Comparative anatomy of selected basal
Kyo Tanoue, Hai-Lu You, and Peter Dodson
Abstract: The dental structure of basal ceratopsians is described. Evolutionary trends in maxillary and dentary teeth of
basal ceratopsians include decrease and possible loss of enamel on the occluding side of tooth crowns, increase in the an-
gle of wear facet, development of a prominent primary ridge and deep indentations on mesial and distal sides of the pri-
mary ridge, and increase in tooth size in neoceratopsians. Premaxillary teeth in the basalmost ceratopsian Yinlong and
basal neoceratopsian Archaeoceratops oshimai exhibit wear facets and denticles along the carina, which imply use for
feeding. Maxillary and dentary teeth of basal ceratopsians were probably not as effective in feeding as those in ceratopsids
because of the relatively less prominent primary ridges. Some dental characters can be used to identify taxon and tooth po-
sition of isolated basal ceratopsian teeth.
Re´sume´ : La structure dentaire des ce´ratopsiens basaux est de´crite. Parmi les tendances e´volutives des dents maxillaires et
dentaires des ce´ratopsiens basaux figurent une diminution et une e´ventuelle disparition de l’e´mail sur la face occlusive des
couronnes des dents, une augmentation de l’angle des facettes d’usure, l’apparition d’une creˆte primaire e´minente et de
profondes indentations sur les coˆte´s me´sial et distal de la creˆte primaire, ainsi que l’augmentation de la taille des dents
chez les ne´oce´ratopsiens. Les dents pre´maxillaires de Yinlong, le plus basal des ce´ratopsiens, et du ne´oce´ratopsien basal
Archaeoceratops oshimai pre´sentent des facettes d’usure et des denticules le long de la care`ne, ce qui indique une fonction
alimentaire. Les dents maxillaires et dentaires des ce´ratopsiens basaux n’e´taient probablement pas aussi efficaces pour
l’alimentation que celles des ce´ratopside´s en raison de leurs creˆtes primaires relativement moins e´minentes. Certains carac-
te`res dentaires peuvent eˆtre utilise´s pour de´terminer le taxon et la position de dents isole´es de ce´ratopsiens basaux.
[Traduit par la Re´daction]
A unique feature of ceratopsids, the most derived ceratop-
sians, is the form of mastication. Derived ceratopsian jaws
contain large numbers of teeth, which are mesiodistally
compressed for close packing in dental batteries in which
files of teeth interlock both vertically and horizontally (Os-
trom 1964; Dodson 1996). In ceratopsids the enamel is con-
fined to the labial side of the maxillary tooth crown and
lingual side of the dentary tooth crown. The dental batteries
exhibit vertical cutting planes, in which the lingual sides of
maxillary teeth occlude against the labial sides of dentary
teeth. (Vertical shear of dental batteries is not found in any
other herbivorous vertebrate taxa.) In contrast, basal ceratop-
sians lack dental batteries; instead, the closely spaced teeth
merely erupt in a single horizontal line. Examination of the
tooth structure of basal ceratopsians should help to elucidate
the evolutionary transformation of dentition within the Cera-
topsia. However, compared with theropod dinosaurs, compa-
rative studies on ornithischian dentitions, including those of
ceratopsians, have rarely been done (Ostrom 1966; Weish-
ampel 1984; Coombs 1990). Dental structure in the basal
Ceratopsia has attracted little attention other than a few thor-
ough descriptions (Zhao et al. 1999; Makovicky and Norell
2006; Godefroit and Lambert 2007). Diagnostic characters
of neoceratopsian teeth from North America were identified
by Chinnery et al. (1998). Moreover, premaxillary teeth are
found only in basal ceratopsians, except for the Psittacosaur-
idae, but the function of these teeth has not been discussed
Recent discoveries of skulls of basal Ceratopsia include Ar-
chaeoceratops (Dong and Azuma 1997), Zuniceratops
(Wolfe and Kirkland 1998), Chaoyangsaurus (Zhao et al.
