial condyle and
cnemial crest are nearly at and together slope laterally, combining with the morphology of
the lateral condyle to dene a depression at the middle of the proximal articular surface.
ere is a pronounced bular crest on the lateral surface of the proximal end of the tibia
(g. 18B–C). e lateral surface of the crest is broken, but original bone surfaces on the ante-
rior and posterior margins are heavily rugose. e broken prole of the crest clearly indicates
that this structure was elongate and robust. It extends for approximately 190 mm proximodis-
tally, terminating only a few millimeters below the proximal edge of the tibia, and is approxi-
mately 26 mm thick anteroposteriorly at its midpoint. Such thickness indicates that the crest
extended as a robust ange, visible as a discrete rectangular sheet in anterior view, as in other
tyrannosauroids (e.g., Lambe, 1917; Parks, 1928; Brochu, 2003). However, in comparison to
other tyrannosauroids, the crest of Dryptosaurus is proportionally elongate, as its broken out-
line nearly reaches the proximal articular surface of the tibia. Comparisons with other tyran-
nosauroids are hampered by the poor preservation of the crest in Dryptosaurus, but if the
extreme proximal extent of the crest is a genuine feature, it is unusual among tyrannosauroids.
Most other taxa possess a crest that terminates further distally, oen 5–15 mm from the proxi-
mal end in individuals of similar size or larger than the Dryptosaurus holotype (e.g., Lambe,
1917; Parks, 1928; Brochu, 2003; Carr et al., 2005). e only exception is Stokesosaurus lang-
hami, which possesses an even more extreme condition in which the bular crest extends to
the proximal end of the tibia (Benson, 2008).
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 33
e distal end of the tibia is attened anteriorly to back the ascending process of the
astragalus. However, surface texture is so poorly preserved that it is impossible to measure the
height of the astragalar facet, contra Carpenter et al. (1997), who provided a measurement of
the ascending process based on a perceived tibial articular surface that we could not identify.
In distal view the tibia is triangular (g. 18F). Its anteroposterior dimension is longest medially,
and laterally the bone thins to a rounded point. e medial region of the distal surface is exca-
vated by a deep, ovoid concavity, which was likely even deeper and more distinct in life since
its surface is eroded.
e lateral malleolus extends further distally than the medial malleolus, and also projects
laterally from the sha as a discrete ange. e distal expansion of the lateral malleolus is a
tyrannosauroid feature also present in Guanlong (IVPP V14531), Stokesosaurus (Benson, 2008),
Eotyrannus (MIWG 1997.550), Appalachiosaurus (Carr et al., 2005), and tyrannosaurids (e.g.,
Parks, 1928; Lambe, 1917; Brochu, 2003). In contrast, basal coelurosaur outgroups possess lateral
and medial malleoli that extend to roughly the same level distally (e.g., Carpenter et al., 2005a,
2005b; Kobayashi and Barsbold, 2005; Rauhut and Xu, 2005; Gohlich and Chiappe, 2006) (note,
however, that several more basal tetanuran outgroups also possess a distally expanded lateral
malleolus, which may be a deeper theropod plesiomorphy retained in tyrannosauroids).
e lateral expansion of the lateral malleolus is normal for theropods, but is subtle when
compared to the condition in many other tyrannosauroids. In taxa such as Raptorex (LH PV18),
Appalachiosaurus (Carr et al., 2005), and all tyrannosaurids except for Alioramus (IGM
100/1844), the mediolateral dimension of the lateral malleolus is greater than 40% of the
mediolateral width of the adjacent sha. Dryptosaurus retains the primitive, less expanded
condition, which is also present in Guanlong (IVPP V14531), Dilong (IVPP V14243), Eotyran-
nus (MIWG 1997.550), and Stokesosaurus (Benson, 2008). In Dryptosaurus, the bular articular
facet on the lateral malleolus faces almost entirely anteriorly, as is typical for theropods with
even a subtly expanded malleolus. e medial surface of the medial malleolus is eroded, but
appears to have been smoothly conuent with the tibial sha, and thus did not project as a
lobate structure. e two malleoli join each other distally to form a concave distal margin of
the tibia in anterior and posterior views.
F: e proximal end of the le bula is present (g. 19; table 2). Assuming that the
bula is approximately the same length as the tibia, it is estimated that slightly more than half
of the bula is preserved. e proximal region is expanded relative to the sha, which tapers
distally, as is normal for theropods. e lateral surface of the bone is convex, as are the anterior
and posterior surfaces.
e medial surface of the proximal region is invaded by a deep, ovoid fossa (g. 19D). Both
the proximal and anterior walls of the fossa are nearly perpendicular to the medial surface, and
thus form an abrupt separation between the fossa and the remainder of the medial surface of
the bula. As a result, the anteroproximal corner of the fossa is most deeply inset. Posteriorly
and distally, however, the fossa becomes gradually shallower to become conuent with the
remainder of the medial bula. Posterior to the fossa, much of the bula is missing, and a large
broken surface is present. is break is also present on the NHM cast, in which it appears as
34 AMERICAN MUSEUM NOVITATES NO. 3717
FIG. 19. Le bula of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in anterior (A), posterior (B),
lateral (C), medial (D), and proximal (E) views. Abbreviations: autr, autapomorphic ridge; fos, medial fossa;
i, iliobularis tubercle. Scale bar = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 35
a smooth notch that could easily be mistaken for original morphology. Distal to the fossa, the
medial surface is at.
