Redescription of the Mongolian sauropod Nemegtosaurus mongoliensis Nowinski (Dinosauria: Saurischia) and comments on Late Cretaceous sauropod diversity

Abstract

The isolated skulls of Nemegtosaurus mongoliensis and Quaesitosaurus orientalis from the Nemegt Basin of Mongolia are among the most complete sauropod cranial remains known from the Late Cretaceous, yet their evolutionary relationships to other neosauropods have remained uncertain. Redescription of the skull of Nemegtosaurus identifies key features that link it and its closely related counterpart Quaesitosaurus to titanosaur sauropods. These include a posterolaterally orientated quadrate fossa, ‘rocker’-like palatobasal contact, pterygoid with reduced quadrate flange and a novel basisphenoid–quadrate contact. Other features are exclusive to Nemegtosaurus and Quaesitosaurus, such as the presence of a symphyseal eminence on the external aspect of the premaxillae, a highly vascularised tooth bearing portion of the maxilla, an enclosed ‘maxillary canal’, orbital ornamentation on the postorbital, prefrontal and frontal, exclusion of the squamosal from the supratemporal fenestra and dentary teeth smaller in diameter than premaxillary and maxillary teeth.

Full text

Journal of Systematic Palaeontology 3(3): 283–318 Issued 24 August 2005
doi:10.1017/S1477201905001628 Printed in the United Kingdom C
The Natural History Museum
Redescription of the Mongolian
Sauropod NEMEGTOSAURUS
MONGOLIENSIS Nowinski (Dinosauria:
Saurischia) and comments on Late
Cretaceous Sauropod diversity
Jeffrey A. Wilson
University of Michigan, Museum of Paleontology and Department of Geological Sciences, 1109 Geddes
Road, Ann Arbor, Michigan 48109–1079, USA
SYNOPSIS The isolated skulls of Nemegtosaurus mongoliensis and Quaesitosaurus orientalis from
the Nemegt Basin of Mongolia are among the most complete sauropod cranial remains known from
the Late Cretaceous, yet their evolutionary relationships to other neosauropods have remained
uncertain. Redescription of the skull of Nemegtosaurus identifies key features that link it and its
closely related counterpart Quaesitosaurus to titanosaur sauropods. These include a posterolaterally
orientated quadrate fossa, ‘rocker’-like palatobasal contact, pterygoid with reduced quadrate flange
and a novel basisphenoid–quadrate contact. Other features are exclusive to Nemegtosaurus and
Quaesitosaurus, such as the presence of a symphyseal eminence on the external aspect of the
premaxillae, a highly vascularised tooth bearing portion of the maxilla, an enclosed ‘maxillary canal’,
orbital ornamentation on the postorbital, prefrontal and frontal, exclusion of the squamosal from the
supratemporal fenestra and dentary teeth smaller in diameter than premaxillary and maxillary teeth.
Re-examination of Late Cretaceous sauropod distributions in the light of this well-supported
phylogenetic hypothesis has important implications for their diversity at the end of the Mesozoic
in Asia and elsewhere. Cretaceous Asian sauropod faunas consist solely of titanosauriforms, which
probably migrated there from other landmasses during the Late Jurassic, during which time neosaur-
opods were absent from Asia. Globally, narrow-crowned titanosaurs and rebbachisaurids radiated
during the Cretaceous, but only titanosaurs survived into the latest Cretaceous. These late-surviving
sauropods flourished on most continental landmasses until the end of the Maastrichtian.
KEY WORDS vertebrate palaeontology, evolution, palaeobiogeography, Mesozoic, Asia, Titanosauria
Contents
Introduction 284
Institutional abbreviations 284
Systematic palaeontology 284
Sauropoda Marsh, 1878 284
Macronaria Wilson & Sereno, 1998 284
Titanosauria Bonaparte & Coria, 1993 284
Nemegtosauridae Upchurch, 1995 284
Nemegtosaurus Nowinski, 1971 284
Nemegtosaurus mongoliensis Nowinski, 1971 285
Description 286
Preservation 286
Dermal roof complex 286
Palatal complex 297
Braincase 300
Lower jaw 301
Teeth 305
Reconstruction 306
Phylogenetic affinities of Nemegtosaurus 308

284 J. A. Wilson
Previous cladistic hypotheses 309
Nemegtosauridae 311
Effect of ambiguous features 313
Implications for Late Cretaceous sauropod diversity 313
Origin of Asian Cretaceous sauropod fauna 313
Titanosaur predominance in the Late Cretaceous 315
Acknowledgements 315
References 315
Introduction
The Central Asiatic Expeditions (1922–1930), Mongo-
lian Palaeontological Expeditions (1946–1949), Polish–
Mongolian Palaeontological Expeditions (1963–1971) and
Soviet-Mongolian Palaeontological Expeditions (1969–
1979) to the Gobi Desert of Mongolia discovered a wealth
of fossil material that documented both the end of the
age of dinosaurs and the beginning of the age of mam-
mals in Asia (Efremov 1948; Kielan-Jaworowska & Dovchin
1968/69; Kurochkin & Barsbold 2000). These expeditions to
Mongolia brought to light numerous new dinosaurs, such
as ankylosaurs, ornithopods, ceratopsians and pachyceph-
alosaurs, as well as many coelurosaurian theropods (see
Benton et al. 2000). Subsequent studies have suggested that
many Late Cretaceous Mongolian ornithischian and thero-
pod genera are closely related to genera from similar-aged
horizons in western North America, implying multiple dis-
persals across Beringia (Maryanska & Osm´
olska 1975; Rus-
sell 1993; Sereno 2000; Upchurch et al. 2002). Also present
in the Late Cretaceous Mongolian dinosaur fauna were
three sauropod genera (Opisthocoelicaudia,Quaesitosaurus,
Nemegtosaurus) that had no hypothesised correlates in North
America, despite the presence of at least one Late Cretaceous
sauropod in the western USA (Alamosaurus: Gilmore 1922).
These three Mongolian sauropod genera have received less
attention than their ornithischian and theropod counterparts,
despite representing some of the best preserved sauropod re-
mains known from Late Cretaceous-aged sediments of north-
ern (Laurasian) landmasses.
The Mongolian sauropod Opisthocoelicaudia was ini-
tially described as Camarasaurus-like (Borsuk-Bialynicka
1977; McIntosh 1990), whereas Nemegtosaurus and
Quaesitosaurus were described as Dicraeosaurus-like
(Nowinski 1971; McIntosh 1990). These designations were
consistent with what was then known of the Late Jurassic
Asian sauropod fauna, which included genera considered
by McIntosh (1990) to be camarasaurid (Euhelopus)and
diplodocid (Mamenchisaurus). Thus, Mongolia appeared
to have representatives of both narrow-crowned (dip-
lodocids, titanosaurs) and broad-crowned (camarasaurids,
brachiosaurids) sauropod groups (e.g. Janensch 1929; Romer
1956; McIntosh 1990). More recently, cladistic analyses of
sauropod relationships have demonstrated that broad crowns
are primitive for Sauropoda (Upchurch 1995) and that narrow
crowns evolved at least twice independently within the group
(Salgado et al. 1997; Wilson & Sereno 1998). This revised
context of sauropod phylogeny has produced new hypotheses
for the relationships of the three Mongolian sauropods that
differ from traditional views. Cladistic analyses that have in-
cluded the genus agree that Opisthocoelicaudia is a titanosaur
(e.g. Upchurch 1995, 1998; Salgado et al. 1997; Sanz et al.
1999; Curry Rogers & Forster 2001; Wilson 2002), that
Euhelopus is either a derived titanosauriform (Wilson &
Sereno 1998; Wilson 2002) or a basal neosauropod
(Upchurch et al. 2004) distantly related to Camarasaurus and
that Mamenchisaurus is a non-neosauropod (Upchurch 1995,
1998; Wilson 2002). The phylogenetic affinities of Nemegto-
saurus and Quaesitosaurus, in contrast, remain unresolved.
Originally described as Dicraeosaurus-like, Nemegto-
saurus and Quaesitosaurus alternatively have been resolved
by cladistic analyses as the monophyletic sister-taxon of dip-
lodocoids (diplodocids, dicraeosaurids and others) (Yu 1993;
Upchurch 1998, 1999; Upchurch et al. 2002, 2004), the basal
members of a clade including diplodocoids and titanosaurs
(Upchurch 1995) and, most recently, as titanosaurs (Calvo
1994; Salgado & Calvo 1997; Wilson 1997; Curry Rogers &
Forster 2001; Wilson 2002). Lack of consensus on the phylo-
genetic affinities of these taxa probably stems from ambiguity
resulting from preservational distortion of the skulls, uncer-
tainty surrounding the higher-level relationships of narrow-
crowned sauropods and lack of comparative Late Cretaceous
skull material. General similarities between the skulls have
likewise contributed to this problem (Upchurch 1999).
Below, Nemegtosaurus is redescribed and features dia-
gnosing the genus and supporting its placement within
Titanosauria are identified. Based on this revision, the af-
finities of Quaesitosaurus and other Asian sauropods are re-
assessed and a diagnosis and definition of Nemegtosauridae
are proposed. The implications of the titanosaur affinities of
Nemegtosauridae for Late Cretaceous sauropod diversity in
Asia and worldwide are discussed.
Institutional abbreviations
GSI, Geological Survey of India, Kolkata; IVPP, Institute of
Vertebrate Palaeontology and Palaeoanthropology, Beijing;
PIN, Russian Academy of Sciences, Moscow; PVL, Fun-
daci´
on Miguel Lillo, Universidad Nacional de Tucum´
an, San
Miguel de Tucum´
an; Z. PAL, Palaeobiological Institute of the
Polish Academy of Sciences, Warsaw.
Systematic palaeontology
SAUROPODA Marsh, 1878
MACRONARIA Wilson & Sereno, 1998
TITANOSAURIA Bonaparte & Coria, 1993
NEMEGTOSAURIDAE Upchurch, 1995
NEMEGTOSAURUS Nowinski, 1971
TYPE SPECIES.Nemegtosaurus mongoliensis Nowinski 1971.

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
285
Figure 1 Map of Mongolia showing the sites that produced Nemegtosaurus mongoliensis (Nemegt Uul), Opisthocoelicaudia skarzynskii
(Altan Uul IV) and Quaesitosaurus orientalis (Shar Tsav). Locality data from (Kielan-Jaworowska 1969; Kurochkin & Barsbold 2000); Mongolia
map based on Shupe et al. (1992). Scale bar =500 km.
DIAGNOSIS AND OCCURRENCE.As for the species.
Nemegtosaurus mongoliensis Nowinski, 1971 (see
Figs 2–16)
1971 Nemegtosaurus mongoliensis Nowinski: 59, figs 1–8,
pls 8–14.
HOLOTYPE.Based on a nearly complete skull lacking only
the dorsal margin of the narial region and portions of the
mid-palate (palatines and posterior vomer), articulated with
nearly complete left and right lower jaws lacking only the
prearticular and articular (Z. PAL MgD-I/9).
OCCURRENCE.Nemegt Formation, Upper Cretaceous (mid-
Maastrichtian; Jerzykiewicz & Russell 1991) of the Gobi
Desert, Mongolia (Fig. 1).
REVISED DIAGNOSIS.Nemegtosaurus mongoliensis is char-
acterised by the following autapomorphies: presence of a
spur on the posterior squamosal and a conspicuous fossa
surrounding the preantorbital fenestra. Other features cannot
be scored in closely related taxa (i.e. Quaesitosaurus)and
are thus ambiguous autapomorphies. These include the pres-
ence of an accessory fenestra positioned anterodorsal to the
preantorbital fenestra, a jugal foramen and a coronoid fora-
men. For a fuller discussion of characters shared by Nemeg-
tosaurus and closely related forms, see ‘Nemegtosauridae’
below.
REFERRED SPECIMENS.Cranial remains of several Asian
sauropods have been referred to Nemegtosaurus or included
as members of Nemegtosauridae. First used by Upchurch
(1995), Nemegtosauridae is phylogenetically defined by
Upchurch et al. (2004: 303) as a stem-based clade including
diplodocoids more closely related to Nemegtosaurus than to
Diplodocus. Although this definition specifies a small clade
within the phylogenetic framework supported by Upchurch
et al. (2004), which places Nemegtosaurus within the Dip-
lodocoidea, the same definition specifies a much larger group
(Macronaria) under the topology supported here and else-
where (Curry Rogers & Forster 2001; Wilson 2002). The
phylogenetic definition of Nemegtosauridae is discussed in a
later section (see ‘Nemegtosauridae’ below). Nemegtosaur-
idae currently includes several slender-toothed forms found
in Asia.
Dong (1977) created the new species Nemegtosaurus
pachi for a narrow-crowned tooth discovered in Upper Creta-
ceous strata of the Turpan Basin, Xinjiang, China. Nemeg-
tosaurus pachi (IVPP V4879) resembles N. mongoliensis in
its possession of longitudinally striated enamel near the base
of the tooth (Dong 1977: pl. 2, fig. 8), but this feature is not
diagnostic, as evidenced by its presence in both the narrow-
crowned forms ‘Titanosaurus’rahioliensis (Mathur &
Srivastava 1987: 564; pl. 3, fig. 6) and cf. Alamosaurus (Kues
et al. 1980: figs 4–5), as well as the broad-crowned form
Mamenchisaurus sinocanadorum (IVPP V10603; pers. obs.).
For the same reason, the isolated teeth described by Dong
(1997) from the Upper Cretaceous beds of the Mazongshan
Area of Gansu Province, China cannot yet be referred to
Nemegtosauridae.
In a reinterpretation of Nemegtosaurus and Quaesito-
saurus as titanosaurs, Wilson (1997) suggested that these
two genera might represent the same species. Although a
close relationship between Nemegtosaurus and Quaesito-
saurus is recognised here, several cranial differences sup-
port retention of separate genera (see ‘Nemegtosauridae’
below).
Maryanska (2000: 458) reported a skull referable to
the genus Nemegtosaurus. This undescribed skull is housed
in the Geological Institute of the Mongolian Academy of
Sciences in Ulaanbaatar. Its provenance, completeness and
association with other remains have not yet been published.
Thus far, however, specific features linking this new skull
to Nemegtosaurus have not been recognised and additional
comparisons are required to confirm its referral to an existing
Late Cretaceous Asian genus.
Most recently, Buffetaut et al. (2002) interpreted new
remains of the Early Cretaceous Phuwiangosaurus as sup-
porting its membership in Nemegtosauridae. These include

286 J. A. Wilson
jaw fragments and a well-preserved braincase collected from
separate localities in the Sao Khua Formation (discussed in
‘Relationship to other Mongolian Sauropods’, below). Al-
though Buffetaut et al. refer Phuwiangosaurus and other
Early Cretaceous Asian sauropods (Huabeisaurus,Mongo-
losaurus) to Nemegtosauridae, they note primitive characters
that differentiate them from the Late Cretaceous Nemegto-
saurus.
Description
The following description emends and supplements that
provided by Nowinski (1971), based on personal observation
of the holotype (Z. PAL MgD-I/9) at the Polish Academy of
Science, Warsaw. Observations of Quaesitosaurus are based
on examination of the holotype (PIN 3906/2) at the Russian
Academy of Science in Moscow.
Preservation
Nearly the entire skull of Nemegtosaurus is preserved; only
portions of the external narial border and the middle portion
of the palate have been weathered away. The skull was a
single unit prior to preparation (Fig. 2), but since that time it
has been separated into four pieces: an anterior skull block
that includes the upper snout and anterior palate; a posterior
skull block that includes braincase, posterior palate, skull
roof and temporal region; and the two jaw rami. Most of the
skull has been completely cleared of matrix, but matrix still
fills the braincase and temporal region of the posterior skull
block.
The shape of the skull has been distorted by deforma-
tion during preservation. Transverse compression is readily
observed in both skull blocks. The posterior skull block
has a triangular cross-section in anterior view formed
by the flat skull roof and the approximated lower por-
tions of the skull (quadratojugal and jugal). The anterior
skull block bears signs of transverse compression be-
cause the midline elements of the snout have been forced
past one another and the teeth have been displaced (see
‘Premaxilla’, below). Forward and upward shearing of the
right side of the skull is evidenced by the dislocated
lower jaw and crushed lateral temporal region visible in
the unprepared skull (Fig. 2), as noted by Salgado &
Calvo (1997), Calvo et al. (1998b) and Upchurch (1999).
This deformation can be recognised in other skull elements,
such as the snout bones, as discussed below. More specific
comments about the preservation and distortion of each skull
element is provided below.
Dermal roof complex
The dermal roof shield is made up of tooth-bearing ele-
ments (premaxilla, maxilla), median roofing elements (nasal,
frontal, parietal), circumorbital elements (postorbital, pre-
frontal, lacrimal, jugal) and temporal elements (squamosal,
quadratojugal). The skull has suffered some transverse com-
pression, which has affected the shape of the supratemporal
openings as well as the positions of the tooth-bearing ele-
ments. The cheek and eye regions of Nemegtosaurus are well
preserved on both sides of the posterior skull block (Figs 3
& 4). Elements comprising them, in particular those of the
lateral temporal region, have suffered deformation resulting
Figure 2 Skull and lower jaws of Nemegtosaurus mongoliensis
prior to preparation in right lateral (A), left lateral (B), and anterior
(C) views. Photographs are from Nowinski (1971: pl. 8; pl. 13, fig.2).
Scale bar =20 cm.
from the forward shearing of the right side of the skull. This
deformation has altered the shape of the various lateral skull
openings on the right side, which formed the basis of Nowin-
ski’s (1971: fig. 1) skull reconstruction, the source for most
comparisons (e.g. McIntosh 1990; Salgado & Calvo 1992;
Upchurch 1995, 1999).
Premaxilla
The bodies of the right and left premaxillae are nearly com-
pletely preserved, except for an eroded area near the base
of the ascending or dorsal processes. Very damaged por-
tions of the premaxillary ascending processes are preserved
in contact with the dorsal process of the maxilla. Their distal
extremes and contact to the body of the premaxilla, however,
are missing.