1999), Liaoceratops (Xu et al. 2002), Hongshanosaurus
(You et al. 2003), Magnirostris (You and Dong 2003),
Lamaceratops, Platyceratops (Alifanov 2003), Prenoceratops
(Chinnery 2004), Auroraceratops (You et al. 2005), Yinlong
(Xu et al. 2006), Xuanhuaceratops (Zhao et al. 2006),
Yamaceratops (Makovicky and Norell 2006), and Cerasinops
(Chinnery and Horner 2007), whose occurrences range from
the Upper Jurassic (Oxfordian; Xu et al. 2006) to the Upper
Cretaceous (Campanian; Chinnery and Horner 2007). Subse-
quent preparation of many of these specimens has revealed
new information of the teeth and jaws, allowing us to con-
duct a comparative study of the early evolution of the cera-
Received 3 December 2008. Accepted 20 June 2009. Published
on the NRC Research Press Web site at cjes.nrc.ca on 12 August
Paper handled by Associate Editor H.-D. Sues.
K. Tanoue.1 Canadian Museum of Nature, P.O. Box 3443, Stn
‘‘D’’, Ottawa, ON K1P 6P4, Canada.
H. You. Institute of Geology, Chinese Academy of Geological
Sciences, 26 Baiwanzhuang Road, Beijing 100037, China.
P. Dodson. School of Veterinary Medicine and Department of
Earth and Environmental Science, University of Pennsylvania,
3800 Spruce Street, Philadelphia, PA 19104-6045, USA.
1Corresponding author (e-mail: firstname.lastname@example.org).
Can. J. Earth Sci. 46: 425–439 (2009) doi:10.1139/E09-030 Published by NRC Research Press
AMNH, American Museum of Natural History, New
York, N.Y., USA; CAGS, IG, IGCAGS, Institute of Geol-
ogy, Chinese Academy of Geological Sciences, Beijing,
China; CMN, Canadian Museum of Nature, Ottawa, Ontario,
Canada; IVPP, Institute of Vertebrate Paleontology and Pa-
leoanthropology, Beijing, China; PKUP: Peking University
Paleontological Collections, Beijing, China.
Materials and methods
Dentitions of nine basal ceratopsian genera were exam-
ined and measured. Phylogenetic relationships among these
genera are shown in Fig. 1. In this study, Yinlong and
Chaoyangsaurus are considered the basalmost Ceratopsia,
which are here defined as outgroup to Psittacosauridae +
Neoceratopsia (Xu et al. 2006). Psittacosaurus and Hon-
gshanosaurus compose Psittacosauridae. Hongshanosaurus
is a newly discovered psittacosaurid that differs from Psitta-
cosaurus in having greater preorbital length relative to basal
skull length (approximately one half) than Psittacosaurus
(less than 40%; Sereno 2000) and elliptical external naris,
orbit, and infratemporal fenestra, whose long axes are ori-
ented posterodorsally (You et al. 2003; You and Xu 2005).
The mandible of Hongshanosaurus is deeper than the man-
dible in any species of Psittacosaurus (Tanoue et al. in
press). The adult specimen (IVPP V12617) examined in this
study is attributed to Hongshanosaurus since the preorbital
length is approximately half of the basal skull length and
the orbit and infratemporal fenestra are elliptical, with the
long axes sloping anteroventrally as in the holotype (IVPP
V12704). IVPP V12617 is unlikely to have undergone dis-
tortion, for the skull and mandible are symmetrical and the
delicate palatal structure is preserved intact (Dodson et al.
in press). In this study Liaoceratops, Archaeoceratops,
Auroraceratops, Leptoceratops, and Protoceratops represent
the basal Neoceratopsia. Dental terminology used is shown
in Fig. 2.
Yinlong (IVPP V14530) has three premaxillary teeth on
each side (Fig. 3A; Xu et al. 2006). The well-preserved left
first and right first and second premaxillary teeth are larger
than the maxillary teeth in labial view. The lengths of the
crown and root are greater than the width. The tooth crown
base widens for the first 2 to 3 mm apically in labial and
mesial views, then tapers toward the tip (Fig. 3B). Most of
the occlusal surface is flat, except at the base where it is lin-
gually concave in the first teeth on both sides (Fig. 3C). The
labial side is mesiodistally convex as in Chaoyangsaurus.
Xu et al. (2006) reported serrations only on the distal carina
of the right second tooth, but the mesial carina is also ser-
rated (Fig. 3B). The serrated region is longer and more
prominent on the distal carina than on the mesial carina.
The left maxillary tooth row of the type specimen of Yin-
long downsi is not fully exposed. Thirteen teeth make up the
labially convex right maxillary tooth row (Xu et al. 2006).