e bula is mediolaterally thickened dorsal to the fossa, and the proximal articular surface
is crescentic (g. 19E). Its lateral margin is convex, its medial margin concave, and it tapers in
mediolateral width as it extends posteriorly. e articular surface for the distal femur is troch-
lear, with a smooth concave region between anterior and posterior convexities. e entire
proximal surface slopes medially.
e most conspicuous feature of the bula is a pronounced muscle scar on the margin
where the anterior and lateral surfaces meet, slightly proximal to the broken distal edge of the
bone (g. 19A). is scar, the iliobularis tubercle, is large, prominent, rugose, and elongate
proximodistally, and its long axis trends proximolaterally-distomedially. Importantly, the tuber-
cle is bifurcated: it is comprised of lateral and medial crests that are separated by a depressed
fossa. is morphology is especially clear in anterior view. Bifurcated iliobularis tubercles are
present in Appalachiosaurus (Carr et al., 2005) and tyrannosaurids (e.g., Brochu, 2003), but are
absent in basal tyrannosauroids such as Guanlong (IVPP V14531) and Dilong (IVPP V14243).
e bifurcated tubercle of Dryptosaurus was rst recognized by Carr et al. (2005), who described
it as a derived feature shared with tyrannosaurids.
Immediately proximal to the tubercle, the anterior margin of the bula is marked by an
accessory ridge that trends proximomedially (g. 19A). is ridge is nearly as swollen, rugose,
and prominent as the ridges that dene the iliobularis tubercle itself. Although this region of
the bula is poorly preserved, the ridge appears to be a genuine structure. It is absent in other
tyrannosauroids (e.g., Brochu, 2003; Carr et al., 2005) and is here considered an autapomorphy
of Dryptosaurus. Unlike the iliobularis tubercle, this crest is a single structure and does not
bifurcate to enclose a smooth fossa.
A: e le astragalus is well preserved distally but is missing most of its ascend-
ing process (g. 20; table 2). As in other coelurosaurs, the distal condyles are positioned on the
anterior surface of the tibia when the two bones are articulated. erefore, the condyles project
mostly anteriorly rather than cupping the tibia ventrally, and in proximal view the ascending
process and condyles form a smooth, single articular surface that lies against the anterior sur-
face of the tibia.
Only the base of the ascending process is preserved, but it is clear that the process was
essentially as wide as the condylar region of the bone, unlike the condition in Guanlong in
which the ascending process is reduced to a discrete, tonguelike ange (IVPP V14531, V14532).
e anterior surface of the ascending process is invaded by a deep, discrete, ovoid fossa imme-
diately above the condyles (g. 20A). e fossa clearly continued up the ascending process for
some distance, as it is still present at the broken proximal margin of the astragalus. Fossae are
common on the anterior surface of the ascending process in theropods, but derived tyranno-
sauroids exhibit a unique condition in which the fossa is especially deep, well dened, centered
on the astragalus, and surrounded by a second, more extensive broad fossa that covers most of
the ventral region of the ascending process. is morphology is present in Raptorex (Sereno et
al., 2009), Appalachiosaurus (Carr et al., 2005), and tyrannosaurids (e.g., Lambe, 1917; Parks,
36 AMERICAN MUSEUM NOVITATES NO. 3717
1928; Brochu, 2003). In contrast, the basal tyrannosauroids Guanlong (IVPP V14531, V14532)
and Dilong (IVPP V14243) exhibit only a shallow, subtle fossa, comparable to that in close coe-
lurosaurian outgroups (e.g., Carpenter et al., 2005a, b; Kobayashi and Barsbold, 2005; Rauhut
and Xu, 2005). In Dryptosaurus, the lateral edge of the ascending process projects somewhat
proximomedially, unlike the more proximodistally vertical morphology in Appalachiosaurus
(Carr et al., 2005) and tyrannosaurids (e.g., Lambe, 1917; Parks, 1928; Brochu, 2003). As only
the base is preserved, however, it is unknown whether this trend continued proximally or
whether the lateral margin straightened out.
Distally, the medial condyle is more strongly convex both distally and anteriorly, and there-
fore appears expanded relative to the lateral condyle (g. 20D). e medial condyle is 82 mm
long anteroposteriorly at its midpoint and 64 mm wide mediolaterally, and its long axis is antero-
medially oriented. e lateral condyle is 81 by 55 mm with a straight anteroposterior long axis.
is condyle would have been larger in life, however, since the calcaneum would have formed
its lateral margin. e distal surface of the astragalus is smooth and deeply inset as a trochlear
surface between the two condyles. As a result, the distal margin of the astragalus is concave in
FIG. 20. Le astragalus of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in anterior (A), posterior
(B), medial (C), distal (D), and lateral (E) views. Abbreviations: ap, ascending process; calc, calcaneum articu-
lation; ext, extensor groove; b, bula articulation; fos, fossa on anterior surface; lip, lip separating bula and
calcaeum articulations; lc, lateral condyle; mc, medial condyle. Scale bar = 50 mm.