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
287
v
paof
m
ect
pt
pt
la
os
ls
j
amf
l ampr pm
n
prf
fr
q
pop
po
sq
qj
stfo
pm
m8
m6
m5
m3
m2
pm3
pm4
la
Figure 3 Stereopairs and interpretive line drawing of the prepared skull of Nemegtosaurus mongoliensis (Z. PAL MgD-I/9) in left lateral view.
The anterior (snout) portion of the skull has been rotated slightly out of contact with the posterior portion of the skull (compare with Figure 2). In
this and other line drawings, cross-hatching indicates broken bone, pattern indicates matrix, grey tone denotes elements from the opposite side
of the skull and light tone lines indicate anatomy obscured by reconstructed areas. Abbreviations used in Figures 3–15, 17, 18: addfo, adductor
fossa; amf, anterior maxillary fenestra; ampr, anteromedial process; an, angular; aof, antorbital fenestra; as, articular surface (used in
conjunction with other abbreviations, e.g. as ect); asaf, anterior surangular foramen; aspr, ascending process; bo, basioccipital; bpt,
basipterygoid process; bs, basisphenoid; bt, basal tuber; cor, coronoid; d, dentary; den f, dental foramen; ect, ectopterygoid; eo-op,
exoccipital–opisthotic; en, external nares; f, foramen; fm, foramen magnum; fo, fossa; fr,frontal;gr, groove; j, jugal; l,left;la, lacrimal; la f,
lacrimal foramen; ls, laterosphenoid; ltf, lateral temporal fenestra; ltfo, lateral temporal fossa; m, maxilla; mgr, Meckel’s groove; n, nasal; oc,
occipital condyle; or, ornamentation; os, orbitosphenoid; p, parietal; pal, palatine; paof, preantorbital fenestra; pm, premaxilla; po, postorbital;
pop, paroccipital process; prf,prefrontal;psaf, posterior surangular foramen; pt, pterygoid; ptf, post-temporal foramen; q, quadrate; qfo,
quadrate fossa; qj, quadratojugal; ri,ridge;sa, surangular; sh,shelf;so, supraoccipital; sp,spur;spl, splenial; sq, squamosal; sr, sclerotic ring;
stf, supratemporal fenestra; stfo, supratemporal fossa; sy, symphysis; v, vomer; Arabic numerals indicate tooth position; Roman numerals
indicate openings for cranial nerves. Scale bars =10 cm.
The particular arrangement of the bones and bone frag-
ments of the snout offers some information as to the principal
forces of deformation that acted on them during preserva-
tion. Due to the transverse compression of the skull, the left
premaxilla has been shifted medially and interposed between
the right premaxilla and its teeth (Figs 5 & 6). The first tooth
on the right premaxilla has been shifted over the midline,
so that it is partially bordered by the labial portion of the
left premaxilla. The four teeth of the right premaxilla are
likewise shifted laterally by nearly one alveolus – the fourth
tooth is positioned at the junction between the premaxilla and
maxilla. The forward shearing of the right side of the snout,
which shifted the anteromedial process of the right max-
illa to a position anterior of the left premaxillary ascending

288 J. A. Wilson
m
j
pt
pal
pm
f
qj
bo
ltf
q
sa
pop
q
qfo
pt
fo
m8
paof
la
?m9
sq
po
m7
m5
m2 pm4
stfo
prf
os
sr
ls
fr
n
m4
gr
Figure 4 Stereopairs and interpretive line drawing of the prepared skull of Nemegtosaurus mongoliensis (Z. PAL MgD-I/9) in right lateral
view. See Figure 3 for abbreviations. Scale bars =10 cm.
process, apparently took place after compression, because it
likewise affected the left premaxilla. By virtue of its intercal-
ation with its opposite, the left premaxilla was drawn slightly
out of articulation with the body of the maxilla as the right
premaxilla was sheared forward. This deformation, as well as
the truncation of the dorsal process of the premaxilla creates
the impression of a step in the dorsal margin of the snout in
lateral view (Fig. 4), a preservational artifact that Salgado &
Calvo (1997: fig. 8) incorporated into their reconstruction of
Nemegtosaurus.
The premaxilla is triangular in anterior view, with
a broad ventral (alveolar) margin and a body that tapers
dorsally. Along the length of their symphyseal margin, each
premaxilla bears a paramedian ridge that is approximately
the breadth of one alveolus (Fig. 5). The ridge is flanked by a
shallow groove formed by bone that bears a coarse, transverse
orientation. Due to damage, the dorsal extent of the parame-
dian ridges and flanking grooves cannot be ascertained. Each
premaxilla has four alveoli arranged in a fairly flat arch. Be-
cause little of their symphysis is visible, the orientation of
the premaxillae relative to the axis of the skull cannot be
determined with certainty. The shape of the symphyseal por-
tion of the dentaries, however, suggest that they were nearly
transversely orientated (see ‘Dentary’ below). The exposed
portion of the medial face of the right premaxilla (Fig. 3)
reveals a relatively narrow symphyseal contact, although this
must be confirmed on additional specimens. A portion of the
ascending process of the left premaxilla is preserved adja-
cent to the left maxilla. It overlaps the anteromedial process
of the maxilla and partially covers an elongate opening in
the maxilla (see ‘Maxilla’ below). The ascending process of
the premaxilla is long, straight and flat. No margin of the
external naris is preserved.
There are dental foramina on the medial surface of the
premaxilla, corresponding to each of the four teeth. They
are teardrop-shaped and increase in size laterally. Only three
dental foramina on the right premaxilla are visible in ventral
view, however, because the opening associated with the first

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
289
Figure 5 Stereopairs and interpretive line drawing of the snout of Nemegtosaurus mongoliensis (Z. PAL MgD-I/9) in anteroventral view. The
left premaxilla has been interposed between the right maxilla and its teeth by compressive forces acting during fossilisation. See Figure 3 for
abbreviations. Scale bars=10 cm.
tooth is obscured by the left premaxilla (Fig. 6). Low ridges
extend toothward from the ventral margin of each dental
foramen. These ridges separate shallow depressions. Close
packing of the right premaxillary teeth is due to preserva-
tional compression. Those of the left side are separated by
gaps of 4–6 mm (Fig. 6).
In lateral view, the body of the premaxilla has a gently
rounded anterior margin (Figs 3 & 4). Unfortunately, the
critical region between the body and ascending process of
the premaxilla is not preserved, precluding assessment of a
stepped or gradual transition between the two. No narial fossa
is preserved on the preserved portion of the premaxilla, but its
absence cannot be confirmed in this specimen. The bodies of
the premaxilla and maxilla have gently sinuous articular mar-
gins that indicate alternating overlap between the two along
their length (Figs 3 & 5). The maxilla overlaps the prema-
xilla near the upper and lower thirds of their contact, but
the premaxilla overlaps the maxilla along the middle third.
The transition between these sections is marked by a slight
punctuation of what is, for the most part, a sinuous margin.

290 J. A. Wilson
Figure 6 Stereopairs and interpretive line drawing of the snout of Nemegtosaurus mongoliensis (Z. PAL MgD-I/9) in ventral view. Right stereo
at top. See Figure 3 for abbreviations. Scale bars =10 cm.
Maxilla
Portions of both maxillae are preserved. In general, the tooth-
bearing region of the maxilla is better preserved than is the
more delicate cheekward region, which is preserved only on
the right side (Fig. 4). The alveolar margin of the maxilla
is nearly completely preserved on the left side and enough
of its post-dentigerous portion is preserved to reconstruct its
shape (Figs 3 & 4). The ascending process of the maxilla
is preserved up to the anterior maxillary fenestra, but absent
distal to that opening. As a consequence, the shape of the two
openings it borders, the external naris and antorbital fenestra,
cannot be determined.
Transverse compression has approximated the right and
left maxillae such that their palatal shelves are separated by
only a few centimetres (Fig. 6) and forward shearing of the
right side of the skull brought a portion of the right maxillary
anteromedial process anterior to the left premaxillary ascend-
ing process. The left anteromedial process was detached from
the remainder of the maxilla but is preserved in correct ana-
tomical position on the palatal surface of the skull (Fig. 6).
Nowinski (1971: 65), however, associated the left maxillary
anteromedial process with the adjacent vomer and described
them together as a ‘T-shaped vomer’. As noted by Upchurch
(1999), the crossbar belongs to the maxilla (see ‘Vomer’,

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
291
below) and it closely resembles that of Quaesitosaurus (see
Fig. 17).
The maxillary tooth row terminates anterior to the pre-
served portion of the antorbital fenestra, whose anterior
limit cannot be determined. Despite this incompleteness, Ne-
megtosaurus did not possess the anteroposteriorly elongate
antorbital fenestra that characterises Rapetosaurus (Curry
Rogers & Forster 2004). The tooth row probably terminated
anterior to the antorbital fenestra, a feature also present in dip-
lodocoids (Upchurch 1998, 1999; Wilson 2002). There are
eight teeth preserved on the left side and either eight or nine
on the right. The uncertainty stems from damage in the pos-
terior region of the snout (Fig. 6). Nowinski (1971: table 1)
and McIntosh (1990: 393) listed maxillary tooth counts of
eight, but there appears to be one missing tooth from the left
side. Nine maxillary teeth and four premaxillary teeth would
match the dentary tooth count, which is 13 (Nowinski 1971:
70; see ‘Dentary’ below). Eight teeth were preserved in asso-
ciation with the Rapetosaurus maxilla and Curry Rogers &
Forster (2004) estimated that there would have been a total of
12–14 maxillary teeth. With four premaxillary teeth, which
are standard in all sauropods, this estimate implies a 5–8 tooth
mismatch between the 16–18 upper teeth and the 11 lower
teeth in Rapetosaurus. The dentary of the titanosaur Malaw-
isaurus bears 15 alveoli (Jacobs et al. 1993), implying a max-
illary tooth count of 11 if there were four premaxillary teeth
and equal numbers of upper and lower teeth. In this context,
the reduced maxillary tooth count in Nemegtosaurus,Quaes-
itosaurus (see Fig. 17) and Rapetosaurus may be a diagnostic
feature, but more titanosaur tooth counts are needed. The
entire tooth-bearing portion of the maxilla is coursed by deep
vascular grooves that run towards the alveolar margin of the
jaw. Some of these grooves anastamose distally. Nearly all of
these openings are positioned ventral to the anterior maxillary
foramen and anterior to the last tooth. This highly vascular-
ised region of the snout is delimited by a shallow transverse
groove (Figs 3 & 4). A highly vascularised anterior maxilla is
also present in Diplodocus (Wilson & Sereno 1998: fig. 6B);
less pronounced neurovascular grooves are present in the
maxillae of Apatosaurus (Berman & McIntosh 1978: fig. 7A)
and Dicraeosaurus (Janensch 1935–1936: fig. 108). Quaes-
itosaurus (PIN 3906/2) also bears a highly vascularised
snout, but this feature cannot be determined in other ti-
tanosaurs because complete maxillae have not yet been
described.
Just posterior to the tooth row and to the transverse
groove is an enlarged oval depression that spans nearly 10 cm
along its anteroposterior axis. Within this depression are two
openings, which Nowinski (1971: fig. 2) termed ‘infraorbital
fenestrae’. Madsen et al. (1995: 9) regarded them as an-
terior maxillary fenestrae, but Upchurch (1999: 112, fig. 2)
regarded them as preantorbital fenestrae. The larger, more
posteriorly positioned opening exits from a space enclosed
by the medial portion of the maxilla, which is here termed
the ‘palatal canal’. I consider this to be the preantorbital
opening and the much smaller, more anteriorly placed open-
ing to be an accessory foramen. The palatal canal is trian-
gular in cross-section, with its base formed by the palatal
shelf and its sides formed by medial and lateral aspects of
the maxilla. The lateral wall of the canal is extremely thin
(1–2 mm) near the exit of the preantorbital fenestra. The me-
dial wall of the canal is thicker and apparently contiguous
with the maxillary ascending and anteromedial processes.
In most sauropods, the anteromedial process is tab-like and
medially orientated, whereas the ascending process is elong-
ate and posterodorsally oriented. In Nemegtosaurus,how-
ever, these two processes are merged into a single, medial
sheet of bone that underlies the ascending process of the
premaxilla. An elongate opening enters the palatal canal at
the junction of these processes and the body of the max-
illa. This large opening (45 mm long, 10 mm wide) was re-
ferred to as the ‘intermaxillary foramen’ by Nowinski (1971:
fig. 1) and as the ‘subnarial foramen’ by Upchurch (1999:
111, fig. 2). Unlike the subnarial foramen, which passes
between the premaxilla and maxilla in saurischians (Ser-
eno & Novas 1993), the opening in Nemegtosaurus enters
the maxilla via the palatal canal. I regard this opening as the
anterior maxillary foramen, based on topological correspond-
ence with that opening in other sauropodomorphs (Plateo-
saurus,Camarasaurus, Wilson & Sereno 1998: figs 5A, 7C;
Brachiosaurus, Janensch 1935–1936: fig. 42). The subnarial
foramen may have been located just below the anterior max-
illary foramen, to which it was linked by a short groove.
However, only the maxillary portion of such an opening is
preserved; position of the subnarial foramen must be con-
firmed in other specimens. It is not known whether the sub-
narial foramen in Nemegtosaurus is reduced in size, lost, or
modified.
In lateral view, the ventral margin of the maxilla is
arched immediately posterior to the tooth row (Figs 3 & 4).
The curve of the post-dentigerous maxilla appears smooth
and it is both longer and more arched than in other sauro-
pods. At its posterior extreme, the maxilla overlaps the jugal
along a margin that is roughly concave posteriorly. The elong-
ate ventral process of the maxilla extends below the jugal,
which does not contribute to the ventral margin of the skull.
Although it appears that the maxilla contacted the quadrato-
jugal, the nature and extent of this contact cannot be determ-
ined because the latter has been shifted forward and out of
place. The posterior maxilla is much deeper than, and less
arched than, the corresponding portion of the Rapetosaurus
maxilla, which may be autapomorphic in these regards (Curry
Rogers & Forster 2004: figs 1, 3–4).
Nasal
Portions of the right and left nasals are preserved along their
contacts with the prefrontal and frontal (Fig. 7). Nearly the
entirety of the nasal ventrolateral process is preserved on the
left side of the skull. Its distalmost tip, however, is miss-
ing and may have extended further alongside the prefrontal
(Fig. 8). The midline contact of the nasals – and thus part
of the margin of the external naris – has not been preserved
(Fig. 7), but small portions of the right and left anterior
process offers some information on the three-dimensional
orientation of the naris (Figs 3 & 4).
The ventrolateral process of the nasal is tongue-like and
dorsoventrally deep. At its distal extreme, the ventrolateral
process of the nasal contacts the anteromedial surface of
the lacrimal, which separates it from the prefrontal. It has
a smooth internal surface that represents the posterolateral
margin of the external naris, which was retracted to a posi-
tion between the prefrontals, as reconstructed by Nowinski
(1971: fig. 2) and Upchurch (1999: fig. 6D). The size and
shape of the external naris, however, was not preserved. Now-
inski (1971: 66) stated his uncertainty about the presence of
an internarial bar, but reconstructed confluent external nares

292 J. A. Wilson
la
n
stfo
stf
as prf
as fr
prf
fr
po
ptf
p
or
eo-op
pop
so
sq
Figure 7 Stereopairs and interpretive line drawing of the posterior portion of the skull of Nemegtosaurus mongoliensis (Z. PAL MgD-I/9) in
dorsal view. Right stereo at top. See Figure 3 for abbreviations. Scale bars =10 cm.
(1971: fig. 2). Upchurch (1995) scored the external nares as
confluent and dorsally facing, but more recently (1999: 111)
has stated that neither can be determined. Curry Rogers &
Forster (2004: 127) regarded the external nares of Nemeg-
tosaurus as fully retracted and confluent, in agreement with
the presumed condition in Rapetosaurus. The anterior pro-
cess of the nasal is poorly preserved in Nemegtosaurus,but
nonetheless indicates a broad internarial bar (>4cm) that
would have tapered anteriorly. In lateral view, the anterior
process diverges from the lateral process, indicating that the
external nares were somewhat laterally orientated (Figs 3 &
4 and see Fig. 16).
Posteriorly, the nasal overlaps the frontal, which is ex-
tremely thin at its anterior extreme. Nearly half of this contact
has been preserved on the right side, but slightly less is pre-
served on the left. From this limited evidence, it appears
that the frontal–nasal contact was angled slightly postero-
medially, as suggested by Nowinski (1971: 65), but better
specimens are required to confirm this.
Frontal
The frontals are completely preserved on both right and left
sides, but the right side has been slightly compressed trans-
versely and the left has been damaged in the region of its
prefrontal articulation (Fig. 7).
An interdigitated suture clearly separates right and left
frontals, which are each transversely elongate and together
hexagonal in dorsal view. The lateral margin of the frontal
is concave where it flanks the prefrontal but convex at its
greatest breadth near mid-length (Fig. 7). Its orbital mar-
gin bears roughened ornamentation that continues anteriorly
and posteriorly on the prefrontal and postorbital, respectively
(Figs 7 & 8).
The frontal is positioned between the nasal and pari-
etal along the midline and between the prefrontal and post-
orbital along the orbital margin of the skull. As discussed
above, the frontal–nasal contact is not well preserved, but
the frontals appear to have been concave forward in dorsal
view. Although the frontals are fairly flat transversely, a shal-
low but marked anteromedial depression is present near their
contact with the nasals, as in Saltasaurus (PVL 4017-162)
and Quaesitosaurus (PIN 3906/2). The paramedian doming
present on the frontals of Rapetosaurus (Curry Rogers &
Forster 2004: fig. 13) is not present in Nemegtosaurus.The
prefrontal–frontal articulation is also concave forward, but
asymmetrically so, with a greater forward excursion on the
orbital margin (Fig. 7). The parietal–frontal contact is relat-
ively straight but angled slightly posterolaterally. The suture
is interdigitated and was probably a butt-joint, as in other
sauropods. Although at its lateral extreme the frontal ap-
proaches the supratemporal opening, it does not participate
in the supratemporal fossa. The frontal–postorbital contact is
vertical and planar, but orientated anterolaterally rather than
posterolaterally.
Ventrally, the frontal contacts elements of the lateral
wall of the braincase. Its suture with the orbitosphenoid

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
293
prf
la
la
j
n
fr
stfo
po
os
ls
p
l
lll
ll
lV
V
qj
pop
pt
qfo
q
sq
sp
ltfo
Figure 8 Stereopairs and interpretive line drawing of the posterior portion of the skull of Nemegtosaurus mongoliensis (Z. PAL MgD-I/9) in
left lateral view. See Figure 3 for abbreviations. Scale bars =10 cm.
anteriorly is interdigitated, whereas that with the laterosphe-
noid appears smooth. A well defined ridge projects anter-
olaterally from the underside of the frontal at the anterior
extreme of its orbitosphenoid contact. Present in many
sauropods, this ridge separates the lateral, orbital portion of
the skull from its medial, narial portion.