The diastema between the right third premaxillary and the
first maxillary teeth measures approximately 4 mm. The
right tooth row is 59 mm long. The maxillary teeth are
tightly packed with slight overlap of the crowns. The distal
end of each tooth is lateral to the mesial end of the subse-
quent tooth. The unworn crown of a left maxillary tooth is
ovate (Figs. 3D, 3E). The low primary ridge is wide at the
base and tapers apically toward the crown apex. The base
of the primary ridge is confluent with the low cingulum. On
Fig. 1. Phylogenetic tree of the Marginocephalia compiled from
You et al. (2003, 2005), Makovicky and Norell (2006), Xu et al.
(2006), and Chinnery and Horner (2007).
Fig. 2. Schematic diagram of a ceratopsian tooth with dental termi-
nology. (A) Dentary tooth in lingual view. (B) Dentary tooth in oc-
426 Can. J. Earth Sci. Vol. 46, 2009
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both mesial and distal sides of the primary ridge are shallow
depressions, bounded basally by the cingulum. Secondary
ridges develop within both depressions. The secondary
ridges on the mesial indentation are much longer and more
prominent than those on distal depression. The secondary
ridges extend toward the primary ridge near the occlusal
margin of the crown, but curve to be subparallel to the pri-
mary ridge dorsally, near the tooth base.
Dentary teeth are not exposed, except for the rostral part
of the right tooth row and the partial crown of an erupting
tooth in the left dentary (Figs. 3F, 3G). This partial crown
can be observed ventral to the middle of the dentary tooth
row on the medial side of the left dentary. It is 5.7 mm
wide and 6.0 mm high. The exposed portion is triangular,
and its dorsal margin bears denticles. The primary ridge is
poorly developed and only slightly wider than the secondary
ridges. Secondary ridges are short. Four and five secondary
ridges develop mesial and distal to the low primary ridge,
respectively, supporting denticles on the occlusal edges of
The dentition of Chaoyangsaurus is relatively well pre-
served in the holotype (Fig. 4A; IGCAGS V371). Two teeth
compose the premaxillary tooth row (Figs. 4A, 4B).
Although the ventral margin of the premaxilla just posterior
to the second premaxillary tooth is not preserved on either
side, it is unlikely that this specimen bore a third premaxil-
lary tooth, since it lacks any evidence of an alveolus. The
two peg-shaped premaxillary teeth are close to each other,
with little or no space between them (Zhao et al. 1999).
The root is cylindrical. The base of the premaxillary tooth
crown is slightly wider than the root. The crown tapers api-
cally. It is enameled on both the labial and lingual surfaces.
Along the occlusal margin, the carina extends in a mesiodis-
tal direction. The left second premaxillary tooth preserves a
small wear facet on the lingual side of the tip, which is
The left maxillary tooth row preserves seven distal teeth
(Figs. 4C, 4D). Mesial teeth are missing due to poor preser-
vation of the rostral portion of left maxilla. The preserved
tooth row measures 36 mm in length. Nine teeth compose
the 37 mm long right maxillary tooth row, from which the
second tooth is missing. The maxillary tooth rows are
slightly concave labially (Fig. 4C). There is a slight overlap
in the maxillary teeth, with the distal end of a tooth labially
overlapping the mesial end of the subsequent tooth in oc-
clusal view. The maxillary tooth crown is ovoid in labial
(Fig. 4D) and lingual views. The length of the maxillary
tooth crown is greater than the width. The height of the
crown is greater than the length in the mesial teeth, and
smaller in the distal teeth (Zhao et al. 1999). In occlusal
view, the labial surface is more convex than the lingual sur-
face. The occlusal edge of an unworn, erupting maxillary
tooth preserves about 10 denticles. A low vertical ridge ex-
tends at the midline on the labial surface (Fig. 4D). This
ridge probably represents a remnant of the primary ridge
(Zhao et al. 1999). However, this is uncertain due to the
preservation of the specimen. On both sides of this low
ridge are shallow depressions (Fig. 4D; Zhao et al. 1999:
Fig. 4B). A low cingulum separates the crown from the
Fig. 3. Yinlong downsi (IVPP V14530). (A) Skull and mandible in
right lateral view. (B) Left second premaxillary tooth in lingual
view. (C) Left first premaxillary tooth in mesial view. (D, E)
Photograph (D) and line drawing (E) of left maxillary tooth in la-
bial view. (F, G) Photograph (F) and line drawing (G) of left den-
tary tooth in lingual view. cin, cingulum; den, denticles; mxt,
maxillary teeth; pmxt, premaxillary teeth; pr, primary ridge; sr,
secondary ridge; wf, wear facet. Scale bars = 5 mm in B, C, D, and
Tanoue et al. 427
Published by NRC Research Press
cylindrical root, which is only slightly narrower than the
crown (Zhao et al. 1999). Enamel covers both sides of the
maxillary teeth. The enamel thickness appears to be about
the same on both the labial and the lingual sides (Zhao et
al. 1999). The wear facet is sloped.