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 37
anterior and posterior views, more so than gured by Carpenter et al. (1997: g. 9A). In proxi-
mal view, the condyles are marked by a trochlear surface where they sit against the anterior face
of the tibia.
e medial surface of the astragalus is crescent shaped and mostly at, although some
regions are gently convex (g. 20C). e lateral surface exhibits a heavily damaged facet for the
calcaneum ventrally (g. 20E), and this articulation is expressed in distal view as a concave
notch on the lateral surface of the lateral condyle (g. 20D). It is clear that the bula made
contact with only the lateral surface of the astragalus, including the lateral surface of the
ascending process, as there is no discrete bular cup on the proximal surface of the astragalus.
Contact between the bula and astragalus is more extensive in basal theropods, in which the
astragalus cups the tibia and bula distally (e.g., Welles and Long, 1974; Madsen, 1976).
Unfortunately, because the lateral surface is broken distally in Dryptosaurus, it is dicult
to trace the sutural relationships between the astragalus, bula, and calcaneum here. However,
there is a pronounced lip that separates the ascending process from the condyles on the lateral
surface (g. 20E). Dorsal to this lip is a broad, smooth contact site for the bula on the lateral
surface of the ascending process, which faces strongly laterally and slightly anteriorly. It is likely
that this lip would have separated the articular facets for the bula and calcaneum, and there-
fore the calcaneum would have articulated with the entire lateral surface of the lateral condyle
itself. e lip is sharp and pronounced, and projects as an anterolaterally oriented corner at its
anterior point. Its dorsal surface is punctured by a deep pit, which forms the bottom of the
bular articulation on the ascending process. is pit, although reduced, is probably homolo-
gous to the more extensive bular cup on the proximal surface of the astragalus in more basal
theropods. Ventral to the lip, there is no discrete socket on the lateral surface of the condyle,
which is present and receives a peg from the calcaneum in Appalachiosaurus and tyrannosau-
rids (Carr et al., 2005). On the contrary, in Dryptosaurus the lip on the astragalus overlaps the
calcaneum proximally, which is an autapomorphy among tyrannosauroids because it is absent
in other taxa such as Appalachiosaurus (Carr et al., 2005), Tarbosaurus (ZPAL MgD-I/29), and
Tyrannosaurus (Brochu, 2003).
Carr et al. (2005: 134) described the ventrolateral buttress, the ridge dening the antero-
distal corner of the bular articulation, as “weakly developed” in Dryptosaurus. In comparison,
Appalachiosaurus was described as having a more prominent buttress, a dierence used to
support the taxonomic separation of these two genera. Dryptosaurus does appear to have a
weaker buttress in comparison to the sharp, pronounced ridge in Appalachiosaurus (Carr et
al., 2005: g. 18A). is region of the astragalus has been abraded in Dryptosaurus, however,
and its apparently weaker buttress may be artifactual.
M: Two metatarsals are preserved (gs. 21–22), including a complete le meta-
tarsal IV (AMNH FARB 2438, the only bone of the holotype not cataloged under ANSP 9995)
and a fragmentary metatarsal III that has not hitherto been described.
e le metatarsal IV is long and gracile, with convex anterior, lateral, and medial surfaces
(g. 22; table 2). e posterior surface, on the contrary, is at proximally before becoming
convex distally. As a result, the proximal cross section of the sha is semiovoid, much wider
38 AMERICAN MUSEUM NOVITATES NO. 3717
mediolaterally than anteroposteriorly, with the lateral and medial surfaces reduced to thin,
pronounced crests. Unfortunately, little original bone surface is preserved in this region, raising
the possibility that the posterior surface was not so extremely at in life. However, the few pre-
served regions of original bone are at, so we tentatively identify the attened posterior surface,
and the associated mediolateral thickening of the sha relative to its anteroposterior thickness,
as an autapomorphy of Dryptosaurus. Other tyrannosauroids, such as Guanlong (IVPP V14531),
Eotyrannus (MIWG 1997.550), Appalachiosaurus (Carr et al., 2005), and Tyrannosaurus (Bro-
chu, 2003), have a much more convex posterior surface, resulting in a more ovoid or triangular
cross section of the sha at its midpoint.
e proximal end of the metatarsal is expanded relative to the sha. In proximal view, its
lateral margin is strongly convex and its anterior and posterior margins more weakly so (g.
22E). e medial margin is interrupted by a small, discrete, concave notch, against which
FIG. 21. Metatarsal III (le or right) of the holotype of Dryptosaurus aquilunguis (ANSP 9995) in anterior
(= extensor) (A), posterior (= exor) (B), lateral or medial (C), and opposite lateral or medial (D) views.
Abbreviations: at, at posterior surface; fos, fossa; ridge, anterior surface reduced to thin ridge. Scale bar =
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 39
FIG. 22. Metatarsal IV of the holotype of Dryptosaurus aquilunguis (AMNH 2438) in anterior (= extensor)
(A), lateral (B), posterior (= exor) (C), medial (D), proximal (E), and distal (F) views. Abbreviations: ac,
anterior condyle; mlp, medial ligament pit; mt III, articular facet for metatarsal III; pc, posterior condyles.
Scale bar = 50 mm.