294 J. A. Wilson
Figure 9 Stereopairs and interpretive line drawing of the posterior portion of the skull of Nemegtosaurus mongoliensis (Z. PAL MgD-I/9) in
posterior view. Right stereo at top. See Figure 3 for abbreviations. Scale bars =10 cm.
Parietal
The parietals are nearly completely preserved, except for a
median gap near their contact with the supraoccipital. As
the posteriormost element of the skull roof, the parietal
has a broad exposure both dorsally and posteriorly
(Figs 7 & 9).
In dorsal view, the anterior and posterior margins of
the parietal are roughly parallel to one another. No pari-
etal or postparietal openings are present (contra Nowinski
1971: fig. 2). The parietal has a short, flat midline suture
and two lateral processes that contact the postorbital in front
of and behind the supratemporal fenestra. The anterior lat-
eral process is shorter and narrower than the longer and
thicker posterior process. This double contact excludes the
squamosal from the margin of the supratemporal fenestra
(Nowinski 1971; Upchurch 1995). Rapetosaurus does not
have a well developed anterior lateral process and the frontal
appears to have bordered the supratemporal fenestra (Curry
Rogers & Forster 2004: figs 2, 15C). Anteriorly, the pari-
etal is flat transversely, but posteriorly it is raised so that
the posterior wall of the supratemporal opening is visible in
lateral view although the opening itself is not (Fig. 8). The
supratemporal fenestra is elliptical, with its long axis canted
10–15◦posterior of the transverse axis of the skull. Right
and left openings are separated from the midline of the
skull by a distance surpassing the greatest diameter of
each.
In posterior (occipital) view, the parietal is sand-
wiched between the supraoccipital, postorbital, exoccipital–
opisthotic and squamosal (Fig. 9). The well developed
occipital fossa on the posterior parietal is bounded by a raised
ridge of bone. The parietal is strap-shaped in posterior view,
and its greatest height is less than that of the foramen mag-
num. The parietal has a broad, flat contact with the postor-
bital laterally and is contacted by a narrow isthmus of the
squamosal just ventral to this. By virtue of these contacts,
the parietal is excluded from the margin of the post-temporal
fenestra.

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
295
Figure 10 Stereopairs and interpretive line drawing of the posterior portion of the skull of Nemegtosaurus mongoliensis (Z.PAL MgD-I/9) in
anterior view. Right stereo at top. See Figure 3 for abbreviations. Scale bars =10 cm.
Postorbital
The postorbitals are complete on both sides of the skull
(Figs 3 & 4). As a result of the transverse compression that
has bent inwards their ventral processes (Fig. 10), both post-
orbitals have been pulled out of articulation with the frontals
(Fig. 7). The right postorbital has suffered additional crush-
ing, which has artificially reduced the size of the right supra-
temporal fenestra. The left appears undistorted.
The postorbital is T-shaped in lateral view, with an
elongate ventral process and relatively short anterior and
posterior processes (Fig. 8). The anterior process is approx-
imately half as deep as the posterior process, giving the post-
orbital the appearance of a percussion hammer. The anterior
process of the postorbital contacts the frontal and parietal me-
dially, separating the orbit from the supratemporal fenestra.
The anterior process is heavily ornamented, especially near
its contact with the frontal (Figs 7 & 8), as in Quaesitosaurus
(PIN 3906/2). It meets the frontal in a deep, vertical suture
that is directed anterolaterally. The posterior process of the
postorbital is dorsoventrally deep and contacts the anterior
process of the squamosal laterally (Fig. 8) and the lateral
processes of the parietal dorsally (Fig. 7). The posterior pro-
cess separates the supratemporal and lateral temporal fenes-
trae. Its articulation with the squamosal is planar, near ver-
tical and orientated transversely. This condition differs from
the tongue-and-groove postorbital–squamosal articulation of
other dinosaurs. Although the postorbital is unknown in Ra-
petosaurus, the presence of a triangular notch on the lateral
aspect of the squamosal suggests it had a typical postorbital–
squamosal contact (Curry Rogers & Forster 2004: fig. 18). In
dorsal view, the posterior process of the postorbital contacts
both termini of the C-shaped portion of the parietal, form-
ing the lateral margin of the supratemporal opening (Fig. 7).
A shallow supratemporal fossa is present on the postorbital.

296 J. A. Wilson
There is no orbital ornamentation on the posterior process of
the postorbital.
The ventral process of the postorbital, which separates
the lateral temporal fenestra from the orbit, is slightly con-
cave along its orbital margin. The ventral process is narrow
anteroposteriorly and broad transversely towards its prox-
imal end. The ventral process narrows transversely towards
its distal end, where its anteroposterior breadth exceeds its
transverse breadth. The ventral process bears ornamentation
near its junction with the anterior and posterior processes, but
this texture disappears near its midlength (Fig. 8). The distal
end of the ventral process bears a rounded margin, which
contacts the posterodorsal process of the jugal. As evidenced
by the matrix separating it from the jugal on the right side, the
postorbital has been displaced anteriorly out of articulation
(Fig. 4). The postorbital–jugal contact is better preserved on
the left side, where it appears that the postorbital contacted
the lacrimal to exclude the jugal from the margin of the orbit
(Figs 3 & 8).
Prefrontal
The prefrontal is complete on the right side and nearly so
on the left, which lacks only its distal tip (Figs 7 & 10).
Both prefrontals have been damaged somewhat near their
connection to the frontal.
The prefrontal is tongue-shaped and downwardly
curving. It contacts the frontal posteriorly, the nasal me-
dially and the lacrimal anteroventrally. The prefrontal forms
the anterodorsal margin of the orbit and is marked by heavy
ornamentation in that region (Fig. 8). In contrast to the ridge-
like ornamentation of the frontal, the prefrontal orbital orna-
mentation consists of numerous small, pointed projections.
The prefrontal of Rapetosaurus bears much more subtle
ornamentation near its contact with the frontal (Curry
Rogers & Forster 2004: fig. 11).
Posteriorly, the prefrontal is embraced by an embay-
ment in the frontal (Fig. 7). The posterior margin of the
prefrontal is smooth and lacks the posteromedially orient-
ated ‘hook’ characteristic of diplodocids (Upchurch 1998;
Wilson 2002). The anterior process of the nasal parallels the
prefrontal and excludes it from the margin of the external
naris (Fig. 7). The prefrontal and nasal were separated by the
lacrimal anteriorly. The prefrontal overlaps the lateral sur-
face of the lacrimal on a well developed articular facet that
is bounded by a low lip.
Lacrimal
The right lacrimal is nearly complete except for a narrow
interval at midlength that was lost during preparation of the
specimen (see Figs 2(A) and 4). A small portion of the prox-
imal left lacrimal is preserved adjacent to the nasal and a
small piece of the distal end is present in articulation with
the jugal (Figs 3 & 8).
The lacrimal is a nearly vertical element that separates
the orbit from the antorbital fenestra. It bears no ornament-
ation, unlike other elements bordering the orbit (namely the
prefrontal, frontal & postorbital). The lacrimal has a pos-
terior projection that separates the nasal and prefrontal. The
lacrimal is broad anteroposteriorly in lateral view and its
base rests on the lateral surface of the jugal. The proximal
lacrimal, which bears a lacrimal canal whose anterior exit
is visible medially (Fig. 8), is approximately twice as broad
transversely as anteroposteriorly. A shallow groove is present
on the medial aspect of the lacrimal. The posterior ridge that
defines this groove is a continuation of the ventral ridge on
the frontal, which separates the orbital and nasal cavities.
The anterior process of the lacrimal is incomplete but ap-
pears to have been well developed in Nemegtosaurus. It does
not appear to have been as elongate as that of Rapetosaurus,
which may be autapomorphic (Curry Rogers & Forster 2004:
fig. 7).
Nowinski (1971: fig. 2) reconstructed the lacrimal on
the margin of the external naris, as did both Upchurch (1999:
fig. 2B) and Curry Rogers & Forster (2004: 126). However,
the lateral process of the nasal and the dorsal process of the
maxilla are not completely preserved and it cannot yet be
determined whether they contacted to exclude the lacrimal
from the external naris. I have conservatively reconstruc-
ted a nasal–maxilla contact as is present in other sauropods
(Figs 3 & 4 and see Fig. 16).
Jugal
The jugal is well preserved on both sides of the skull, lacking
only portions of its antorbital margin and its posterior process
(Figs 3, 4 & 8). The medial aspect of both the left and right
jugal is covered by matrix. The jugal connects temporal,
circumorbital and tooth bearing elements of the skull and,
in doing so, approaches or borders the margins the lateral
temporal fenestra, orbit and antorbital fenestra.
The jugal is a large, flat element marked by a pos-
terior embayment that represents the anteroventral margin
of the lateral temporal fenestra. On either side of this em-
bayment extend processes of unequal length, the longer of
which projects posterodorsally along the posterior surface
of the postorbital. The shorter, posteroventral process is not
completely preserved on either side, but its brevity can be
confirmed from its overlap facet on the anterior process of
the quadratojugal (Fig. 8). One foramen opens on the jugal,
just below its contact with the lacrimal. This opening is pre-
served on both sides of the skull of Nemegtosaurus (Figs 3 &
4). This opening is absent in Rapetosaurus (Curry Ro-
gers & Forster 2004: fig. 6), and its presence cannot be
confirmed in Quaesitosaurus, which does not preserve a
jugal.
Anteriorly, the jugal is overlapped by the maxilla. As
mentioned above, the bone in the cheek region is extremely
thin, but it appears that the maxilla did not exclude the jugal
from the antorbital fenestra, contra reconstructions by Now-
inski (1971: fig. 1) and Salgado & Calvo (1997: fig. 8). The
jugal of Rapetosaurus maintains short orbital and antorbital
margins (Curry Rogers & Forster 2004:125–126).
Squamosal
The squamosal is nearly completely preserved on both sides,
lacking only the distal terminus of its ventral process (Figs 3,
4 & 8). The medial aspect of both the left and right squamosal
is obscured by matrix. The left squamosal has been disartic-
ulated from the quadrate head and its ventral process has
been displaced medially to a position within the quadrate
fossa, along with a piece of the quadrate (Fig. 9). The right
squamosal is also deformed, having been levered forward
with the forward shearing of the right side of the skull. Con-
sequently, the squamosal ventral process has been pushed
into the lateral temporal fenestra. Deformation has artifi-
cially reduced the angle of the apex of the lateral temporal
fenestra on both sides.

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
297
The squamosal forms the posterodorsal corner of the
skull, contacting the skull roof, occipital and temporal ele-
ments. The squamosal is triradiate, with an elongate vent-
ral process, an abbreviate anterior process and a narrow,
medially-directed occipital process. The anterior process of
the squamosal contacts the postorbital to form the temporal
bar, which separates the lateral temporal and supratemporal
fenestrae. The postorbital–squamosal contact is vertical and
flat, which differs from the tongue-and-groove articulation of
other dinosaurs. Although it forms part of the temporal bar,
the squamosal does not enter the margin of the supratem-
poral fenestra, which is enclosed solely by the parietal and
postorbital. This condition is shared by Quaesitosaurus (PIN
3906/2) but is not present in Rapetosaurus (Curry Rogers &
Forster 2004: 134).
The posterior aspect of the squamosal bears a medially-
directed occipital process that separates the lateral portions of
the parietal and exoccipital–opisthotic, excluding the former
from the border of the post-temporal fenestra (Fig. 9). The
posterior surface of the squamosal is overlapped by the paroc-
cipital process, a contact that is marked by a sharply defined
facet bounded by a ridge. The squamosal ridge and those of
the postorbital and parietal bound a shallow occipital fossa.
The occipital process of the squamosal wraps around the
posterior aspect of the quadrate head, which is visible pos-
teriorly (Fig. 9). In addition to the squamosal, the quadrate is
supported posteriorly by the paroccipital process. The nature
of the squamosal–quadrate articulation cannot be assessed in
this specimen because this region has not yet been prepared,
but it probably resembled that of other saurischians, in which
the squamosal has a well developed socket that receives the
convex head of the quadrate.
The ventral process of the squamosal forms much of
the posterior margin of the lateral temporal fenestra and a
shallow lateral temporal fossa is preserved on its anterior
aspect (Fig. 8). In lateral view the ventral process is con-
cave posteriorly, which results in an anteriorly-broadening
lateral temporal fenestra. The ventral process is not com-
pletely preserved on either side, but it probably contacted the
posterior quadratojugal, based on the relationship between
these bones preserved on the left side (Fig. 8). A squamosal–
quadratojugal contact was suggested by Nowinski (1971)
and Upchurch (1999), but Madsen et al. (1995) concluded
that they did not contact. Together, the ventral process of the
squamosal and the dorsal process of the quadratojugal form
the lateral boundary of a broad quadrate fossa (Fig. 9), as
indicated by Nowinski (1971: 67). A small ‘spur’ is present
on the posterior surface of the ventral process, near the base
of the three-bone junction involving the paroccipital process
and the quadrate (Fig. 8). This spur is unknown elsewhere in
Sauropoda and may represent an autapomophy of Nemegto-
saurus.
Quadratojugal
The quadratojugal has been crushed forwards on both sides,
the right side more so than the left (Figs 2–4). Neverthe-
less, the body and anterior process of the quadratojugal are
well preserved on both sides. Its dorsal process has been
damaged on the right side, but is better preserved on the left
(Fig. 8). The left quadratojugal is not obscured by other skull
elements and provides most of the valuable characters of this
element.
The quadratojugal forms the ventrolateral corner of the
skull and of the lateral temporal fenestra. Like the squamosal,
it overlies the quadrate. The quadratojugal–quadrate contact
is marked by a surface of roughened bone on the distolateral
quadrate. In lateral view, the posterior portion of the quad-
ratojugal is rounded and projects beyond the dorsal process.
The bone in this region bears weak transverse striae that can
be seen on both right and left sides. Preserved portions of
the dorsal process of the quadratojugal indicate that it was
elongate and probably contacted the anterior surface of the
squamosal, but the shape of the element and the nature of
its contact with the quadrate and squamosal is not certain
(Figs 3 & 4). The body and dorsal process of the quadrato-
jugal form the ventrolateral boundary of the quadrate fossa.
The anterior process of the quadratojugal is tongue-
shaped, with a slightly convex dorsal margin and a sinuous
ventral margin (Fig. 8), as in Quaesitosaurus (Kurzanov &
Bannikov 1983: fig. 1). This condition is present but less pro-
nounced in Diplodocus, which also bears an elongate anterior
process (Wilson & Sereno 1998: fig. 6A). The dorsal margin
of the anterior process of the quadratojugal is overlapped by
the jugal, a contact is marked by a distinct facet that deepens
anteriorly. As discussed above, the quadratojugal contacted
the posterior end of the maxilla to exclude the jugal from the
ventral skull margin, but the nature and extent of that contact
is unknown (see ‘Maxilla’, above).
Palatal complex
The palatal complex of Nemegtosaurus is represented by the
paired vomers, pterygoids, quadrates and ectopterygoids. Of
these elements, only the incomplete vomers and the quadrate
have been adequately figured (Nowinski 1971: pls 11–12);
other palatal elements have received less attention and the
palate of Nemegtosaurus has never been reconstructed. In
part, this is attributable to the incomplete preservation of
the palatal elements, but also to their derived morphology.
The supposed absence of an ectopterygoid has provided ad-
ditional difficulty. Below the five palatal elements are re-
described and reinterpreted.
Vomer
Fragments of the right and left vomer are preserved, but
the nature of their connection to other palatal elements pos-
teriorly and to the dermal skull anteriorly was not preserved
(Fig. 6). Nowinski (1971: 64) described the vomer as a single
element comprising an elongate median process and a trans-
versely orientated crossbar positioned just posterior to the
premaxillae. McIntosh & Berman (1975: 195) questioned
this interpretation and suggested that Nowinski’s T-shaped
element was actually two elements; the elongate portion rep-
resenting part of one paired vomer (they did not specify
right or left) and the crossbar representing part of some other
element. Upchurch (1999: 113) agreed that the elongate pro-
cess represented one of the vomers and that, the crossbar
represented ‘broken portions of the vomerine processes [of
the maxilla]’. Re-examination of the palate indicates that the
crossbar represents the left maxillary anteromedial process
(Fig. 6; see ‘Maxilla’, above) and the elongate portion repres-
ents the paired right and left vomers. No posterior divergence
can be recognised between the two vomers, which are incom-
plete. In other sauropods, these elements diverge posteriorly