The dentary tooth rows are concave labially (Fig. 4E).
The right tooth row, bearing 11 teeth, is 46 mm long. The
left tooth row, which is 39 mm long, preserves only nine
teeth. Lack of an alveolus distal to the last tooth indicates
that the distal teeth of the left tooth row are completely pre-
served. The length of the dentary teeth increases distally up
to the seventh tooth in the left tooth row and eighth tooth in
the right tooth row, and decrease to the last tooth. The
length is greater than the width, as on the maxillary teeth.
The tooth crown is round or oval in lingual view (Fig. 4F).
The occlusal margin bears several denticles. The denticles
are supported by short secondary ridges extending ventrally
toward the tooth base. The wear facet is concave apicola-
bially. Fully exposed dentary teeth including the root show
that these teeth are single-rooted.
The dentitions of five of the eight named species of Psit-
tacosaurus, P. lujiatunensis, P. major, P. mongoliensis, P.
neimongoliensis, and P. sinensis, were examined in this
study. None of the specimens studied possess premaxillary
teeth. The maxillary and dentary teeth are single-rooted.
There is only one replacement tooth in each tooth position,
as in other basal ceratopsians. Although the maxillary and
dentary teeth are closely spaced, there is a small gap be-
tween the basal portions of the tooth crowns in some speci-
mens. The crown length is greater than the width in all
The tooth rows of most species are nearly straight in oc-
clusal view, but in P. sinensis and P. lujiatunensis, they are
concave labially (Sereno and Chao 1988; Zhou et al. 2006).
The maxillary tooth crowns are aligned at a shallow angle to
the long axis of the tooth row. The distal end of each crown
labially overlaps the mesial end of the subsequent crown
(Sereno et al. 1988; Sereno 1990; Zhou et al. 2006; You et
al. 2008). Although no fully exposed unworn maxillary
tooth crown was observed in this study, the crown is oval
in labial view in P. meileyingensis and P. mongoliensis (Se-
reno et al. 1988). A low primary ridge separates the mesial
and distal lobes on the labial side of the crown. It tapers to-
ward the apex, except for P. lujiatunensis (PKUP V1053)
and P. sinensis (IVPP V738), in which the widths of the pri-
mary ridges remain constant (Figs. 5A, 5B). In P. neimongo-
liensis (Fig. 5C; IVPP 12-0888-2) and P. major (Fig. 5D;
CAGS-IG-VD-004), long deep grooves are present on both
sides of the primary ridge (You et al. 2008). The mesial
lobe is flat and broader than the distal lobe. The distal lobe
is swollen and can be more prominent than the primary
ridge. Several secondary ridges, separated from each other
by shallow longitudinal grooves, extend toward the tooth
base from the occlusal margins of the two lobes. They do
not reach the cingulum. The mesial lobe displays more sec-
ondary ridges than the distal lobe. Denticles are present at
the occlusal ends of the secondary ridges and can be ob-
served in relatively unworn crowns (Fig. 5B; Sereno and
Chao 1988; Sereno et al. 1988). In relatively well-preserved
maxillary teeth, enamel layers cover both labial and lingual
surfaces of the crown, with the thicker layer on the labial
side (Sereno and Chao 1988; Sereno 1990; Averianov et al.
Fig. 4. Chaoyangsaurus youngi (IGCAGS V371). (A) Skull in right
lateral view. (B) Left premaxillary teeth in labial view. (C) Left
maxillary tooth row in occlusal view. (D) Left maxillary tooth row
in labial view. (E) Mandible in dorsal view. (F) Left dentary teeth
in lingual view. dt, dentary teeth. See Fig. 3 for other abbreviations.