40 AMERICAN MUSEUM NOVITATES NO. 3717
metatarsal III articulated. e notch continues distally on the medial surface of the sha as a
concave fossa, which quickly expands in anteroposterior width before gradually tapering and
becoming less inset into the medial surface distally.
e shape of the proximal articular surface, especially the shape and size of the notch for
metatarsal III, is nearly identical to that of derived tyrannosauroids such as Albertosaurus
(Holtz, 1995), Appalachiosaurus (Carr et al., 2005), Gorgosaurus (Lambe, 1917), Raptorex
(Sereno et al., 2009), Tarbosaurus (Maleev, 1974), and Tyrannosaurus (Brochu, 2003), all of
which have an arctometatarsalian pes in which metatarsal III is “pinched” between metatarsals
II and IV (see Holtz, 1995; Snively et al., 2004 for more details). In contrast, more basal tyran-
nosauroids without an arctometatarsalian pes, such as Eotyrannus (MIWG 1997.550), exhibit
a more planar medial margin of the proximal fourth metatarsal, as in large-bodied basal tet-
anurans (e.g., Brusatte et al., 2008). In sum, the morphology of the proximal metatarsal IV
suggests that Dryptosaurus possessed an arctometatarsalian pes, which is borne out by the
morphology of metatarsal III (see below). Carpenter et al. (1997) noted that the presence of a
proximal notch for metatarsal III is a characteristic of arctometatarsalian taxa, but argued that
Dryptosaurus was not arctometatarsalian, at least in the style of derived tyrannosaurids, because
it lacks the at, expanded facet on the medial surface distally that articulates with metatarsal
III, as well as the laterally kinked distal end of metatarsal IV that accommodates the expanded
distal end of metatarsal III. However, we note that the medial surface is heavily eroded distally,
and very little original bone surface remains. us, the presence or absence of the medial facet
cannot be determined. Additionally, although the distal end is not sharply kinked, it is curved
laterally relative to the proximal end.
Distally, metatarsal IV is heavily eroded and the posterior condyles are missing (g. 22F).
e anterior condyle is teardrop shaped and curves medially as it continues anteriorly, such
that its medial surface is nearly at whereas its lateral surface is broadly convex. Anteriorly the
condyle tapers to a thin point, as in Eotyrannus (MIWG 1997.550) and Appalachiosaurus (Carr
et al., 2005), but unlike the more gently rounded condition in Albertosaurus (Parks, 1928),
Gorgosaurus (Lambe, 1917), Tarbosaurus (ZPAL MgD-I/206), and Tyrannosaurus (Brochu,
2003), in which the anterior surface is nearly at. In concert with this morphology, the distal
surface is more elongate anteroposteriorly relative to its mediolateral width in Dryptosaurus
and basal tyrannosauroids than it is in tyrannosaurids (Brusatte et al., 2010: character 305).
e distal articular surface itself is smooth and highly convex, and the medial ligament pit was
evidently large, deep, and set in a broad fossa. e corresponding region is eroded on the lateral
side, but if present, the lateral pit would have been much smaller.
A very poorly preserved bone appears to represent the midsha region of metatarsal III,
although it is dicult to determine whether this is a le or right element (g. 21). Importantly,
one surface of this fragment is reduced to a thick ridge, whereas the opposite surface is at and
one of the remaining (either lateral or medial) surfaces is marked by an elongate, concave fossa.
ese details match well with the morphology of metatarsal III of arctometatarsalian tyranno-
sauroids, in which the anterior surface is reduced to a ridge where it ts in between metatarsals
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 41
II and IV, the posterior surface is at and extensively contributes to the palmar surface of the
metatarsus, and the lateral and medial surfaces tightly brace against the surrounding metatar-
sals (e.g., Brochu, 2003: g. 100). is morphology, especially the triangular cross section of
the bone due to the thick ridge, would not be expected in a bula (another possible identica-
tion for a long, thin bone fragment), which in tyrannosauroids has a more crescentic cross
section without a conspicuous ridge.
D S: H O
When Cope (1866, 1867, 1868a, 1868b, 1869) rst described the holotype of Dryptosaurus,
it was among the most complete skeletons of a large theropod dinosaur known to science. With
little comparative material at hand, Cope and other contemporary paleontologists noted simi-
larities between Dryptosaurus and other fragmentary remains of large theropods, including
Megalosaurus from the United Kingdom (e.g., Cope, 1866; Leidy, 1868; Lydekker, 1888). Dryp-
tosaurus was oen allied with Megalosaurus and Deinodon, both wastebasket genera that sub-
sumed material from what are now known as numerous theropod taxa widely spread in time
and space, in the family Megalosauridae or Deinodontidae (e.g., Osborn, 1902; Hay, 1902;
Gilmore, 1920; Huene, 1926; Romer, 1956, 1966; Kuhn, 1965). Other authors, however, argued
that Dryptosaurus was unusual enough to warrant its own family, Dryptosauridae, which in
time became a wastebasket taxon for fragmentary theropod remains from across North Amer-
ica (e.g., Marsh, 1890, 1896). is concept has remained in favor until relatively recently, as
Carpenter et al. (1997) retained a monotypic Dryptosauridae aer they were unable to con-
dently place Dryptosaurus within any other theropod clade.
Beginning in the middle of the 20th century, and especially gaining steam in the 1970s,
paleontologists began to recognize similarities between Dryptosaurus and the characteristic
Late Cretaceous tyrannosaurids Albertosaurus and Tyrannosaurus from western North America
(e.g., Gilmore, 1946; Steel, 1970; White, 1973). Baird and Horner (1979) formally referred
Dryptosaurus to Tyrannosauridae, but this was based on the tyrannosaurid anities of a femur
from North Carolina that was tentatively referred to Dryptosaurus, not the holotype. Molnar
(1980) regarded Dryptosaurus as an “albertosaur,” an informal term used to group this genus
with the tyrannosaurids Albertosaurus and Daspletosaurus. However, despite these referrals,
the tyrannosaurid anities of Dryptosaurus remained controversial. Russell (1970) disputed
any connection between Dryptosaurus and Late Cretaceous tyrannosaurids, arguing that the
larger hand and more gracile femur of Dryptosaurus precluded assignment to Tyrannosauridae.