298 J. A. Wilson
to embrace the tips of the anterior processes of the pterygoids
(e.g. Camarasaurus;Madsenet al. 1995: fig. 5B).
The anterior contact of the vomers cannot be observed
in this specimen. Their preserved position, however, suggest
that the vomer contacted the anteromedial process of the
maxilla (Fig. 6). A posteriorly directed vomerine process is
not present on the posterior aspect of the premaxilla, as it is
in Camarasaurus (Madsen et al. 1995: fig. 7).
Ectopterygoid
In sauropod outgroups (Prosauropoda and Theropoda), the
palatine and ectopterygoid flank the vomer and pterygoid
in a fairly consistent fashion. The palatine of Plateosaurus
contacts the maxilla laterally, the pterygoid medially and the
vomer anteriorly (Galton 1984). In Herrerasaurus, the palat-
ine additionally contacts the jugal laterally (Sereno & Novas
1993: fig. 8D). In both genera, the ectopterygoid contacts
the jugal laterally and the pterygoid medially to form the
transverse palatal hook. The palatine and ectopterygoid do
not contact one another and the post-palatine fenestra is re-
latively large. These topological relationships are conserved
in sauropods, but with one notable difference. Perhaps re-
lated to the overall infraorbital shortening of the eusauropod
skull, the ectopterygoid’s contact with the skull margin is
shifted forward onto the maxilla (Wilson & Sereno 1998).
By virtue of the approximation of the lateral processes of the
ectopterygoid and palatine on the maxilla, the postpalatine
foramen is much smaller in neosauropods than in their out-
groups and there is a novel posteromedial contact between
the ectopterygoid and palatine.
Only one paired marginal element is preserved in posi-
tion on the anterior palate of Nemegtosaurus (Fig. 6). Be-
cause remnants of nearly all of the other cranial elements
have been preserved, it has been assumed that one of the
marginal palatal elements did not ossify. Nowinski (1971:
64–65) identified the preserved element as the palatine, a
decision followed by McIntosh & Berman (1975), McIntosh
(1990) and Madsen et al. (1995). Nowinski (1971: 58) further
considered the ectopterygoid unossified and regarded this as
diagnostic of the genus. However, neither the anterior pteryg-
oids nor the posterior vomers have been preserved, indicating
that the mid-palate has been damaged. For this reason, a pre-
servational absence for either the palatine or ectopterygoid
cannot be ruled out.
The shape and sutural connections of the preserved mar-
ginal element may allow discrimination between the three
alternative explanations for why only one element was pre-
served: one element was not preserved, one element did not
ossify, or two elements fused into one composite element.
The preserved marginal palatal bone is elongate, straight and
orientated sub-parallel to the skull midline. Its anterior and
posterior articular extremes are flattened and orientated or-
thogonally to one another. The anterior end is dorsoventrally
compressed and tongue-shaped in ventral view, contacting
the maxilla on the underside of the palatal shelf (Fig. 6). The
element has been forwardly displaced on both sides of the
skull and extends beyond the anterior extreme of the pal-
atal shelf. In contrast, the posterior end of the element is
transversely compressed and has a vertically-orientated con-
tact with the pterygoid. Between its extremes, the element is
nearly cylindrical in cross-section. The shape of its anterior
end most closely resembles that of a typical sauropod pal-
atine, as does its elongate, uncurved shape. Unlike a typical
palatine, however, this element has no medially directed pro-
cess to abut the pterygoid medially or the vomer anteriorly.
Moreover, the palatine typically does not contribute to the
transverse palatal hook in reptiles. However, the participation
of this element in the transverse palatal hook and its anterior
connection to the palatal shelf matches the connections of
the sauropod ectopterygoid. However, the ectopterygoid is
typically hooked and orientated orthogonal to the skull axis,
as its alternative name ‘transversum’ suggests.
The topological connections of the element best agree
with those of an ectopterygoid, despite the morphological
differences listed above. I identify it here as such, raising
the question of the absence of the palatine on both sides of
the skull, as well as the nature of its connection to the max-
illa. The morphology of Quaesitosaurus is informative in
this regard. As in Nemegtosaurus, there is but one marginal
element, the ectopterygoid, preserved on both sides of the
skull of Quaesitosaurus. Its morphology is identical – it is a
strap-shaped element twisted 90◦at midlength and connect-
ing to the underside of the palate and the transverse palatal
hook. The ectopterygoid of Quaesitosaurus,however,hasnot
been forwardly displaced and a shallow facet for a second
marginal element, which I suggest is the palatine, is present
on the underside of the palatal shelf on the right side (see
Fig. 17). This facet, if also present in Nemegtosaurus, would
be obscured by the forwardly displaced ectopterygoid. Thus,
it appears that a strap-shaped ectopterygoid is present in both
Nemegtosaurus and Quaesitosaurus and that the absence of
the palatine is preservational, rather than phylogenetic.
Palatine
A fragment attached to the ventral portion of the right pteryg-
oid may represent the posterior tip of the palatine (Figs 4,
10 & 11). This piece is uninformative otherwise.
Pterygoid
The posterior (quadrate) and ventral (ectopterygoid) pro-
cesses of the pterygoid are preserved on both sides of the
skull (Fig. 11). The anterior (palatine) process is nearly com-
pletely preserved but is partially covered by matrix and ob-
scured by adjacent elements. Transverse compression of the
skull has altered the orientation of the pterygoids so that
they are nearly vertically orientated and separated by only
a narrow gap (Fig. 10). The apparent difference in pteryg-
oid orientation between Nemegtosaurus and Rapetosaurus
(Curry Rogers & Forster 2004: 139) is a preservational
artifact.
The pterygoid is platelike and its three processes are
coplanar and arranged symmetrically about a central point.
Nemegtosaurus and Rapetosaurus share this feature in com-
mon (Curry Rogers & Forster 2004), in contrast to other
sauropods in which the pterygoid processes are not coplanar
(e.g. Diplodocus, McIntosh & Berman 1975: fig. 4). The
pterygoid contacts the vomer anteriorly, the palatine and
opposite pterygoid anteromedially, the ectopterygoid vent-
rally and it is sandwiched by the basipterygoid processes
and quadrate posteriorly. The ventral (ectopterygoid) pro-
cess and posterior (quadrate) processes are nearly collin-
ear,astheyareinRapetosaurus (Curry Rogers & Forster
2004: fig. 26). The ventral process contacts the ectopteryg-
oid at nearly a right angle to form the transverse palatal
hook. The pterygoid wraps around the posterior portion of
the ectopterygoid, extending further laterally than medially

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
299
Figure 11 Stereopairs and interpretive line drawing of the posterior portion of the skull of Nemegtosaurus mongoliensis (Z.PAL MgD-I/9) in
ventral view. Right stereo at top. See Figure 3 for abbreviations. Scale bars =10 cm.
(Figs 9 & 11). The transverse palatal hook is positioned near
the middle of the preserved portion of the antorbital fenestra
(Figs 3 & 4).
The posterior process, or quadrate flange, is intercalated
between the quadrate and braincase, as visible in posterior
and ventral views (Figs 9 & 11). Its contact with the quad-
rate is flat and vertically orientated. The articular surface
of the basipterygoid bears a ventrally concave, ‘rocker-like’
shape (Fig. 9) that is shared by Rapetosaurus and Quae-
sitosaurus. The pterygoid lacks the hook-like process that
encloses the basipterygoid process in some sauropods (e.g.
Dicraeosaurus,Camarasaurus).
Quadrate
The quadrate is nearly completely preserved on both sides of
the skull, but it is fairly damaged in the region of the quad-
rate fossa. The quadrate is visible in anterior and posterior
views, but it is partially obscured by the quadratojugal and
squamosal in lateral view (Figs 8–11). The right quadrate
has been shifted anteriorly by the forward shearing of that
side of the skull, as noted previously (Salgado & Calvo 1997;
Upchurch 1999; compare Figs 3 & 4).
The quadrate forms the upper condyle of the jaw
joint and contacts the temporal elements (quadratojugal,
squamosal), occipital elements (paroccipital process, basal
tuber) and the palate (pterygoid). In posterior view, the quad-
rate cants medially from its articulation with the socket of
the squamosal on the posterodorsal corner of the skull to
contact the basal tubera (Fig. 9). Ventral to this contact, the
quadrate bends back laterally to its contact with the quadrato-
jugal at the posteroventral corner of the skull. Consequently,
the axis of the quadrate appears bent in posterior view, with
the angle of that bend occupied by the quadrate fossa. The
novel palate–braincase contact, via the quadrate and basal
tuber, is restricted to Nemegtosaurus and related forms such
as Rapetosaurus (Curry Rogers & Forster 2004: fig. 1A) and
Quaesitosaurus (see Fig. 18). The quadrate–basal tuber con-
tact, which is not a preservational artifact (contra Salgado &
Calvo 1997: 42), can be recognised in isolated braincases
by well-marked rugosities derived from this articulation (see
‘Basisphenoid’, below). The quadrate fossa is deep (con-
tra Madsen et al. 1995: 15), broad and finished laterally
by the squamosal and quadratojugal. When these latter ele-
ments are not preserved, the quadrate fossa appears to face
laterally, as was the case in Quaesitosaurus, which was re-
constructed with a laterally facing quadrate fossa termed a
‘resonator depression’ by Kurzanov & Bannikov (1983: 92;
fig.1).
The pterygoid flange of the quadrate is only partially
visible in the left lateral and ventral views (Figs 8 & 11) due
to the close approximation of the quadrate and basal tubera
and to matrix. The quadrate distal condyle is complete on
the left side (Fig. 8). As seen in ventral view, the condyle is
kidney-shaped, with a convex posterior margin and slightly
concave anterior margin (Fig. 11). The condyle is undivided
and its medial aspect is narrower anteroposteriorly than is the
lateral. The long axis of the quadrate condyle is orientated

300 J. A. Wilson
anteromedially, but this may be due to postmortem rotation of
the quadrate from its original position. The quadrate condyle
of Quaesitosaurus – which is similar in other respects but
uncrushed – is transversely oriented (PIN 3906/2). As seen
in posterior view, the quadrate condyle of Nemegtosaurus
is bevelled such that the medial side hangs lower than the
lateral side (Fig. 9).
Braincase
The braincase is firmly co-ossified and well preserved.
Sclerotic plates are present on the right side of the skull,
held in position by matrix (Figs 4 & 10). The left side of the
skull is nearly free of matrix, as is the occiput and the ventral
aspect of the braincase. The endocranial cavity is completely
filled by matrix.
Basioccipital
The basioccipital is the median element that forms the oc-
cipital condyle. It contacts the paired exoccipital–opithotics
on either side of the foramen magnum and abuts the median
basisphenoid anteriorly near the basal tubera (Figs 9 & 11).
The basioccipital contribution to the metotic foramen, which
conveys cranial nerves IX–XI, cannot be determined because
it is obscured by matrix. Cranial nerve XII is completely en-
closed within the exoccipital–opisthotic and does not pass
through the basioccipital (Fig. 9).
Both the basioccipital and the exoccipital–opisthotic
contribute to the occipital condyle. The basioccipital forms
the main body of the occipital condyle and the exocccipital–
opisthotic its ‘shoulders’, based on the faint sutures pre-
served (Fig. 9). The occipital condyle is downwardly ori-
entated when the supraoccipital is orientated vertically. The
dorsal margin of the occipital condyle is concave where it
forms the floor of the foramen magnum, but the remainder is
strongly convex. The condyle is slightly broader transversely
than deep dorsoventrally (60 ×50 mm) and more than twice
as broad as the foramen magnum. The surface of the oc-
cipital condyle is rugose and was probably covered by a
modest layer of articular cartilage. The suture between the
basioccipital and basisphenoid cannot be identified, so the
contribution of the former to the basipterygoid processes
cannot be determined.
Basisphenoid
The basisphenoid is well preserved but only visible in pos-
terior and ventral views because of matrix (Figs 9 & 11). This
large, median element floors part of the braincase and forms
part of the basal tubera, basipterygoid processes and the
parasphenoid rostrum. It contacts the basioccipital and quad-
rate posteriorly, the pterygoid ventrally and the prootic and
laterosphenoid dorsally.
The basisphenoid of Nemegtosaurus has two contacts
with the palate: the plesiomorphic basipterygoid–pterygoid
contact and a novel basisphenoid–quadrate contact that
is evidenced by the bevelled, roughened surface of the
basal tubera. This contact is not due to postmortem de-
pression (see ‘Quadrate’ above). This contact does not
appear to be present in Saltasaurus, in which the basal
tubera are transversely narrow and smooth (Powell 1992:
fig. 1).
The basipterygoid processes are ventrally directed and
diverge from one another at an angle less than 45◦(Fig. 9). A
single, transversely elongate opening for the internal carotid
artery emerges from between the basipterygoid processes, as
in Quaesitosaurus (Figs 9 & 11). A second, median opening
is present on the ventral surface of the basisphenoid fur-
ther anteriorly. Distally, the basipterygoid processes are ex-
panded anteroposteriorly and are narrow transversely (45 ×
∼10 mm). Their articular surface for the pterygoid is antero-
posteriorly convex and fits into a similarly concave ‘rocker’
facet in the pterygoid (see ‘Pterygoid’, above). This facet
is unlike the plesiomorphic socketed contact of most sauro-
pods (e.g. Brachiosaurus, Janensch 1935–1936: figs 27–30)
or the hooked contact present in Camarasaurus (Madsen
et al. 1995: fig. 5E) and Dicraeosaurus (Janensch 1935–
1936: fig. 105). The rocker-like basipterygoid process–
pterygoid contact is shared by Nemegtosaurus,Quaesito-
saurus (PIN 3906/2) and Rapetosaurus (Curry Rogers &
Forster 2004: figs 23–27).
Supraoccipital
The supraoccipital is a median basicranial element that forms
the dorsal margin of the foramen magnum. The supraoccipital
contacts the exoccipital–opisthotic and the occipital process
of the parietal laterally on the occiput; it contacts the skull
roof via the parietal dorsally (Figs 7 & 9).
The supraoccipital is triradiate, with a relatively flat
ventral surface and dorsal and lateral projections separated
by a sharp notch. In posterior view, the supraoccipital con-
tacts the parietal along the L-shaped notch formed by the
lateral aspect of its dorsal process and the dorsal aspect of its
lateral process (Fig. 9). The remainder of the lateral process of
the supraoccipital is enclosed by the exoccipital–opisthotic,
which extends medially to form the lateral margin of the fo-
ramen magnum. Whereas the course of its suture with the
parietal is well marked, that with the exoccipital–opisthotic
is difficult to trace in places. Consequently, the supraoccipital
contribution to the dorsal margin of the foramen magnum is
difficult to determine. The supraoccipital bears a vertically-
orientated median ridge that begins near the middle of the
element and extends dorsally to its summit. This supraoc-
cipital ridge can be seen in dorsal view as a small, triangular
process posterior to the parietals (Fig. 7).
Prootic
The prootic cannot be examined because it is obscured by
matrix.
Exoccipital–Opisthotic
This paired composite element forms the breadth of the
occiput, extending laterally as the paroccipital processes.
The right exoccipital–opisthotic is nearly completely pre-
served, lacking only the distalmost portion of the paroc-
cipital process. The left is less complete and lacks the distal
extreme of the paroccipital process. In posterior view, the
exoccipital–opisthotic fits between the supraoccipital, pari-
etal, squamosal and basioccipital (Fig. 9). Anteriorly, the
exoccipital–opisthotic is appressed against the prootic, which
forms part of the lateral wall of the braincase.
The occiput is broadest across the paroccipital pro-
cesses, which expand dorsoventrally and anteroposteriorly
at their distal ends. Prior to this distal expansion is a smooth
dorsal notch that forms the lower margin of the post-temporal
foramen, which was bounded dorsally by the squamosal