Scale bars = 5 mm in B, C, D, and F.
428 Can. J. Earth Sci. Vol. 46, 2009
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2006). The wear facet on the lingual side of the crown is
oblique rather than vertical.
The number of teeth in the dentary tooth row usually
equals the number in the maxillary tooth row. The size of
the dentary teeth is comparable to that of the maxillary
teeth. Unworn dentary tooth crowns are circular to oval in
outline in lingual view (Sereno and Chao 1988; Averianov
et al. 2006; You et al. 2008). The crown is symmetrical,
with the bulbous primary ridge in the middle of the lingual
surface (Fig. 5E). The primary ridge is more prominent on
the dentary teeth than on the maxillary teeth (Sereno 1990).
It narrows toward the apex, as in the maxillary tooth crown.
Mesial and distal lobes are of about the same size. Several
secondary ridges supporting the denticles along the occlusal
margin are present on both lobes, but they do not reach the
basal half of the lobe (Fig. 5E). These short secondary
ridges are visible on the labial surfaces of relatively unworn
dentary teeth (Fig. 5C). The largest number of denticles, 10
in each lobe, has been reported in P. xinjiangensis (Sereno
and Chao 1988). The enamel layer is thicker on the lingual
side than on the labial side of the tooth (Sereno and Chao
1988; Sereno 1990; Averianov et al. 2006). The wear facet
is sloped to occlude against that of maxillary tooth.
The adult specimen of Hongshanosaurus (IVPP V12617)
was examined. As in all other psittacosaurids, Hongshano-
saurus lacks premaxillary teeth (Fig. 6A). Juvenile specimen
was not included in this study.
The maxillary tooth rows are poorly preserved compared
with the dentary tooth rows in IVPP V12617. The maxillary
tooth row probably consists of eight or nine teeth. Five func-
tional teeth are preserved in both maxillary tooth rows
(Fig. 6A). Several replacement teeth are partially exposed,
with one for each tooth position. The length of the maxillary
tooth crown is greater than its width. The primary ridge is
low and widens toward the tooth base (Fig. 6B). The pri-
mary ridge is shifted distally, resulting in a wider mesial
lobe than distal lobe. Short vertical secondary ridges extend
from the occlusal margin of the crown. At least three secon-
dary ridges are present on the mesial lobe, and one secon-
dary ridge on the distal lobe. In unworn maxillary teeth, the
occlusal margin bears denticles supported by the secondary
ridges. The enamel covers the labial and lingual surfaces of
the crown, but it is thicker on the labial side (You and Xu
Both left and right dentary tooth rows bear 10 teeth and
are 38 mm and 41 mm long, respectively (Fig. 6C). They
extend along the medial side of the dentaries. The tooth
rows are slightly concave labially in occlusal view. In most
dentary teeth, the distal end of the tooth crown labially over-
laps the mesial end of the subsequent crown. Unworn den-
tary tooth crowns are round or oval in lingual view
(Fig. 6D). The bulbous primary ridge of a dentary tooth
crown is widest at the base and tapers dorsally. It is more
prominent than that of a maxillary tooth, as in Psittacosau-
rus (Fig. 6D; Sereno 1990). The primary ridge separates the
mesial and distal lobes of the crown. It generally extends
near the midline, but it is shifted distally in some teeth. As
many as eight mesial and six distal denticles are present. A
secondary ridge stretching toward the tooth base from the
Fig. 5. Psittacosaurus. (A) Right maxillary teeth of Psittacosaurus
lujiatunensis (PKUP V1053) in labial view. (B) Right maxillary
tooth row of P. sinensis (IVPP V738) in labial view. (C) Left max-
illary and dentary teeth of P. neimongoliensis (IVPP 12-0888-2) in
labial view. (D, E) P. major (CAGS-IG-VD-004). (D) Right maxil-
lary tooth row in labial view. (E) Right dentary tooth row in lingual
view. Scale bars = 5 mm. pr, primary ridge; sr, secondary ridge.
Tanoue et al. 429
Published by NRC Research Press
occlusal margin of the dentary tooth crown supports each
denticle. The secondary ridges develop only in the apical
third of the crown. Both labial and lingual surfaces of the
crown are enameled, with the enamel layer being thicker on
the lingual side (You and Xu 2005). The wear facet is flat
and slopes labially.