Carpenter et al. (1997: 571) followed suit, asserting that the teeth and astragalus of Dryptosau-
rus diered in morphology from those of tyrannosaurids, thus “cast(ing) doubt on the anity
of Dryptosaurus with the tyrannosaurids.” In his review of problematic large theropods, Molnar
(1990) noted a few features shared between Dryptosaurus and tyrannosaurids, but concluded
that Dryptosaurus could not be placed into any currently known theropod clade.
42 AMERICAN MUSEUM NOVITATES NO. 3717
One of the primary problems confronting theropod paleontologists, from the time of
Cope (1866) until the more recent redescription of Carpenter et al. (1997), was poor sampling,
specically the lack of basal tyrannosauroid taxa—species intermediate in phylogenetic posi-
tion, body size, and morphological features between generalized coelurosaurs and the derived,
colossal Late Cretaceous tyrannosaurids. Authors such as Russell (1970), Molnar (1990), and
Carpenter et al. (1997) were correct in identifying both similarities between Dryptosaurus and
tyrannosaurids, as well as dierences. is confounded many researchers, including Carpenter
et al. (1997), who used a typological argument to exclude Dryptosaurus from Tyrannosauridae
based only on the absence of characters, ignoring shared derived features that unite these taxa.
e discovery of numerous basal tyrannosauroids during the rst decade of the 21st century,
which have been studied in a rigorous phylogenetic context, illuminate an important pattern:
the mixture of similarities and dierences between Dryptosaurus and tyrannosaurids are
expected in an “intermediate” tyrannosauroid taxon on the stem toward the large-bodied
Tyrannosauridae. In other words, Dryptosaurus shares many derived characters with tyran-
nosaurids not seen in non-tyrannosauroid coelurosaurs and some basal tyrannosauroids such
as Guanlong, Dilong, and Eotyrannus (Hutt et al., 2001; Xu et al., 2004, 2006). It lacks other
characters that unite taxa such as Albertosaurus, Daspletosaurus, and Tyrannosaurus into
Tyrannosauridae proper. erefore, it possesses a mixture of primitive and derived features,
which in the context of a phylogenetic analysis support the placement of Dryptosaurus on the
tyrannosaurid “stem” (e.g., Holtz, 2004; Brusatte et al., 2009, 2010; Carr and Williamson,
e rst authors to recognize this emerging pattern were Holtz (2004) and Carr et al.
(2005). e discovery of the Campanian tyrannosauroid Appalachiosaurus, also a midsized
taxon from eastern North America showing both similarities and dierences with derived
tyrannosaurids, was a key to realizing the tyrannosauroid anities of Dryptosaurus. Carr et al.,
(2005: 139) argued that Dryptosaurus was a tyrannosauroid based on the possession of two
unique, derived characters seen only in this clade: an enlarged posterior surangular foramen
and a bifurcated iliobularis scar on the bula. In addition, the generalized morphology of
metatarsal IV was described as “comparable” to that of tyrannosaurids. Carr et al. (2005) also
included Dryptosaurus in a phylogenetic analysis, which placed it as the most basal tyranno-
sauroid (however, it is worth noting that other basal tyrannosauroids such as Eotyrannus,
Dilong, and Guanlong were not included, and several recently described basal tyrannosauroids
were unknown at the time). Holtz (2004), who based his analysis on the characters discussed
in a prepublication version of Carr et al. (2005), also included Dryptosaurus in a phylogenetic
analysis, nding it in a comparable basal tyrannosauroid position. More recently, Brusatte et
al. (2009, 2010) and Carr and Williamson (2010) have recovered similar results. ese latter
studies include a full array of basal tyrannosauroids, and place Dryptosaurus as intermediate
between basal taxa (i.e., Guanlong, Proceratosaurus, Dilong, Eotyrannus) and Tyrannosauri-
dae proper (i.e., Albertosaurus, Alioramus, Daspletosaurus, Gorgosaurus, Tarbosaurus, Tyran-
nosaurus). Contemporary theropod paleontologists, therefore, have reached a consensus:
Dryptosaurus is an intermediate-grade tyrannosauroid (g. 23).
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 43
T T A D
Although the work of Carr et al. (2005) and others has made it clear that Dryptosaurus is
a tyrannosauroid, the specic characters supporting this placement have been only briey dis-
cussed in the literature. Carr et al. (2005) and Holtz (2004) noted the two synapomorphies
listed above, and Carr et al. (2005) also included specic comparisons between Dryptosaurus
and Appalachiosaurus in their osteological description of the latter taxon. Other features that
place Dryptosaurus within the tyrannosauroid clade have been encoded in phylogenetic datas-
ets, but have been listed only in data matrices and not discussed or described in detail. Here,
we review numerous features of the skeleton that support the tyrannosauroid anities of Dryp-
tosaurus. Most of these characters are included in the phylogenetic analysis of Brusatte et al.
(2010), and many of them in the earlier analyses of Brusatte et al. (2009) and Carr and Wil-
liamson (2010), and thus explicitly support the tyrannosauroid placement of Dryptosaurus in
numerical cladistic studies (g. 23). In the following discussion, we rely explicitly on the phy-
logenetic dataset and analysis of Brusatte et al. (2010).