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
301
(Fig. 9). The rugose bone at the distal paroccipital process
is sharply demarcated. In lateral view, the distal end of the
paroccipital process is fusiform, with its anteroposteriorly
thickened dorsal half fitting in a well-marked facet in the pos-
terior aspect of the squamosal and its concave ventral half
accommodating the quadrate head (Fig. 8). The distovent-
ral tip of the paroccipital process is not completely pre-
served on either side in Nemegtosaurus.InQuaesitosaurus,
the completely preserved right paroccipital process is pro-
longed ventrally by a smooth, narrow prong (see Fig. 18).
This distoventral prong is preserved in other titanosaur brain-
cases, including Rapetosaurus (Curry Rogers & Forster
2004: fig. 19), Antarctosaurus wichmannianus (Huene 1929:
fig. 1), Saltasaurus (Powell 1992: fig. 1) and cf. Ant-
arctosaurus septentrionalis (Chatterjee & Rudra 1996:
fig. 11).
The exoccipital–opisthotic forms the lateral margin of
the foramen magnum and part of its ventral margin. Sutures
with the basioccipital within and lateral to the occipital con-
dyle, however, are difficult to discern. Cranial nerve XII exits
through the exoccipital–opisthotic lateral to the occipital con-
dyle. A conspicuous ridge crosses the exoccipital–opisthotic
from the dorsolateral margin of the foramen magnum to
midlength on the paroccipital process (Fig. 9). This ridge
may have formed the lower boundary of the occipital fossa.
A similar ridge is present in Quaesitosaurus (see Fig. 18),
which preserves low proatlantal facets that are apparently
reduced or absent in Nemegtosaurus.
Laterosphenoid
The lateral wall of the braincase is best seen on the left side
of the skull (Fig. 8). The laterosphenoid forms the posterior
portion of the braincase sidewall. The laterosphenoid is trans-
versely orientated and partially separates the temporal region
of the skull from the orbital region via its contact with the
frontal and postorbital. Anteriorly, the laterosphenoid con-
tacts the orbitosphenoid along the border of cranial nerves
IV, V and a large opening dorsal to IV, as in Quaesitosaurus
(PIN 3906/2) and other sauropods. Two deeply impressed
grooves pass anteroventrally and posteroventrally from the
opening of cranial nerve V. These grooves mark the exits
of the inferior orbital (V2) and mandibular (V3) branches of
the trigeminal nerve, respectively. The opening for cranial
nerve IV is slit-shaped and transversely orientated; a larger
opening of unknown identity is located dorsal to it. The exit
for cranial nerve III is relatively small and positioned along
the line connecting cranial nerves II and V. A small opening
for cranial nerve VI is located anteroventral to that for cra-
nial nerve V. A similar arrangement of cranial nerves II–V is
present in Quaesitosaurus.
Orbitosphenoid
The paired orbitosphenoids meet anteriorly to close the brain-
case (Fig. 10). They contact the laterosphenoid posteriorly
and the basisphenoid ventrally. Large, paired openings for
cranial nerve I pass through a dorsal breach in the orbito-
sphenoid symphysis. Paired openings for cranial nerve II
exit through the orbitosphenoid just anterior to that for cra-
nial nerve III (Fig. 8). A smaller opening posterolateral to
these may have carried cranial nerve VI. At its posterior mar-
gin, the orbitosphenoid forms the anterior border for cranial
nerves IV, V and a large opening dorsal to IV; their posterior
border is finished by the laterosphenoid. The ventral portion
of the orbitosphenoid is obscured by matrix.
Lower jaw
Both right and left lower jaws are well preserved (Figs 12–
14). Nearly all of the bones are complete on one or both mand-
ibles (dentary, surangular, angular, coronoid) or can be re-
constructed from preserved portions on both jaws (splenial),
but neither the prearticular nor articular were preserved on
either. The lower jaw is deepest in the coronoid region, which
is expanded dorsally and somewhat ventrally. The jaw rami
are shallow between the coronoid and tooth row, but then
deepen anteriorly. No external mandibular fenestra is present.
Thirteen teeth are preserved in each jaw ramus, these are
discussed in a subsequent section (see ‘Teeth’, below). The
articulated mandibles form an elongate, U-shaped structure,
due to the inward curvature of the dentaries (Nowinski 1971:
pl. 14, fig. 1A).
The right lower jaw, which is the source of most recon-
structions (e.g. Nowinski 1971; McIntosh 1990) has experi-
enced forward crushing, which has displaced the completely
preserved angular out of its articulation with the dentary
(Fig. 2). A portion of the right surangular remains adhered
to the posterior skull block (Figs 4, 9 & 11).
Dentary
Both dentaries are well preserved, although preservational
distortion has shifted them out of their symphyseal articula-
tion (Fig. 2C). The complete disarticulation of the dentary
symphysis may suggest a weak connection between the two
elements; no other skull elements were as distorted.
The dentary forms the anterior half of the mandible,
contacting its opposite at the symphysis, the surangular and
angular posterolaterally and the splenial and coronoid me-
dially. In lateral view, the dentary deepens both anteriorly
and posteriorly (Fig. 13). The dorsal margin of the dent-
ary is nearly horizontal, but the ventral margin is concave,
accounting for the expansion at either end of the element.
The anterior half of the dentary is dentigerous, but only the
posterior teeth are visible laterally; the anterior half of the
tooth row is orientated nearly transversely. Numerous vas-
cular foramina open in the anterior region of the dentary,
similar to the upper snout. The dentary extends posteriorly
as two asymmetrical processes separated by an embayment
(Fig. 13). The shorter, posterodorsal process reaches the base
of the coronoid eminence and overlaps the surangular. The
longer, posteroventral process extends to a position below
the summit of the coronoid eminence and overlaps the an-
gular and part of the surangular. The posterior portion of the
splenial extends beyond the posterior dentary on the medial
side of the lower jaw (Fig. 12; see ‘Splenial’, below).
A conspicuous Meckelian groove is visible medially
(Figs 12 & 14). From its elevated position posteriorly, the
groove descends to the ventral portion of the dentary. Near
the symphysis, the groove rises slightly to form a prominent
notch extending up the ventral third of the articular surface.
The dentary symphysis is narrow transversely and lacks the
roughened sutural surface present in other sauropods (e.g.
Camarasaurus;Madsenet al. 1995: figs 41–42). The long
axis of the dentary symphysis is perpendicular to the long
axis of the lower jaw (Nowinski 1971: 70), as in Quaesito-
saurus and Antarctosaurus wichmannianus but unlike other

302 J. A. Wilson
d11
d10
dden f mgr f
an
spl
d9
d8
d7
d6
d1
sy
sa
f
cor
gr
addfo
d13
Figure 12 Stereopairs and interpretive line drawing of the right lower jaw of Nemegtosaurus mongoliensis (Z.PAL MgD-I/9) in medial view. See
Figure 3 for abbreviations. Scale bars =10 cm.
Figure 13 Stereopairs and interpretive line drawing of the left lower jaw of Nemegtosaurus mongoliensis (Z.PAL MgD-I/9) in lateral view. See
Figure 3 for abbreviations. Scale bars =10 cm.
sauropods, in which the axis cants forward (Calvo 1994).
The tooth row begins just posterior to the symphysis. Thir-
teen teeth are present in each dentary; replacement foramina
are visible on the medial margin of the tooth row. Posterior
to the tooth row, the dentary is overlapped medially by the
coronoid, to which it is partially fused (see ‘Coronoid’, be-
low).
The lower jaws are nearly U-shaped in dorsal view
(Nowinski 1971: pl. 14, fig. 1A). All of the curvature is
accommodated by the dentaries, which bend medially near

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
303
Figure 14 Stereopairs and interpretive line drawing of the left lower jaw of Nemegtosaurus mongoliensis (Z.PAL MgD-I/9) in medial view. See
Figure 3 for abbreviations. Scale bars =10 cm.
the middle of the tooth row. This condition resembles that
in Rapetosaurus (Curry Rogers & Forster 2004: 140) and
Quesitosaurus (PIN 3906/2) and is distinct from the rect-
angular dentary present in diplodocoids such as Diplodocus
(McIntosh & Berman 1975: fig. 5C).
Surangular
The surangular is nearly completely preserved on the left
side (Figs 13–14), but the thin central portion of the element
is damaged, as it is in Quaesitosaurus. Much less of the right
surangular is preserved, owing to its impingement against
the quadratojugal during fossilisation (Figs 2A & 12). The
surangular forms the coronoid process and the outer wall of
the adductor fossa.
The anterior portion of the surangular is clasped by the
dentary laterally and the coronoid medially. The postero-
dorsal process of the dentary does not extend as far posteri-
orly as does the coronoid, which extends to near the summit of
the coronoid process (Fig. 12). Following the shape of these
two elements, the coronoid process rises sharply to reach its
summit at approximately two-thirds jaw length. From this
summit, the coronoid process descends flatly (nearly con-
cave) before curving downwards towards the articular. The
dorsal margin of the surangular is thickened medially and
borders an elliptical coronoid fossa that stretches from the
posterior surangular foramen to within the adductor fossa
(Nowinski 1971: pl. 14, fig. 1A). The bone in this fossa is
thin and incompletely preserved. The less complete right sur-
angular preserves the margin of a large opening in this region,
which Nowinski termed the ‘mandibular vacuity’ (1971:
fig. 2). This opening was originally complete on the left sur-
angular (Fig. 2A), but its margins have been since damaged
(Fig. 13). This large, elliptical opening corresponds in pos-
ition to the anterior surangular foramen. A smaller opening
positioned near the arched portion of the posterior coronoid
process (Figs 12–15) represents the posterior surangular fora-
men. Although the surangular is not completely preserved in
Quaesitosaurus, the presence of a depression and extremely
thin bone in this region suggests the presence of an enlarged
anterior surangular foramen; the presence of a posterior sur-
angular foramen cannot be determined. Rapetosaurus bears a
large ‘surangular foramen’ on the anterior half of the surangu-
lar, as well as a smaller, posterior opening (Curry Rogers &
Forster 2004: 142, fig. 30). These resemble in shape and
position the anterior and posterior surangular foramina of
Nemegtosaurus.
Angular
The angular is well preserved on both lower jaws. This strap-
shaped postdentary element is exposed both medially and
laterally and it forms the floor of adductor fossa (Figs 12–14).
It is overlapped by both the surangular and dentary laterally
and by the splenial medially. The prearticular and articular
would have also contacted the splenial medially, but these
elements were not preserved.
The angular is deepest near its midlength; it tapers pos-
teriorly towards the jaw joint and anteriorly towards its ar-
ticulation with the dentary. Its ventral surface is slightly con-
cave posterior to the level of the coronoid process. Medially,
a sharply defined shelf marks the articular surface for the
prearticular, which is not preserved on either side (Figs 12 &
14). Although the articular is not preserved, it would have ar-
ticulated in the medial expansion near the posterior extreme
of the angular. The angular did not form an extended retroar-
ticular process, as it does in Diplodocus. In lateral view, the
ventral portion of the angular is developed into a low ridge
that borders a shallow fossa (Fig. 13).

304 J. A. Wilson
Figure 15 Detail of the left lower jaw of Nemegtosaurus mongoliensis (Z.PAL MgD-I/9) in medial view. See Figure 3 for abbreviations. Scale
bar =10 cm.
Splenial
Both splenials are nearly complete, but the left is better pre-
served than the right (Figs 12 & 14). The splenial is an
anteromedial jaw element that forms part of the floor and
inner wall of the adductor fossa and of the Meckelian canal.
It contacts the coronoid and surangular dorsally and the an-
gular and dentary ventrally. It would have also contacted the
prearticular posteriorly, but this element is not preserved.
The splenial is tetraradiate, with two closely approxim-
ated anterior processes and two widely divergent posterior
processes. The anteroventral process of the splenial is not
completely preserved on either side, but it can be reconstruc-
ted based on the shape of the Meckelian groove of the dentary,
which it overlies. The anteroventral process was probably
elongate, triangular and would have extended anteriorly to
the level of the penultimate dentary tooth position (Figs 12 &
14). The adjacent anterodorsal process is more completely
preserved, especially on the right side. The anterodorsal pro-
cess is rounded and elongate; it extends to the level of the
posterior-most dentary tooth position. The anterodorsal pro-
cess is itself divided into two smaller, tonguelike processes
by a conspicuous notch that is preserved on both sides. A
small groove extends from this notch towards the posterior
margin of the tooth row (Figs 12, 14 & 15). Posteriorly, the
dorsal margin of the splenial follows the upward curvature of
the coronoid, terminating before the summit of the coronoid
process. The shape of the posterior portion of the splenial
roughly parallels that of the dentary. A narrow posterodorsal
process is separated from the broad, elongate posteroventral
process by a broad embayment. The posterodorsal process
of the splenial is divided distally into two short, smaller
processes. The posteroventral process of the splenial is rel-
atively straight, tongue-shaped and overlaps the angular. A
slit-shaped, anteroposteriorly elongate splenial foramen is
preserved on both sides (Figs 12 & 14).
Coronoid (=‘Intercoronoid’)
The presence of an elongate, flat element positioned flush
against the posterior portion of the lower tooth row has been
recognised in the sauropods Brachiosaurus,Camarasaurus
and Mamenchisaurus. A similar element is also present (but
was not identified) in the lower jaw of Omeisaurus maoi-
anus (Tang et al. 2001a: fig. 13). In Camarasaurus and Bra-
chiosaurus, this element covers all but dentary teeth 1–4,
whereas in Omeisaurus and Mamenchisaurus it covers all
but dentary teeth 1–8. So far, it has not been identified in
any skull of Shunosaurus (Zhang 1988; Chatterjee & Zheng
2002) or Diplodocus (McIntosh & Berman 1975) and it has
been regarded as absent in Nemegtosaurus (Madsen et al.
1995: 32). It is found behind the tenth tooth in Plateo-
saurus (Brown & Schlaikjer 1940: fig. 4), which suggests
its presence and posterior position may be primitive for saur-
opodomorphs. In these basal forms, the posterior position of
this element may be correlated with the greater number of
teeth present in the lower jaw. This lower jaw element has
been called the ‘complementare’ (Janensch 1935–1936), ‘in-
tercoronoid’ (Madsen et al. 1995) and ‘coronoid’ (Russell &
Zheng 1993; Ouyang & Ye 2002). Because ‘complementare’
is synonymous with ‘coronoid’ (Romer 1956), only ‘coron-
oid’ and ‘intercoronoid’ remain as alternative names for this
element. Brown & Schlaikjer (1940: 4–5) applied the term
‘intercoronoid’ to describe in dinosaurs ‘a jaw element which
was present only in some of the early amphibians of the Car-
boniferous’ that has ‘exactly the same relationship with the
other lower jaw elements in amphibians and early reptiles’.
Brown & Schlaikjer (1940) did not specify which early am-
phibians or which lower jaw elements they were comparing,
but it is probable that they were referring to the middle of
the three coronoid elements present in basal tetrapods. The
anterior and middle coronoids are positioned at the anterior
and posterior ends of the tooth row, respectively, whereas the
posterior coronoid was positioned near the coronoid emin-
ence (Laurin 1998). The smaller coronoid element preserved
in Camarasaurus near the coronoid eminence probably cor-
responds to the posterior coronoid, whereas the elongate ele-
ment appressed against the posterior end of the tooth row
in Brachiosaurus,Mamenchisaurus and Omeisaurus corres-
ponds to the middle coronoid of amphibians. This element
will be referred to as ‘coronoid’ here.
A modified coronoid is present in Nemegtosaurus
(Figs 12–15). This elongate element is positioned just an-
terior to the coronoid eminence of the surangular and can
be distinguished from adjacent elements. Here, the coronoid