The dentition of Liaoceratops was examined in three
specimens: IVPP V12738, IVPP V12633, and CAGS-IG-
VD-002 (Xu et al. 2002; You et al. 2007: Figs. 1C, 2C).
The holotype (IVPP V12738) is an adult skull and the other
two are juvenile skulls, with CAGS-IG-VD-002 being the
smallest of the three specimens.
Two premaxillary teeth are preserved on both sides in the
holotype (Fig. 7A). Sockets are present rostrolateral to the
right first tooth and caudolateral to the second right tooth,
which may represent alveoli for additional premaxillary
teeth, but no alveoli are preserved for more than two teeth
in the left premaxillary tooth row. In both juvenile speci-
mens the premaxillary tooth rows each contain three teeth
(You et al. 2007). The premaxillary teeth are peg-shaped
with a cylindrical root (Fig. 7B). Both labial and lingual
sides of the premaxillary tooth crown are covered with en-
amel. The carina extends along the mesiodistal axis of the
crown. The first right premaxillary tooth exhibits the ser-
rated distal carina reported by Makovicky and Norell
(2006). Both the mesial and distal carinae of the left second
tooth display denticles, as in Yamaceratops and a new spe-
cies of Archaeoceratops (Fig. 7B; Makovicky and Norell
2006; You et al. in press). The serrated region is longer on
the distal carina than on the mesial carina. No wear facets
are present on any premaxillary tooth.
The left and right maxillary tooth rows of the holotype
(adult) skull consist of 11 and 12 teeth, respectively. All
maxillary tooth rows of juvenile specimens contain 10 teeth.
No unworn tooth is preserved in the holotype. Enamel cov-
ers the labial surface of the crown, but not the lingual sur-
face. The primary ridge is shifted distally (Fig. 7C) and is
confluent with the cingulum at the base. The primary ridges
are poorly differentiated from secondary ridges on the max-
illary teeth of the juvenile skulls. Two or three secondary
ridges are located mesial to the primary ridge and at least
one lies distally. In occlusal view, the maxillary teeth show
steep but not vertical wear facets.
At least 12 teeth are preserved in the left dentary tooth
row of the holotype (Fig. 7D). The right dentary tooth row
consists of 15 teeth. Only the lingual surface of the oval
crown is covered with enamel. The primary ridge is mesial
to the midline in lingual view (Fig. 7E). The primary ridges
in the dentary teeth of juvenile mandibles are only slightly
more prominent than the secondary ridges, as is also the
case in the maxillary teeth. As many as six and seven secon-
dary ridges develop mesial and distal to the primary ridge,
respectively. The wear facets are concave dorsolabially in
Fig. 6. Hongshanosaurus houi (IVPP V12617). (A) Skull in left
lateral view. (B) Left maxillary teeth in labial view. (C) Mandible
in dorsal view. (D) Left dentary teeth in lingual view. dt, dentary
teeth. See Fig. 3 for other abbreviations.
430 Can. J. Earth Sci. Vol. 46, 2009
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all three specimens. Some dentary teeth exhibit wear facets
with shelf structure.
Well-preserved teeth can be observed in the holotype of
Archaeoceratops oshimai (Fig. 8A; IVPP V11114). The den-
tition of a new species of Archaeoceratops will be described
by You et al. (in press). Three teeth compose the premaxil-
lary tooth row (Figs. 8A, 8B). All three right premaxillary
teeth are preserved, but only the second premaxillary tooth
remains on the left side. In occlusal view, the second tooth
is slightly labial to the first tooth and the third tooth is
slightly lingual to the first tooth. Both labial and lingual
sides of the crown are covered with enamel (You and Dod-
son 2003). The premaxillary teeth are peg-shaped. In labial
view, the premaxillary teeth first narrow ventrally from the
base, then widen, and the crowns then taper to a narrow
apex. The length is greater than the width. The carina ex-
tends mesiodistally along the occlusal edge. No denticles
were observed along the carina, unlike in the new species
of Archaeoceratops, Liaoceratops, and Yamaceratops (Ma-
kovicky and Norell 2006; You et al. in press). The right sec-
ond and third teeth exhibit ventrolabially sloping, oval wear
facets on the lingual side of the apex of the crown (Fig. 8B).