Dryptosaurus possesses three characters that currently optimize as synapomorphies of the
tyrannosauroid clade (i.e., present in all members of the clade, including basal forms such as
FIG. 23. Phylogenetic relationships of tyrannosauroids, based upon the cladistic analysis of Brusatte et al.
(2010). Dryptosaurus is an “intermediate” tyrannosauroid that is more derived than basal taxa such as Guan-
long and Dilong, but outside of the derived Tyrannosauridae that includes Tyrannosaurus, Albertosaurus, and
other colossal Late Cretaceous taxa.
44 AMERICAN MUSEUM NOVITATES NO. 3717
Guanlong and Dilong). e anteromedial process (= palatal process) of the maxilla is elongate
and extends posteriorly at least to the level of alveolus 4, forming an extensive “secondary pal-
ate” (e.g., Holtz, 2004). e mesiodistal crest of the femur is robust and medially expanded,
such that it is visible in posterior view as a pronounced ange. e lateral malleolus of the tibia
is expanded distally relative to the medial malleolus. is character is seen in all tyrannosau-
roids except for Dilong (IVPP V14243) and Alioramus (IGM 100/1844; Brusatte et al., 2009),
and is optimized as a tyrannosauroid synapomorphy lost in these two taxa. Finally, a fourth
character is also seen in other tyrannosauroids. e ventral neurovascular row on the dentary
is anteroposteriorly elongate, extending far posterior to the midlength of the bone. is is not
an unequivocal tyrannosauroid character, as it is also present in Compsognathus (Ostrom,
1978) and some dromaeosaurs (e.g., Norell et al., 2006), but shorter rows are present in most
other coelurosaurs, including ornithomimosaurs (e.g., Ji et al., 2003; Kobayashi and Barsbold,
2005), compsognathids (Hwang et al., 2004), and troodontids (e.g., Makovicky et al., 2003;
Norell et al., 2009).
Dryptosaurus also possesses several ingroup tyrannosauroid characters, which are not pres-
ent in basal taxa such as Guanlong, Proceratosaurus, and Dilong and thus unite Dryptosaurus
with Eotyrannus, Stokesosaurus, Appalachiosaurus, Raptorex, Xiongguanlong, and Tyrannosau-
ridae as a more derived clade. e surangular shelf is robust and prominently oset laterally,
and it is positioned directly above the posterior surangular foramen and projects solely laterally.
More basal tyrannosauroids, in contrast, have a subtle shelf that is located far dorsal to the
foramen (e.g., Xu et al., 2004, 2006). In distal view, the anterior surface of the lateral condyle
of the femur is marked by a discrete bulge. is character is absent in Guanlong (IVPP V14531)
and Dilong (IVPP V14243), in which the anterior surface of the condyle is a single, broadly
convex margin, but present in Stokesosaurus (OUMNH J.3311: Benson, 2008), Xiongguanlong
(FRDC-GS JB16-2-1), Raptorex (LH PV18), and tyrannosaurids (e.g., Brochu, 2003).
Additionally, Dryptosaurus possesses an intermediate condition for three ordered charac-
ters in the analysis of Brusatte et al. (2010), supporting a phylogenetic placement between basal
tyrannosauroids such as Guanlong and Dilong and more derived taxa such as Raptorex and ty-
rannosaurids. e adductor muscle scar on the surangular faces almost equally dorsally and later-
ally, intermediate between the dorsal condition of Guanlong and Proceratosaurus and the lateral
condition of Tarbosaurus and Tyrannosaurus. e humerus is approximately 38% of the length
of the femur, intermediate between the 50%–70% ratio of Guanlong and Dilong and the 25%–
30% ratio of Raptorex and Tyrannosauridae (even though the ends of both bones are eroded in
Dryptosaurus, only small regions are missing, meaning that Dryptosaurus condently falls within
the two end-member ratios). Similarly, the apex of the deltopectoral crest is located between
25%–35% of the length of the humerus, intermediate between the 40%–50% ratio in Guanlong
and Dilong and the 25% or less ratio in Raptorex and Tyrannosauridae. In both features of the
humerus, Eotyrannus shares the intermediate condition with Dryptosaurus, thus supporting
both taxa as intermediately positioned between basal tyrannosauroids and tyrannosaurids.