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
305
Table 1 Principle measurements of the teeth of Nemegtosaurus mongoliensis.
Upper teeth
Crown measure 1 2 3 4 5 6 7 8 9 10 11 12 13
Length R 46 44 50* 41 40* 26* – – 45 41 30* 34 –
L 41 51 28* – 44* 42 – – 39 39 27* – 30
Anteroposterior width R – – – 10 – 10 – 10 10 – 9 – 9
L1010101010 1010 –1010 – ––
Labiolingual width R 9 9 9 9 – 9 – 9 8 8 8 7 –
L10 10 9 9 10 9 9 – 9 8 – – 6
Wear pattern R 0/1 2 0 3 0 – – – 0 0/1 3 1 0
L2/31 3 – – 3 – –1 1 – –1
Lower teeth
Crown measure 1 2 3 4 5 6 7 8 9 10 11 12 13
Length R 34 25* – – – 30 31 – 29 27 – – –
L 36* 32 38 25 29 34 31* 24 13 – – – –
Anteroposterior width R 8 8 – – 8 8 8 – 6 7 – – –
L88888 8–887866
Labiolingualwidth R8 7 8 7 7.57 7 766 6 ––
L– 7 8 7 7 7 – 767 – ––
Wearpattern R1 2 – 12/32/31 333 0 ––
L0 2 02/32/3 1 0 2–– – ––
Measurements are in millimetres for each tooth position (columns) in both right (R) and left (L) jaws. Asterisks (*) indicate measurements of incomplete crowns.
Wear facet abbreviations: 0, no wear; 1, apical wear; 2, V-shaped wear (symmetrical); 3, V-shaped wear (asymmetrical).
is visible in lateral view, where it extends dorsally above
the margin of the dentary (Fig. 13). It passes anteriorly and
merges with the dentary near the posterior margin of the tooth
row. In medial view, the anterior margin of the coronoid is
difficult to identify, but it may be marked by a small groove
that is preserved on both mandibles, just dorsal to the antero-
dorsal process of the splenial (Fig. 15). If this assessment is
correct, then the coronoid extended to the posterior margin
of the tooth row but did not overlap any alveoli. A foramen
opens in the middle of the coronoid on both sides (Figs 12,
14 & 15). Nowinski (1971: 71, fig. 6) identified this element
as a partially fused coronoid, an identification that is suppor-
ted here. It is not clear whether the modified coronoid element
of Nemegtosaurus, which reaches the summit of the coron-
oid process, has incorporated part of the posterior coronoid
element. A structurally similar coronoid is present in Bonita-
saura (Apestigu´
ıa 2004), Quaesitosaurus (PIN 3906/2) and
possibly Malawisaurus (Jacobs et al. 1993: fig. 1B). Curry
Rogers & Forster (2004:141) note a ‘short, rugose ridge of
bone’ on the oral margin of the mandible that resembles in
shape, position and texture that of Nemegtosaurus and Quae-
sitosaurus.
Teeth
Nearly complete crowns are preserved in both upper and
lower tooth rows of both sides of the skull. The anterior
lower teeth have been displaced somewhat posteriorly as a
result of the postmortem deformation that shifted the lower
jaws forward (Figs 2A & B). The premaxillary teeth have
also been shifted by the transverse compression applied to
the skull (Figs 2C & 5).
As discussed above, there were probably 13 teeth on
each side of the upper and lower jaws. Despite identical
tooth counts, there is a marked difference in upper and lower
tooth breadth – lower crowns are about four-fifths the breadth
of upper crowns (Table 1), a condition that is present in
some diplodocoids (Diplodocus, Holland 1924; Nigersaurus,
Sereno et al. 1999). There are other important differences
between upper lower crowns. Upper crowns are D-shaped
in cross-section and lingually curved, whereas lower crowns
are elliptical in cross-section and slightly labially curved. As
discussed below, tooth-to-tooth wear appears on the lingual
side of upper teeth and on the labial side of lower teeth.
Other aspects of the tooth crowns are identical. Both upper
and lower crowns are narrow and neither expand from their
root. They taper near their apex towards a somewhat blunt
point. Conspicuous ridges are developed on both mesial and
distal edges of upper and lower teeth (Nowinski 1971: 71).
The ridges extend apically from the point at which the crown
tapers to the tip. The enamel is finely wrinkled throughout
the crown, but it is arranged into coarser, longitudinal ridges
at the base of the crown. Tooth crowns do not overlap.
Replacement
Two types of tooth replacement are visible in Nemegto-
saurus. At several tooth positions (LPM2, LPM4, LM2,
RM6, RD10), replacing teeth appear on the lingual surface
of the functional tooth. At other tooth positions, replacing
crowns are pushed out by their roots. Fresh crowns in the
pulp cavities of heavily worn teeth can be observed in both
upper tooth positions (LPM3, LM4) and lower tooth pos-
itions (LD4, LD11; ?RD1, RD2, RD8) in Nemegtosaurus.
In addition to these, Nowinski (1971: 72) reported replacing
tooth tips in the pulp cavities of functional teeth RM 7, RM8
and RD 3, which have since been damaged. In the upper tooth
row, the cycle of replacement appears to alternate between
fresh and heavily worn teeth. The lower rows, in contrast
have fresh teeth alternating with two heavily worn teeth (see
Table 1 ) .
Wear
All but the freshest teeth show signs of tooth-to-tooth con-
tact. Fresh and more lightly worn teeth show fine enamel

306 J. A. Wilson
wrinkling all over the crown. The enamel on moderately
worn teeth, in contrast, smoothes towards the apex of the
crown, although the coarse, longitudinal ridges at the base
are never lost. Fresh and worn teeth also differ in the opacity
of their enamel. Fresh teeth have thicker enamel and appear
dark brown to black in colour, whereas heavily worn teeth
have much thinner enamel and appear light brown. Upper
crowns wear on their lingual surface, lower crowns wear on
their labial surface.
Both V-shaped and apical wear facets are present in
the jaws of Nemegtosaurus (Table 1). This is an uncommon
condition, as sauropods typically bear one or the other but
not both types of wear. It is not yet clear whether V-shaped
and apical wear facets reflect ontogenetic variation, variation
along the tooth row, or both. Most replacing teeth in Nemeg-
tosaurus bear V-shaped wear facets, suggesting that they are
produced later than the apical wear facet. However, two of
these replacing teeth (LPM2, RM6) bear light apical wear.
The difference in crown breadth between upper and lower
teeth may also contribute to variation in wear facets, due to
small differences in alignment along the tooth row. However,
V-shaped and apical wear are not produced in other taxa
with crown-breadth disparity (e.g. Diplodocus). Further in-
vestigation into the microwear will be required to distinguish
between ontogenetic and tooth row variation, as well as to
explain how wear was generated in Nemegtosaurus.
In his study of sauropod feeding mechanisms, Calvo
(1994: 190) examined microwear on the surface of one tooth
and reported long and thin scratches orientated parallel to
the tooth axis. He concluded that the lower jaw of Nemeg-
tosaurus moved in an up-and-down motion to produce an
isognathous bite. Upchurch & Barrett (2000: 100, fig. 4.4)
described similar microwear in Rapetosaurus (referred to as
an ‘unnamed titanosaur from Madagascar’), reporting coarse
scratches extending parallel to the axis of the crown as well
as randomly distributed pits. Upchurch & Barrett (2000:
103) also recognized V-shaped and apical wear facets in
Nemegtosaurus (which they consider a diplodocoid) and sug-
gested a ‘shearing bite was employed’ and that the ‘presence
of mesial and distal [i.e. V-shaped] wear could reflect some
oral processing, or a less precise tooth-to-tooth contact than
found in titanosauroids or diplodocoids’. Again, additional
investigation into the distribution and orientation of wear
facets is required to interpret chewing function in Nemegto-
saurus and other titanosaurs.
Reconstruction
Although the original reconstruction of Nowinski included
lateral, dorsal and posterior views (1971: figs 1, 2, 5A), a
reconstructed ventral view of the skull was never presen-
ted. Salgado & Calvo (1997: fig. 8) presented the only
other reconstruction of the Nemegtosaurus skull, but they
provided only a lateral view. As discussed below, their recon-
struction differs substantially from the original of Nowinski
(1971), most notably in the narial and lateral temporal re-
gions. Few other titanosaur skulls have been reconstructed.
The first was a reconstruction of Antarctosaurus wichman-
nianus by Huene (1929: fig. 31), a species that many have
considered to be an amalgam of both diplodocoid and ti-
tanosaur cranial elements (McIntosh 1990; Jacobs et al.
1993; Sereno et al. 1999; Upchurch 1999; but see Apestigu´
ıa
2004). Although the narial region of the skull was not pre-
served, Huene (1929) reconstructed Antarctosaurus to re-
semble Diplodocus, based on perceived phylogenetic affinit-
ies indicated by narrow tooth crowns. This reconstruction, in
part, led to a long-held notion that diplodocoids and titano-
saurs shared close phylogenetic history, despite few postcra-
nial similarities (McIntosh 1990). Salgado & Calvo (1997:
figs 5B, 7B) provided an alternative reconstruction of the Ant-
arctosaurus skull, as well as that of Quaesitosaurus. Curry
Rogers & Forster (2001: fig. 1A–C) reconstructed the nearly
complete skull of Rapetosaurus in three views, but like Now-
inski (1971) did not present a ventral view. The reconstruc-
ted Rapetosaurus is strikingly similar to Nemegtosaurus,as
discussed below. Hunt et al. (1994: fig. 2) provided a ‘hy-
pothetical reconstruction’ of the titanosaur skull based on
titanosaur material from Malawi, India and Argentina, as
well as Camarasaurus. Several reconstructed aspects of the
skull, including nine premaxillary teeth, do not occur else-
where in Sauropoda, while proposed derived characters such
as a short, high snout and large antorbital fenestra may.
A new reconstruction of the skull of Nemegtosaurus
is presented in Fig. 16. Compression and shearing during
preservation have distorted the shape and configuration of the
skull bones, especially on the right side. The reconstruction
presented here relies on both sides of the skull but more
heavily on the left, relatively undistorted, side. The portion
of the skull between the posterior skull block and the snout
piece was poorly preserved and some key regions such as
the external naris and mid-palate were not preserved at all.
I have reconstructed these regions based on interpretation
of neighbouring bones and comparisons to skulls of other
neosauropods, such as Camarasaurus,Brachiosaurus and
Diplodocus (Wilson & Sereno 1998: figs 6–8). Photographs
of the skull before preparation were used to estimate relative
orientations of snout and posterior skull blocks, as well as
the shape of the antorbital fenestra.
In lateral view (Fig. 16A), the skull is tipped posteriorly
so that the jaw articulation is anterior of the occiput. The
lateral temporal fenestra extends somewhat beneath the or-
bit, as in all sauropods, but is abbreviated antero-posteriorly.
The nares are enlarged, but not so large as the orbit, which is
teardrop-shaped. The nares are retracted to a position just an-
terior of the orbits, as in most sauropods, with their posterior
margin between the prefrontals. The dorsal margin of the
orbit is heavily ornamented. The snout is elongate, as in Bra-
chiosaurus,Rapetosaurus and Diplodocus. As seen in dorsal
view (Fig. 16B), the supratemporal fenestra is narrow antero-
posteriorly and bounded only by the postorbital and parietal.
The nares are relatively broad transversely, as indicated by
preserved portions of the nasal bone (Fig. 6). The palate is
reconstructed for the first time in a titanosaur in Fig. 16D.
The palatine and anterior pterygoid were completely recon-
structed, but the remainder of the palate was well preserved.
The ectopterygoids are elongate elements that differ from
the typically recurved and laterally orientated elements in
other saurischians. The palate contacts the braincase via the
quadrate–basal tubera and pterygoid–basipterygoid contacts.
This novel double palatobasal contact is also visible in pos-
terior view (Fig. 16C).
The reconstructed skull presented differs in several
ways from the original Nemegtosaurus reconstruction of
Nowinski (1971) and the Rapetosaurus reconstruction of
Curry Rogers & Forster (2001). Although Rapetosaurus
bears an exceptionally elongate antorbital fenestra, other

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
307
Figure 16 Reconstruction of the skull and lower jaw of Nemegtosaurus mongoliensis in left lateral (A), dorsal (B), posterior (C) and ventral
(D) views.

308 J. A. Wilson
Figure 17 Composite stereopairs and interpretive line drawing of the right half of the upper snout of Quaesitosaurus orientalis (PIN 3906/2) in
ventral view. Dashed lines separate individual stereopairs. See Figure 3 for abbreviations. Scale bars =10 cm.
aspects of the skulls are quite similar. Both have elongate,
gently sloping snouts, fully retracted and dorsally facing ex-
ternal nares and strongly anteriorly shifted quadrates. The
reconstruction proposed here includes an arched internarial
bar, as indicated by partially preserved nasals, which is prim-
itive for Sauropoda. Likewise, the preservationally distorted
lower lateral temporal fenestra has been restored, pushing the
jaw articulation posterior slightly. In both these regards, the
reconstruction presented here resembles that presented by
Salgado & Calvo (1997). Nevertheless, it differs in not as-
suming Brachiosaurus-like external nares, triangular lateral
temporal fenestra, rounded snout and sharply demarcated
narial fossa.
Phylogenetic affinities of
Nemegtosaurus
Traditional assessments of the phylogenetic affinities of Ne-
megtosaurus and Quaesitosaurus have suggested that they
are close relatives with affinities to Dicraeosaurus. Although
Nowinski (1971: 74) recognised a general resemblance to

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
309
Figure 18 Stereopairs and interpretive line drawing of the posterior half of the skull of Quaesitosaurus orientalis (PIN 3906/2) in posterior
view. Right stereo at top. See Figure 3 for abbreviations. Scale bars =10 cm.
Diplodocus in the shape and proportions of the skull, he
considered more important the specific resemblances to Di-
craeosaurus in the ‘structure of the occipital region, the size
of the supratemporal fossa, the structure of the lacrimal, the
lack of the accessory preorbital opening and the structure of
the teeth.’ Accordingly, he placed Nemegtosaurus in the
Dicraeosaurinae. Kurzanov & Bannikov (1983) followed
this assessment without further discussion. McIntosh (1990:
table 16.1) also classified Nemegtosaurus within the Dicraeo-
saurinae, noting general resemblance to the skull of Dicraeo-
saurus. However, he observed differences in the length and
positioning of the basipterygoid processes and cautioned that
‘serious questions remain’ regarding its affinities because
no Dicraeosaurus-like vertebrae have been found in Upper
Cretaceous rocks (McIntosh 1990: 393).
Previous cladistic hypotheses
Whereas the handful of cladistic analyses of Sauropoda
agree on many aspects of the higher-level relationships of
the group (see Wilson 2002), there has been no consensus
on the phylogenetic affinities of Nemegtosaurus and Quae-
sitosaurus. Upchurch (1995, 1998, 1999), Curry Rogers &
Forster (2001), Wilson (2002) and Upchurch et al. (2004)
have coded one or both of these Mongolian sauropods as ter-
minal taxa in a cladistic analysis. All but Upchurch agree that
the Mongolian skulls Nemegtosaurus and Quaesitosaurus are
members of Titanosauria.
Salgado & Calvo (1992, 1997)
Salgado & Calvo (1992) were the first to refute the traditional
hypothesis that Nemegtosaurus and Quaesitosaurus were di-
craeosaurids. They based this assessment on the absence of
dicraeosaurid synapomorphies in Nemegtosaurus. Symple-
siomorphies retained in Nemegtosaurus and Quaesito-
saurus include short, downwardly projecting basipterygoid
processes, a posteroventrally orientated occipital con-
dyle, unfused frontals, dorsally facing supratemporal fen-
estrae and the absence of median skull roof openings
(Salgado & Calvo 1992: 346). Although Salgado & Calvo
(1992) formally excluded Nemegtosaurus and Quaesito-
saurus from the Dicraeosauridae, they provided no syn-
apomorphies supporting an alternative systematic assign-
ment. Thus, they did not refute the hypothesis that Nemegto-
saurus and Quaesitosaurus could be the immediate outgroup
to diplodocids and dicraeosaurids, a position advocated by
Upchurch (1995, 1998, 1999) and Upchurch et al. (2002).
In a subsequent paper, Salgado & Calvo (1997) provided
synapomorphies linking Nemegtosaurus and Quaesitosaurus
to Antarctosaurus, which they considered to be a titanosaur
but did not include in their phylogenetic analysis (Salgado