Several other characters support a progressively more derived phylogenetic position, plac-
ing Dryptosaurus closer to Raptorex and tyrannosaurids than to taxa such as Eotyrannus, Stokes-
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 45
osaurus, and Xiongguanlong. e enlarged, fenestralike posterior surangular foramen, rst
noted by Carr et al. (2005), is also seen in Raptorex (Sereno et al., 2009) and tyrannosaurids
(e.g., Molnar, 1991; Brochu, 2003), whereas it is only a small foramen in Eotyrannus (MIWG
1997.550), Dilong (IVPP V14243), Guanlong (IVPP V14532), Kileskus (Averianov et al., 2010),
and Pro ceratosaurus (Rauhut et al., 2010). e surangular shelf is oriented horizontally, not
anterodorsally or anteroventrally as in more basal taxa, and there is a discrete fossa on the
lateral surface of the surangular below the glenoid, whereas this region is bare in more basal
taxa. On the manus, the overall similarity in phalangeal shape with derived tyrannosaurids
(e.g., Lambe, 1917; Parks, 1928; Maleev, 1974; Brochu, 2003) suggests that Dryptosaurus most
likely had only two functional digits, as in Raptorex and tyrannosaurids; however, this cannot
be determined conclusively. Also in common with these taxa, the exor tubercle and overall
degree of curvature of the manual unguals are reduced, which likely relate to a loss of grasping
ability in concert with the reduced length of the arm and number of digits. e ischial tubercle
is expressed as a rugose ange, not a subtle convexity or groove as in more basal taxa such as
Guanlong (IVPP V14531) or Stokesosaurus (Benson, 2008), and the accessory trochanter of
the femur is reduced to a subtle mound, unlike the more discrete, angelike morphology in
Guanlong (IVPP V14531) and Xiongguanlong (Li et al., 2010). e iliobularis tubercle is
bifurcated as in Appalachiosaurus and tyrannosaurids, as noted by Carr et al. (2005), and the
anterior surface of the ascending process of the astragalus is marked by a deep, discrete fossa,
as in Raptorex (Sereno et al., 2009), Appalachiosaurus (Carr et al., 2005), and tyrannosaurids
(e.g., Brochu, 2003).
Two characters suggest that Dryptosaurus and tyrannosaurids share a more recent com-
mon ancestor relative to Raptorex, even though the most parsimonious phylogenetic topology
places Raptorex closer to tyrannosaurids (Brusatte et al., 2010). Regardless of their exact opti-
mization, these characters also support a more derived tyrannosauroid placement for Dryp-
tosaurus, and include a pubis that is concave anteriorly and a femur that is longer than the
tibia. Both conditions are seen in Appalachiosaurus (Carr et al., 2005) and tyrannosaurids
(e.g., Lambe, 1917; Parks, 1928; Maleev, 1974; Brochu, 2003), whereas more basal taxa have a
straight or anteriorly convex pubis and a tibia that is longer than the femur.
E N A T
During most of the Late Cretaceous, eastern and western North America were separated
by the Western Interior Seaway, a continental sea that stretched from the present-day Gulf of
Mexico to the Arctic Ocean (Smith et al., 1994). erefore, Dryptosaurus and its contemporary
fauna would have had little, if any, faunal connection with the tyrannosaurid-dominated eco-
systems of western North America during the Maastrichtian. is is also true of the slightly
older (Campanian), eastern North American tyrannosauroid Appalachiosaurus. In light of this
information, our current understanding of tyrannosauroid phylogeny is congruent with the
physical geography of terminal Cretaceous North America: Dryptosaurus and Appalachiosaurus
46 AMERICAN MUSEUM NOVITATES NO. 3717
are not particularly close relatives of coeval western North American tyrannosaurids such as
Albertosaurus and Tyrannosaurus.
e Campanian-Maastrichtian western North American tyrannosauroids form a clade,
Tyrannosauridae, which also includes Asian members. It is tempting to consider whether east-
ern North American tyrannosauroids also form a clade. Indeed, Dryptosaurus and Appalachio-
saurus are extremely similar in body size and general morphology. However, there is currently
no strong evidence that they form a unique eastern clade. No such clade is recovered in the
phylogenetic analysis of Brusatte et al. (2010), which instead places Dryptosaurus and Appala-
chiosaurus as successively closer outgroups to Tyrannosauridae. Furthermore, the two eastern
tyrannosauroids are not proximal outgroups on the tyrannosaurid “stem,” but Raptorex falls
out in between them.
With this said, it is notable that there is minimal overlapping material between Dryptosau-
rus and Appalachiosaurus, and many of the overlapping bones are poorly preserved in both
taxa. Appalachiosaurus is known from a decently preserved skull, whereas little cranial material
is present in Dryptosaurus. Conversely, the forelimb of Dryptosaurus is reasonably well known,
but is completely unknown in Appalachiosaurus. erefore, we consider the exact phylogenetic
relationships of these two taxa as equivocal. It is clear that neither is a tyrannosaurid and that
both are close outgroups to Tyrannosauridae, but whether they form a unique eastern clade
can be condently tested only with the discovery of additional material from eastern North
America. It would not surprise us if such discoveries eventually illuminate a clade of tyrant
dinosaurs restricted to eastern North America.
F P D
Derived tyrannosauroids are well known for their highly reduced forelimb (e.g., Lambe,
1917; Parks, 1928; Russell, 1970; Carpenter and Smith, 2001; Brochu, 2003; Lipkin and Car-
penter, 2009). is feature is rst known, both temporally and phylogenetically, in the small-
bodied (femur length = 338 mm) tyrannosauroid Raptorex from the Hauterivian-Barremian.
In contrast, more basal tyrannosauroids, such as Guanlong (IVPP V14531) and Dilong (IVPP
V14243), possess much larger forelimbs similar to those of other coelurosaurian theropods. In
general, the entire forelimb of Raptorex and tyrannosaurids is reduced: the humerus, radius
and ulna, and hand are all proportionally short (relative to the femur or other comparative
bones), reecting a reduction in the overall range of motion, grasping ability, and presumably
predatory function, of the entire limb (e.g., Lipkin and Carpenter, 2009).