310 J. A. Wilson
et al. 1997: 5).These included the presence of peg-like denti-
tion, sharp wear facets, a vertical dentary symphysis and nar-
row supratemporal fenestrae. Of these, only the presence of
high-angled wear facets currently has a distribution support-
ing this claim. Peg-like dentition and narrow supratemporal
fenestrae are homoplastic (shared by diplodocoids) and the
vertical dentary symphysis is not present in the only other
definitive titanosaur then known from a dentary (i.e. Malaw-
isaurus). In addition, Salgado et al. (1997) suggested cor-
relation amongst features supporting the traditional Nemeg-
tosaurus–dicraeosaurid hypothesis. Hypothesised correlated
characters included the posteriorly inclined quadrate, long
and anteriorly orientated basipterygoid processes, ventrally
orientated occipital condyle and anteriorly orientated olfact-
ory tracts (hence the retracted position of the external nares).
Some of these claims were answered by Upchurch (1999)
and later by Chiappe et al. (2001), who observed the pres-
ence of some but not all of these putative correlated features
in titanosaur embryos. Pachycephalosaurs represent another
counter-example because they have anteriorly orientated ba-
sipterygoid processes, a ventrally deflected occipital condyle
and a posteriorly inclined quadrate, but they retain terminal
nares (pers. obs.).
Claims for Nemegtosaurus-titanosaur affinities are fur-
ther complicated by controversy surrounding the affinities of,
and associations amongst, skeletal remains attributed to Ant-
arctosaurus. Although the postcranial skeleton attributed to
Antarctosaurus wichmannianus is definitively titanosaurian
(McIntosh 1990), the mandible described by Huene (1929:
pl. 29) has been hypothesised to belong to a second, dip-
lodocoid taxon (McIntosh 1990; Jacobs et al. 1993; Sereno
et al. 1999; Upchurch 1999; but see Apestigu´
ıa 2004).
Upchurch (1995, 1998, 1999), Upchurch et al. (2002,
2004)
The only cladistic support for the traditional diplodocoid af-
finities of Nemegtosaurus has been provided by analyses by
Upchurch (1995, 1998) and Upchurch et al. (2002, 2004) as
well as his lengthier treatment of the then-assembled charac-
ter information for and against the hypothesis (Upchurch
1999). As mentioned above, Upchurch (1999: 121–122)
countered claims by Salgado & Calvo (1997) that several
cranial characters are correlated with the rotation of the
braincase in diplodocoids. This result is supported here and
elsewhere (Chiappe et al. 2001).
Upchurch (1999: 118) identified seven characters sup-
porting the diplodocoid affinities of Nemegtosaurus:(1)
premaxilla narrow transversely and elongate anteroposteri-
orly; (2) subnarial foramen elongated along the premaxilla–
maxilla suture; (3) posterior margin of the external naris pos-
terior to anterior end of prefrontal; (4) vomerine (i.e. antero-
medial) processes of the maxillae not visible laterally; (5) loss
of the intercoronoid; (6) mandible subrectangular in dorsal
view; and (7) teeth restricted to the anterior end of the jaws.
Upchurch et al. (2004) did not include characters 4 and 5 but
added two others: (8) premaxilla loses distinction between
body and ascending process and (9) anteroposterior diameter
of supratemporal fenestra is less than 10% occipital width.
The redescription presented above confirms that characters 1,
3, 4, 7 and 9 are indeed present in Nemegtosaurus. The distri-
bution of these features within sauropods requires additional
comment. A narrow premaxilla (character 1) is probably cor-
related with the presence of narrow tooth crowns, since the
number of premaxillary teeth does not vary in sauropods.
Although a premaxilla is not preserved in Rapetosaurus:
presence of narrow tooth crowns suggests a narrow premax-
illa. Fully retracted external nares (character 3) are present in
both diplodocoids (e.g. Diplodocus: Berman & McIntosh
1978) and titanosaurs (Rapetosaurus: Curry Rogers &
Forster 2004) and do not link Nemegtosaurus with either
clade. Likewise, exclusion of the anteromedial processes lat-
erally (character 4), or at least greatly reduced lateral ex-
posure as a narial fossa, is present in diplodocoids (e.g.
Diplodocus) as well as titanosaurs (e.g. Rapetosaurus), so
its presence in Nemegtosaurus does not support either hypo-
thesis. Restriction of the tooth row anterior to the antorbital
fenestra (character 7) is shared between diplodocoids and
Nemegtosaurus. Character 9 is present in both diplodocoids
and derived titanosaurs, as Upchurch et al. (2004: 303) ac-
knowledge. In contrast, the present redescription concludes
that characters 2, 5 and 6 are not present in Nemegtosaurus.
The elongate opening on the anterior snout previously iden-
tified as an elongate subnarial foramen (character 2) by
Upchurch (1999) opens into the maxilla and is here identi-
fied as the anterior maxillary foramen (see ‘Maxilla’ above).
A specialised coronoid (=‘intercoronoid’) is present in
Nemegtosaurus,Quaesitosaurus,Bonitasaura and, possibly,
Rapetosaurus and Malawisaurus (see ‘Coronoid’ above). Fi-
nally, the mandibles of Nemegtosaurus are not subrectangular
in dorsal view (character 6). Although the anteriormost 3–4
teeth are obscured in lateral view, this is distinct from the con-
dition in diplodocids such as Diplodocus and Nigersaurus,
in which most or all teeth are positioned on the transverse
portion of the ramus.
Curry Rogers & Forster (2001)
The discovery of Rapetosaurus provided the first associ-
ation between a skull and titanosaur postcranial skeleton and,
thus, the first opportunity to compare Nemegtosaurus and
Quaesitosaurus to a well-preserved titanosaur skull (Curry
Rogers & Forster 2001, 2004). The strong similarity between
Rapetosaurus,Nemegtosaurus and Quaesitosaurus was re-
flected in a cladistic analysis, in which Curry Rogers &
Forster (2001) listed several shared derived characters linking
these three forms (posterodorsal process of splenial present,
symphysis perpendicular to jaw axis), as well as features
uniting them to Malawisaurus (frontals fused, parietal ex-
cluded from post-temporal fenestra, anteroposteriorly deep
occipital region, basipterygoid processes four times basal
diameter). Among these three, Rapetosaurus and Nemegto-
saurus were found to have a sister-taxon relationship based
on retraction of the nares between the eyes, frontal contri-
bution to supratemporal fossa, anterodorsally facing nares,
absence of a stepped snout profile, ectopterygoid process of
pterygoid positioned caudoventral to the lacrimal and slender
tooth crowns. As discussed in Wilson (2002: 246–247), none
of these features is uniquely derived. Even the perpendicular
orientation of the dentary symphysis relative to the jaw axis
was used to support both the titanosaur (Salgado & Calvo
1997) and diplodocoid (Upchurch 1995, 1998; Upchurch
et al. 2004) hypotheses.
Wilson (2002)
The phylogenetic position of Nemegtosaurus among titano-
saurs suggested by Wilson (2002) and others is defended

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
311
here. This hypothesis nested Nemegtosaurus within a group
of titanosaurs more derived than Malawisaurus. Relatively
high decay indices (4) supported monophyly of the Titano-
sauria and of the group uniting Rapetosaurus,Nemegto-
saurus and Saltasauridae (Wilson 2002: table 12). in ad-
dition, the monophyly of Nemegtosauridae (Nemegtosaurus,
Rapetosaurus) was supported by 13 synapomorphies, five of
which are unique (Wilson 2002: appendix 3).
Nemegtosauridae
The phylogenetic definition of Nemegtosauridae initially
suggested by Upchurch et al. (2004) specified a stem-based
group including all diplodocoids more closely related to
Nemegtosaurus than to Diplodocus. The phylogenetic re-
lationships proposed by Curry Rogers & Forster (2001) and
Wilson (2002), which nest Nemegtosaurus within Titano-
sauria not Diplodocoidea, render this definition synonymous
with Macronaria. Consequently, a new definition of Nemeg-
tosauridae is required to accompany the phylogenetic hy-
pothesis that Nemegtosaurus is a titanosaur. Nemegtosaur-
idae is here phylogenetically defined as the stem-based clade
including all titanosaurs more closely related to Nemegto-
saurus than to Saltasaurus. Following the analyses of Curry
Rogers & Forster (2001) and Wilson (2002), this definition
includes at least Nemegtosaurus,Quaesitosaurus and Ra-
petosaurus. Proposed synapomorphies of Nemegtosauridae
include (Wilson 2002: appendix 2):
Lacrimal with anterior process,
Parietal occipital process short, less than long diameter of
foramen magnum,
Quadrate fossa orientated posterolaterally,
Palatobasal contact ‘rocker’-like, pterygoid with convex ar-
ticular surface,
Pterygoid with reduced quadrate flange, palatobasal and
quadrate articulations approach,
Basisphenoid–quadrate contact,
Dentary symphysis perpendicular to jaw ramus (Salgado &
Calvo 1997; Upchurch 1999),
Tooth crowns do not overlap (shared with Brachiosaurus
and Diplodocus),
Tooth crowns with narrow cross-section (shared with dip-
lodocoids).
Synapomorphies of Nemegtosaurus and Quaesito-
saurus include (Wilson 2002: appendix 3):
Symphyseal eminence on external aspect of premaxillae,
Premaxilla and maxilla with sinuous contact,
Anterior process of the maxilla dorsoventrally deep,
Tooth bearing portion of snout highly vascularised, delim-
ited by transverse groove (shared with Diplodocus),
Palatal shelf on maxilla enclosed to form ‘maxillary canal’,
Postorbital, prefrontal and frontal with orbital ornamenta-
tion,
Prefrontal diverges laterally,
Skull roof broadest across prefrontals,
Squamosal excluded from supratemporal fenestra,
Pterygoid with tongue-and-groove articulation with
ectopterygoid–palatine,
Quadratojugal with sinuous ventral margin,
Intercoronoid partially fused to dentary,
Dentary with weak, anteroposteriorly narrow symphysis,
Meckel’s groove extending onto symphyseal portion of
dentary,
Dentary teeth smaller in diameter than premaxillary and
maxillary teeth (shared with Diplodocus).
Comments on several nemegtosaurid taxa follow.
Quaesitosaurus
Most studies have considered Quaesitosaurus and Nemeg-
tosaurus to be close relatives (e.g. Upchurch 1995), if not
conspecific (Wilson 1997). However, several differences
between the two taxa have been identified. Kurzanov &
Bannikov (1983: 91–92) distinguished Quaesitosaurus from
Nemegtosaurus on the basis of the former’s (1) broader skull,
(2) shorter squamosal that does not contact the quadratojugal,
(3) absence of a parietal aperture, (4) rounded occipital con-
dyle, (5) higher maxillary tooth count (9 versus 8), (6) longer
mandibular tooth row, (7) ‘resonator cavity’ on the quadrate
and (8) canal passing between the basal tubera. McIntosh
(1990: 393) considered the first six to be minor differences
that could be accounted for by preservation or individual
variation. However, he considered the presence of a quadrate
concavity and canal between tubera in Quaesitosaurus to
be generic differences. Salgado & Calvo (1997: 41) further
differentiated Nemegtosaurus and Quaesitosaurus by the
(9) orientation of the basipterygoid processes, which they
suggested are anteroventrally directed in the former and
ventrally directed in the latter. These nine differences are
discussed briefly below.
As suggested by McIntosh (1990), several of the differ-
ences listed by Kurzanov & Bannikov (1983) are preserva-
tional rather than taxonomic. The skulls of Nemegtosaurus
and Quaesitosaurus have been deformed in different ways.
Nemegtosaurus experienced transverse compression and for-
ward shear, whereas deformation in Quaesitosaurus was due
to dorsoventral compression (pers. obs.). These preserva-
tional differences account for the difference in snout breadth
(1). The squamosal is damaged in Quaesitosaurus and neither
its length nor its contact with the quadratojugal (2) can be de-
termined. There is no parietal aperture (3) in Quaesitosaurus,
but it is also absent in Nemegtosaurus (see ‘Parietal’, above).
Differences in the shape of the occipital condyle (4) are
minor and can be attributed to slight damage to that of Ne-
megtosaurus. The number of maxillary teeth (5) in Nemegto-
saurus is uncertain because of damage to the last few alveoli
on both sides of the skull, along with displacement of the
teeth in their sockets. Although Nowinski (1971) originally
recorded eight teeth in the maxilla of Nemegtosaurus,itis
possible a ninth tooth was present, as in Quaesitosaurus.The
apparent difference in the shape of the quadrate (7) results
from the absence in Quaesitosaurus of the quadratojugal and
the ventral process of the squamosal, which form the lateral
margin of the quadrate fossa. The shape of the quadrate is
otherwise similar (see Figs 9 & 16 and see ‘Quadrate’ above).
Differences in the length of the mandibular tooth row (6) and
in the orientation of the basipterygoid processes (9) are
minor. However, the median canal passing between the basal
tubera appears to differentiate Quaesitosaurus from Nemeg-
tosaurus. This canal, which passes between the basal tubera

312 J. A. Wilson
and basipterygoid processes, is open posteriorly in Quaes-
itosaurus but fully enclosed in Nemegtosaurus. Additional
differences (not previously mentioned) include the presence
in Quaesitosaurus of well marked grooves on the anterior
quadrate, lack of a posterior process of the postorbital and
the quadrate articular surface extending from the basal tubera
to the basipterygoid processes. The differences between these
two taxa and the autapomorphies that can be used to distin-
guish them are subtle. Ultimately, retention of both genera
or sinking of Quaesitosaurus into Nemegtosaurus rests on
the taxonomic value given to these differences, which is ar-
bitrary. Perhaps the most conservative decision is to retain
both genera, which does not require revision of existing tax-
onomy, anticipates additional differences that may come to
light when postcranial remains are discovered and recognises
that these taxa come from distinct stratigraphic horizons.
Opisthocoelicaudia
The presence of both the bodyless skull of Nemegtosaurus
(Nowinski 1971) and the skull-less body of Opisthocoeli-
caudia (Borsuk-Bialynicka 1977) in the Nemegt Formation
has led to speculation that they represent the same taxon
(e.g. Paul 1996). Interestingly, the early referral of these
Mongolian forms to the Dicraeosaurinae and Camarasaur-
idae, respectively, forbade this hypothesis. The more recent
referral to Titanosauria of both Opisthocoelicaudia (e.g. Gi-
menez 1992; Salgado & Coria 1993; Upchurch 1995) and
Nemegtosaurus (e.g. Calvo 1994; Wilson 1997; Calvo et al.
1998a) once again allows the possibility that these could
actually pertain to the same genus. However, this hypo-
thesis cannot be evaluated until associated cranial and post-
cranial material is discovered. The Nemegt region has thus
far produced two distinct titanosaur skulls morphs (Nemegto-
saurus,Quaesitosaurus), Opisthocoelicaudia and potentially
a fourth taxon represented by a series of dorsal vertebrae (V.
Alifanov, pers. commu.). Opisthocoelous caudal vertebrae
from the early Late Cretaceous of China attributed to the new
genus Borealosaurus (You et al. 2004) suggest Opisthocoeli-
caudia-like taxa were not restricted to the Nemegt Formation.
Sues & Averianov (2004) have reported a slightly opis-
thocoelous anterior caudal vertebra from the Cretaceous of
Kazakhstan, which may later be shown to have opisthocoeli-
caudiine affinities. The recent discovery of articulated titano-
saur tails with an opisthocoelous, procoelous, amphicoelous
and biconvex vertebrae (Rinconsaurus: Calvo & Gonz´
alez
Riga 2003), underscores the notion that phylogenetic infer-
ences based on a single character are susceptible to homo-
plasy. Sauropod trackways recently recovered from the Ne-
megt Formation (Currie et al. 2003) cannot be attributed to
a specific trackmaker subgroup.
Mongolosaurus
Gilmore (1933) described an associated partial braincase,
fragmentary teeth and anteriormost three cervical vertebrae
as Mongolosaurus haplodon. Despite the well preserved ana-
tomy of this specimen, relatively little has been said of its
affinities. McIntosh (1990: 398) suggested possible diplodo-
cid affinities but classified it as Sauropoda incertae sedis.
Upchurch (1995, 1998, 1999) did not suggest phylogen-
etic affinities for Mongolosaurus and Upchurch et al. (2004:
table 13.1) considered it to be a nomen dubium. Both Barrett
et al. (2002) and Wilson (2002) regarded it as Neosauropoda
incertae sedis, but the former considered it to be a nomen du-
bium. Re-examination of the original material (AMNH 6710)
reveals that the basal tubera are bordered laterally and vent-
rally by a raised lip, which may represent the contact surface
for the quadrate. If so, then Mongolosaurus possesses at least
one synapomorphy of Nemegtosauridae. Mongolosaurus is
diagnosed by at least three autapomorphies pertaining to the
anterior cervical vertebrae. These include an extremely re-
duced neural spine, well developed epipophyses and spino-
prezygapophyseal (sprl) and spinopostzygapophyseal (spol)
laminae aligned nearly horizontally and along the vertebral
axis.
Antarctosaurus wichmannianus
Wichmann (1916) collected cranial and postcranial elements
that he interpreted as representing one individual from Late
Cretaceous rocks in General Roca (R´
ıo Negro Province),
Argentina. Cranial remains include the posterior skull (a
nearly complete braincase, both frontals and parietals and the
upper portion of the left lateral temporal fenestra), a smal-
ler piece pertaining to the right posterior corner of the same
skull (squamosal, parietal, quadrate), an isolated quadrate,
isolated mandibular fragments (a portion of the left dentary
and a complete right dentary in articulation with a partial
splenial, coronoid, angular and prearticular) and many post-
cranial bones. These postcranial remains included a cervical
vertebra, caudal vertebrae, dorsal ribs, a left scapula, portions
of the right forelimb (proximal and distal humerus, proximal
ulna and radius, partial manus), pelvis (partial left ilium, right
ischium, distal right pubis) and left hindlimb (femur, tibia,
astragalus, calcaneum, metatarsals). Huene (1929: 1) stud-
ied these remains in Buenos Aires between 1923 and 1926
and described them as Antarctosaurus wichmannianus.No
holotype was specified, so the elements he described may be
considered the type series, or syntypes (Huene 1929: 66–75).
As mentioned above, the attribution of all of these elements to
a single individual remains problematic, owing to the lack of
documented field associations and derived cranial features of
two disparate sauropod groups (see Salgado & Calvo 1992,
1997; Upchurch 1999). Only the posterior portion of the skull
will be discussed here.
The posterior portion of the skull of Antarctosaurus
wichmannianus (Huene 1929: pls 28–29) displays shared
derived similarities with Nemegtosaurus and other nemegto-
saurids. These include the presence of a pendant non-articular
process on the distal paroccipital process, which is based on
the figures of Huene (1929); the distal paroccipital processes
are apparently no longer present on the specimen (Powell
2003: 45). The presence of a postorbital with reduced pos-
terior process further supports affinities with Nemegtosaurus.
The basal tubera appear to be bevelled, as in Nemegtosaurus,
but the presence of a contact with the quadrate must be con-
firmed. The braincase can be referred to Nemegtosauridae,
but other cranial elements attributed to A. wichmannianus
require further study.
cf. ‘Antarctosaurus’ septentrionalis
An isolated braincase referred to ‘Antarctosaurus’ (ISI R162,
Chatterjee & Rudra 1996: fig. 11) shares the presence of
a quadrate–basal tubera contact with Nemegtosaurus.The
holotypic braincase of ‘Antarctosaurus’septentrionalis (GSI
K27/497) may have had a quadrate–basal tubera contact