Interestingly, the forelimb proportions of Dryptosaurus are unlike those of other tyranno-
sauroids (table 3). e humerus is short relative to the femur (humerus: femur length ratio =
0.375). Although it is not as reduced as those of tyrannosaurids and Raptorex (ratio = 0.268–
0.344), the Dryptosaurus humerus is substantially shorter than in basal tyrannosauroids such
as Guanlong (ratio = 0.597). e ratio of humerus length to femur length is used as an ordered
multistate character in the analysis of Brusatte et al., (2010), and Dryptosaurus is scored for the
intermediate condition. However, the hand of Dryptosaurus is proportionally large. Phalanx I-1
2011 BRUSATTE ET AL.: ANATOMY OF DRYPTOSAURUS AQUILUNGUIS 47
is more than twice as long relative to the femur as those of tyrannosaurids and Raptorex, closer
to the proportions in long-armed basal tyrannosauroids like Guanlong (table 3). is combina-
tion of a large hand and short humerus is unusual among theropods. Other theropods have large
hands and correspondingly enlarged manual ungual phalanges (Rauhut, 2003), including spi-
nosaurids (Charig and Milner, 1997), the megalosaurid Tor v o s auru s (Galton and Jensen, 1979)
and neovenatorid allosauroids (Benson and Xu, 2008; Benson et al., 2010b) (table 3). However,
all of these taxa also have proportionally elongate forelimbs with long humeri, unlike those of
Dryptosaurus and tyrannosaurids. erefore, the combination of a proportionally short humerus
and a large hand is considered an autapomorphy of Dryptosaurus among theropods.
We hypothesize that forelimb reduction in tyrannosauroids may not have proceeded in a
uniform fashion, but rather that the humerus (and perhaps radius and ulna) were shortened
prior to the reduction in manus size. Although current character optimizations support such
a scenario, it is dicult to rigorously test this hypothesis at present, since complete (or near-
complete) forelimbs of other “intermediate tyrannosauroids” phylogenetically proximal to
Dryptosaurus are unknown (e.g., Appalachiosaurus, Eotyrannus, Xiongguanlong).
Similarly, the function of the bizarre forelimb proportions of Dryptosaurus is mysterious.
At the most basic level, it appears as if both the skull and hands were important weapons for
prey acquisition and processing in Dryptosaurus, unlike the condition in coeval western North
American tyrannosaurids in which only the skulls were likely used as predatory armaments
(e.g., Lipkin and Carpenter, 2009). However, it is worth noting that much of the skull is
unknown in Dryptosaurus, and the few skull fragments exhibit some features unlike those of
large-skull, short-armed tyrannosaurids (e.g., ziphodont teeth, more delicately constructed
maxilla and dentary). Similarly, although the hand of Dryptosaurus is large, the ungual exor
tubercles and degree of curvature are reduced compared to more basal tyrannosauroids, indi-
cating a loss of some grasping ability. In sum, neither the skull nor hands of Dryptosaurus were
TABLE 3. Comparative measurements (in mm) of fore- and hind limbs of theropod dinosaurs, illustrating
the unique morphology of Dryptosaurus aquilunguis (ANSP 9995) in which the humerus is proportionally
reduced but the hand is large.
Albertosaurus Parks, 1928 1020 303 85 92 70 0.30 0.083 0.090
Tarbosaurus Maleev, 1974 1011 271 54 33 45 0.27 0.053 0.033
Raptorex Sereno et al., 2009 338 99 26 18 22 0.29 0.077 0.053
Gorgosaurus Parks, 1928 1040 324 98 80 83 0.31 0.094 0.077
Daspletosaurus Russell, 1970 655 225 63 46 40 0.34 0.096 0.070
Guanlong IVPP V14531 352 210 80 74 75 0.60 0.227 0.210
Dryptosaurus ANSP 9995 800 300 160 176 126 0.38 0.200 0.220
Chilantaisaurus Benson and Xu, 2008 1190 580 — 250 — 0.49 — 0.210
Suchomimus MNN GDF 500 1075 560 — 260 — 0.52 — 0.242
48 AMERICAN MUSEUM NOVITATES NO. 3717
as well developed for predatory function as in derived tyrannosaurids and basal tyrannosau-
roids, respectively. Regardless, it is clear that Dryptosaurus possessed a dierent set of predatory
features than contemporary derived tyrannosaurids, which may suggest a dierence (perhaps
subtle) in ecology between these animals. It is hoped that future discoveries of related taxa and
more detailed biomechanical studies may shed light on the unusual forelimb and ecology of
We thank T. Daeschler for access to the holotype at the Academy of Natural Sciences, per-
mission to study the specimen, and hospitality while visiting Philadelphia. We are grateful to
T. Carr and P. Makovicky for their helpful reviews, and thank numerous curators for access to
specimens in their care, most importantly: X. Xu (IVPP), A. Milner and S. Chapman (NHM),
S. Hutt (MIWG), P. Jeery (OUMNH), C. Mehling (AMNH), and P. Sereno (LH). For discus-
sion on nomenclatural issues we thank C. Kammerer, and for discussion on tyrannosauroids
we thank T. Carr, P. Currie, P. Sereno, P. Makovicky, T. Williamson, and X. Xu. M. Ellison pro-
vided gures 1 and 22, and R.B.J.B. took all other photos. S.L.B. is funded by an NSF Graduate
Research Fellowship (Columbia University). R.B.J.B. is funded by a postdoctoral research fel-
lowship at Trinity College, Cambridge. Other support for this project was provided by the
Division of Paleontology, American Museum of Natural History.
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