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
313
(pers. obs.). These specimens are tentatively referred to the
Nemegtosauridae.
Phuwiangosaurus
A newly described braincase referred to Phuwiangosaurus
(Buffetaut et al. 2002: fig. 5) preserves several details that in-
dicate its close relationship to Nemegtosaurus. These include
the novel contact between the quadrate and basal tubera. A
quadrate from the same locality (Buffetaut et al. 2002: fig. 6)
resembles that of Nemegtosaurus, but no shared derived char-
acters could be identified. Similarities between the teeth of
Nemegtosaurus and Phuwiangosaurus cited by Buffetaut &
Suteethorn (1999), do not necessarily indicate close phylo-
genetic relationship because narrow crowns are present in
other sauropod taxa. Phuwiangosaurus has been interpreted
as a basal titanosaur, retaining hyposphene–hypantrum artic-
ulations between dorsal vertebrae and amphicoelous anterior
caudal vertebrae, among other features (Upchurch 1998).
The derived cranial features noted here, however, have not
been included in a phylogenetic analysis and their effect on
tree topology is not yet known.
Bonitasaura
Apestigu´
ıa (2004) recently described a partial titanosaur skel-
eton from Santonian deposits of the Bajo de La Carpa Forma-
tion in Argentina. The associated skeleton includes both cra-
nial and postcranial elements, the former of which include
the anterior portion of the right mandibular ramus, parietal
and frontal. The postcranial remains firmly establish the ti-
tanosaur affinities of Bonitasaura and the cranial remains
facilitate comparisons to Nemegtosaurus. Although the pari-
etal and frontal, which were not described, preserve no de-
rived features linking them to Nemegtosaurus, the lower jaw
does bear such resemblances. In particular, the lower jaw of
Bonitasaura bears a partially fused coronoid bone that rises
to a crest posterior to the tooth row (Apestigu´
ıa 2004: fig. 2C,
E–F). Although not preserved posteriorly, the partially fused
coronoid matches that of Nemegtosaurus (Fig. 15) and Quae-
sitosaurus, suggesting that Bonitasaura is a nemegtosaurid.
Apestigu´
ıa (2004: fig. 2D) interpreted the coronoid as the
bony support for a beak that functioned as a ‘guillotine crest’
that cut against a matching crest on the upper jaw, which was
not preserved in Bonitasaura.InNemegtosaurus, there is no
sharp crest in the post-dentigerous portion of the upper jaw
(Figs 3 & 4), so it is not clear what the purported beaked
structure would have cut against. The function, if any, of
the partially fused coronoids of nemegtosaurids remains un-
known.
Effect of ambiguous features
The nemegtosaurids Nemegtosaurus,Quaesitosaurus and
Rapetosaurus are the only titanosaurs for which nearly com-
plete cranial remains have been described. Of these, only
Rapetosaurus is known from associated postcranial remains.
Malawisaurus is known from a nearly complete skull and
skeleton that have not yet been fully described (Gomani
1998). Two nearly complete adult and several embryonic
titanosaur skulls, each associated with postcranial remains,
have been reported from the Upper Cretaceous of Patago-
nia (Calvo et al. 1997; Martinez 1998; Chiappe et al. 1998),
but also await description. Braincases have been attributed to
several other titanosaurs, but associations to postcranial re-
mains are not well established (Saltasaurus Powell 1992;
Antarctosaurus wichmannianus Huene 1929; Phuwiango-
saurus Buffetaut et al. 2002). Isolated braincases are known
from the Indian ‘Antarctosaurus septentrionalis’ (Huene &
Matley 1933; Chatterjee & Rudra 1996) and the Dongargaon
braincase (Berman & Jain 1982).
The nearly complete skeleton of Rapetosaurus provides
the only link between the isolated Mongolian skulls and
titanosaurs, which are known predominantly from post-
crania. Although numerous characters support the mono-
phyly of Rapetosaurus,Nemegtosaurus and Quaesitosaurus,
nearly all are ambiguous because few titanosaurs are known
from cranial remains. Thus the close relationship among
these three taxa must be treated with caution. As more
well-preserved titanosaur skulls are discovered, the distri-
bution of characters supporting nemegtosaurid monophyly
will likely broaden to diagnose more inclusive groups.
This tendency of character distributions to broaden so that
they diagnose progressively more inclusive groups has been
previously recognised (Grande & Bemis 1998: 19; Sues
1998: 241) and recently termed ‘obsolescence’ by Wilson &
Upchurch (2003). Obsolescence is particularly dangerous
when few characters support a given taxon, as was the
case with ‘Titanosaurus indicus’. Fortunately, the large num-
ber of nemegtosaurid synapomorphies makes it likely that
both higher- and lower-level characters have been identified.
Thus, whereas some nemegtosaurid features will obsolesce
to characterise all titanosaurs, others will probably remain
diagnostic at lower levels.
Implications for Late Cretaceous
sauropod diversity
The redescription of Nemegtosaurus and Quaesitosaurus
presented here supports the titanosaur affinities of these taxa
and identifies novel features supporting the monophyly of
that group and of Nemegtosauridae. As such, Nemegtosaurus
and Quaesitosaurus join the Asian titanosaurs Phuwian-
gosaurus,Opisthocoelicaudia,Borealosaurus,?Jiangshano-
saurus,?Mongolosaurus and ?Huabeisaurus, along with the
Asian titanosauriforms Tangvayosaurus (=‘Titanosaurus
falloti’), Mongolosaurus,Gobititan and Euhelopus (Table 2).
Thus, all Cretaceous Asian sauropods are titanosauriforms,
all Late Cretaceous Asian sauropods are titanosaurs and dip-
lodocoids have not yet been recorded in Asia. These results
imply a taxonomic composition for Cretaceous Asian sauro-
pods that contrasts with those implied by traditional studies
and some cladistic analyses of sauropods, which sugges-
ted a mixed Asian fauna consisting of either camarasaurids,
dicraeosaurids and titanosaurs (McIntosh 1990) or basal dip-
lodocoids, ‘euhelopodids’ and titanosaurs (Upchurch 1998;
Upchurch et al. 2002; Barrett et al. 2002), respectively.
This hypothesis has important implications for the sauro-
pod faunal transition from the Jurassic to the Cretaceous,
both in Asia and globally.
Origin of Asian Cretaceous sauropod fauna
Neosauropod lineages (e.g. Diplodocoidea, Macronaria) first
enter the body fossil record in Upper Jurassic horizons in
North America, Africa and Europe. Neosauropod sister-
group relationships imply a Middle Jurassic origin (Upchurch

314 J. A. Wilson
Table 2 Spatial and temporal distribution of Cretaceous Asian sauropod genera, listed in order of their stratigraphic appearance.
Taxon Formation Country Age Reference
‘Asiatosaurus kwanshiensis’ (n.d.) Napan China Cretaceous Hou et al. 1975
‘Asiatosaurus mongoliensis’ (n.d.) Oshih Mongolia Early Cretaceous Osborn 1924
Mongolosaurus haplodon (?T) On Gong Mongolia Early Cretaceous Gilmore 1933
Phuwiangosaurus sirindhornae (T) Sao Khua Thailand Early Cretaceous Martin et al. 1994
Unnamed (TSF) Kuwajima Japan ?Barriasian-?Hauterivian Barrett et al. 2002
Euhelopus zdanskyi (TSF) Mengyin China ?Neocomian Wiman 1929
Pukyongosaurus millenniumi (TSF) Hasandong South Korea Barremian Dong et al. 2001
‘Chiayusaurus asianensis’ (n.d.) Hasandong South Korea Barremian Lee et al. 1997, 2001
Chiayusaurus lacustris (?TSF) Xinminbao Group China Barremian-Aptian Bohlin 1953
Unnamed (TSF) Xinminbao Group China Barremian-Aptian Dong 1997
Unnamed (TSF) Jinju South Korea Barremian-Aptian Lim et al. 2001
‘Ultrasaurus tabriensis’ (n.d.) Gugyedong South Korea Aptian Kim 1983; Lee et al. 1997
Tangvayosaurus hoffeti (TSF) Gr`es Sup´erieurs Laos Aptian-Albian Allain et al. 1999
Jianshanosaurus lixianensis (?T) Jinhua China Aptian-Albian Tang et al. 2001b
Unnamed (TSF) Ilek Russia Aptian-Albian Averianov et al. 2002
Gobititan shenzhouensis (TSF) Xinminbao Group China Albian You et al. 2003
Unnamed Jiufotang China Albian Wang et al. 1998
Borealosaurus wimani (T) Sunjiawan China early Late Cretaceous You et al. 2004
Huabeisaurus allocotus (?T) Huiquanpu China Late Cretaceous Pang & Cheng 2000
Unnamed Dzarakuduk Uzbekistan Turonian Sues & Averianov 2004
‘Antarctosaurus’jaxarticus’ (n.d.) Dabrazinskaya Svita Kazakhstan Turonian-Santonian Ryabinin 1939
Quaesitosaurus orientalis (T) Barungoyot Mongolia Santonian-Campanian Kurzanov & Bannikov 1983
‘Nemegtosaurus pachi’ (n.d.) Subashi China ?Campanian-Maastrichtian Dong 1977
Nemegtosaurus mongoliensis (T) Nemegt Mongolia Maastrichtian Nowinski 1971
Opisthocoelicaudia skarzynskii (T) Nemegt Mongolia Maastrichtian Borsuk-Bialynicka 1977
(Early and Late Cretaceous genera separated by solid line).
Abbreviations in parentheses after species name indicate phylogenetic affinities: n.d., nomen dubium; T, Titanosauria; TSF, Titanosauriformes.
1995, 1998; Wilson & Sereno 1998), which is supported by
Middle Jurassic ‘wide-gauge’ sauropod trackways (Santos
et al.1994; Day et al. 2002) attributed to the neosauropod
lineage Titanosauria (Wilson & Carrano 1999). Definitive
neosauropods have not yet been reported from the Jurassic of
India, Australia, South America or Madagascar, all of which
have comparatively poor or non-existent Upper Jurassic re-
cords. Asia, by contrast, has well sampled Middle and Upper
Jurassic sediments but no definitive Jurassic neosauropods.
This apparent anomaly has been interpreted as geographical
isolation of Asia during Middle Jurassic until Cretaceous
times (Russell 1993; Upchurch 1995; Buffetaut & Suteethorn
1999; Luo 1999; Barrett et al. 2002; Upchurch et al. 2002;
Zhou et al. 2003). Although this hypothesis explains the ab-
sence of neosauropods from Asia, there are few examples
of endemic Asian dinosaur clades. ‘Euhelopodidae’, for ex-
ample, is often hailed as one such clade (Upchurch 1995,
1998; Barrett et al. 2002), but the monophyly of the group
has been shown to be suspect (Wilson 2002).
Sauropods first appear in Asia in the Upper Trias-
sic (Isanosaurus) and persist in greater numbers into the
Lower Jurassic (Lufeng sauropod, Kunmingosaurus,Gongxi-
anosaurus). The Middle Jurassic fauna of Asia includes
both basal eusauropods (Shunosaurus,Datousaurus,?Ab-
rosaurus) and slightly more derived forms (Bellusaurus,
Klamelisaurus). The Upper Jurassic fauna, in turn, in-
cludes a clade of long-necked omeisaurids (Omeisaurus,Ma-
menchisaurus) that are closely related to Neosauropoda. A
purported Camarasaurus tooth crown from the Upper Jur-
assic of Siberia (Kurzanov et al. 2003) and a brachiosaurid
manus from the Upper Jurassic of China (Dong et al. 2001)
cannot yet be confidently referred to Neosauropoda. Further-
more, although Upchurch et al. (2004) consider Abrosaurus
and Bellusaurus to be neosauropods, the character evidence
supporting this assessment is not yet conclusive.
The abrupt transition from a predominantly or exclus-
ively non-neosauropod Jurassic fauna to an exclusively ti-
tanosauriform Asian Cretaceous fauna (Table 2) has been in-
terpreted as the end of Asia’s geographical isolation (Barrett
et al. 2002; Upchurch et al. 2002). This interesting pattern
raises several questions about the origin of the Cretaceous
Asian titanosauriform fauna. Upchurch et al. (2002) provided
paleogeographical and paleobiogeographical evidence sup-
porting convergence between Europe and Asia during the
Aptian–Albian or perhaps earlier. Barrett et al. (2002) sug-
gested that this exchange may have taken place as early as the
Berriasian, based on the early appearance of titanosauriforms
in Japan (Table 2), but did not specify an origin. Results
from this analysis are consistent with Nemegtosaurus and
Quaesitosaurus forming part of an endemic group because
they are each other’s closest relatives. The recent claim that
Opisthocoelicaudia and Borealosaurus are sister-taxa may
suggest a second endemic Asian titanosaur subclade. The
recent discovery of an Euhelopus-like tooth in Lower Creta-
ceous (Neocomian) of Spain (Canudo et al. 2002) support
the Upchurch et al. (2002) hypothesis of an Early Cretaceous
communication between Europe and Asia. Further support
is provided by the presence of gobiconodontid mammals in
the Early Cretaceous of Spain (Cuenca-Besc´
os & Canudo
2003).

Redescription of
N
EMEGTOSAURUS MONGOLIENSIS
315
Titanosaur predominance in the Late Cretaceous
Zhou et al. (2003: 813) suggested that Asia may have served
as a ‘centre of diversification’ for many taxa, including ti-
tanosauriforms. Although the absence of neosauropods from
the Upper Jurassic of Asia is anomalous in the context of the
records in other landmasses, the appearance of titanosaurs
in the Lower Cretaceous seems to be a more general phe-
nomenon. Titanosaur body fossils make their first appearance
in Lower Cretaceous horizons of North America (Ostrom
1970; Britt et al. 1998), South America (Leanza et al. 2004),
Australia (Coombs & Molnar 1981), Africa (Jacobs et al.
1993) and Europe (Seeley 1869, 1876). In Madagascar and
India, which have poorly sampled Lower Cretaceous strata,
titanosaurs appear with the first sampled Cretaceous rocks,
which are Maastrichtian (Curry Rogers & Forster 2001) or
Turonian (Khosla et al. 2003) in age. These distributions sug-
gest that Titanosauria achieved a near-global distribution by
the Early Cretaceous. The Middle Jurassic first appearance of
‘wide-gauge’ trackways suggest a much earlier origin, which
places the spread of titanosaurs sometime in the Late Juras-
sic when substantial land connections remained. Given these
data, it is surprising that only one Late Jurassic titanosaur
body fossil is known (Janenschia: Janensch 1961).
Other neosauropod lineages survived into the Creta-
ceous, most notably rebbachisaurid diplodocoids, which oc-
cupied Europe (Dalla Vecchia 1998; Pereda-Suberbiola et al.
2003), Africa (Lavocat 1954; Sereno et al. 1999) and South
America (Calvo & Salgado 1995; Bonaparte 1996) until at
least the early Campanian, based on a rebbachisaurid inter-
pretation of the lower jaw of Antarctosaurus wichmannianus
(Leanza et al. 2004). Although many basal titanosauriforms,
individual basal diplodocoids (Amazonasaurus: Carvalho
et al. 2003), dicraeosaurids (Amargasaurus:Salgado&
Bonaparte 1991) and brachiosaurids (‘French’ Bothriospon-
dylus: Lapparent 1943) survived into the Early Cretaceous,
they did not appear to diversify. Titanosaurs and rebbachi-
saurids – both predominantly narrow-crowned clades – ap-
pear to be the only two sauropod clades that flourished dur-
ing the Cretaceous (Barrett & Upchurch 2005). Moreover, in
several cases they are preserved in the same formation, such
as the Candeleros and Lohan Cura Formations of Argen-
tina (Leanza etal. 2004), the Tiour´
aren Formation of Niger
(Sereno et al. 1999) and the ‘Bale’ locality in Croatia (Dalla
Vecchia 1998). By the latest Cretaceous, titanosaurs were the
predominant (if not exclusive) sauropods worldwide, repres-
ented on all continental landmasses except Antarctica, which
has not yet yielded sauropod fossils. If connections between
landmasses were severed by the Cenomanian (e.g., Smith
et al. 1994; Hay et al. 1999; Sereno et al. 2004), then the
survival and predominance of titanosaurs on each landmass
may have been independent. Confirmation of this pattern
will require further sampling of Cretaceous horizons, as well
as a detailed framework of titanosaur interrelationships to
evaluate their Late Cretaceous endemicity.
Acknowledgements
I am especially grateful to Z. Kielan-Jaworowska, H.
Osm´
olska and M. Borsuk-Bialynicka for assistance and ac-
cess to specimens in their care at the Polish Academy of
Sciences (PAN) and for permission to redescribe material in
their care. I thank W. Sicinski for assistance at the PAN and
for providing me with photographs of the unprepared skull of
Nemegtosaurus. I thank V. Alifanov for assistance and hospit-
ality at the Russian Academy of Sciences and Xiqaio Xu for
a translation from the Chinese of the relevant portion of Dong
(1977). I thank P. Barrett and P. Upchurch for critical reviews
of this paper; K. Curry Rogers, M. Lamanna, L. Salgado,
P. Sereno and G. P. Wilson also provided valuable comments.
This research was funded by grants from the Dinosaur Soci-
ety and the Scott Turner Fund.
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