When the skull and mandible are articulated, the premaxil-
lary tooth rows are lateral to the posterior half of the dorsal
margin of the predentary (Fig. 8A). Thus, the lingual surfa-
ces of the premaxillary crowns will have contacted the outer
face of the lower beak.
The left maxillary tooth row comprises 13 teeth and is
46 mm long. Fourteen teeth compose the 47 mm long right
maxillary tooth row. In occlusal view, the tooth rows are
lenticular, widest a little distal to the middle of the tooth
rows and tapering both mesially and distally. Enamel covers
both surfaces of the crown. Unworn crowns are oval in la-
bial view. Unlike the new species of Archaeoceratops, in
which the maxillary teeth lack a primary ridge, maxillary
teeth of A. oshimai show a primary ridge (Fig. 8C; You et
al. in press). However, it is only slightly broader than the
secondary ridges. The primary ridge is slightly distal to the
midline of the crown, and merges basally with the cingulum.
In some teeth, the primary ridge curves ventrodistally. Three
secondary ridges are present on each side of the primary
ridge, forming denticles along the occlusal margin
(Fig. 8C). The secondary ridges stretch dorsally toward the
tooth base, and toward the primary ridge. Roots of some
teeth are partially exposed. They narrow slightly toward the
root apex. Replacement teeth for the left 10th and 12th, and
right eighth and 10th tooth positions are partially exposed on
the medial surface of the maxilla. Only one replacement
tooth is present for each tooth position. The wear facets of
maxillary teeth are steeply inclined.
The labially concave dentary tooth rows are aligned along
the medial borders of the dentaries in occlusal view. Four-
Fig. 7. Liaoceratops yanzigouensis (IVPP V12738). (A) Skull in
left lateral view. (B) Left premaxillary teeth in labial view. (C) Left
maxillary teeth in labial view. (D) Mandible in dorsal view.
(E) Right dentary teeth in lingual view. dt, dentary teeth. See Fig. 3
for other abbreviations. Scale bars = 5 mm in B, C, and E.
Tanoue et al. 431
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teen teeth compose the 55 mm long left tooth row. The right
tooth row comprises 14 teeth and is 53 mm long. Both the
labial and lingual sides of the dentary teeth are covered
with enamel, as is the case on the maxillary teeth. Each den-
tary preserves two or three small rostral teeth, which are iso-
lated from each other and from the closely packed tooth
rows (Fig. 8D). When the skull and mandible are articulated,
the first two dentary teeth are mesial to the first maxillary
tooth. The first maxillary tooth occludes with the third den-
tary tooth. In lingual view, the subtriangular crowns of the
first two teeth lack both primary and secondary ridges and
only show denticles along the apical margins. The third
tooth also displays a subtriangular crown outline, but there
are secondary ridges on the lingual surface. The fourth tooth
preserves a wear facet, but it lacks the primary ridge. The
primary ridges are preserved on teeth distal to the fifth tooth
on both dentaries. With the exception of the third tooth, the
teeth of the closely packed tooth row exhibit oval crowns.
The primary ridges in teeth 5–14 extend vertically in the
mesial third of the crown (Fig. 4E). However, the primary
ridge is only slightly more prominent than the secondary
ridges, which converge basally toward the primary ridge.
Three secondary ridges are located mesial and four distal to
the primary ridge. In the new species of Archaeoceratops,
the dentary teeth lack primary ridges (You et al. in press).
The cingulum is poorly developed in A. oshimai. Unworn
erupting teeth display more denticles than the number of
secondary ridges. Only one replacement tooth is present for
each tooth position. The wear facet is sloped as in maxillary
The teeth of Auroraceratops (Fig. 9A; IG-2004-VD-001)
are the largest among the known Chinese basal ceratopsians.
The premaxillary teeth are better preserved on the left side
(Fig. 9B). Three or four left premaxillary teeth are present.
There is a shallow depression mesial to the preserved teeth,
which possibly is an alveolus for the first tooth. The second
and third teeth are aligned along the lateral margin of the pre-
maxilla in occlusal view, with the first alveolus and fourth
tooth lingual to them. The length and width of the second
tooth are 7.2 mm and 5.2 mm, respectively. The length of
the third tooth is 7.6 mm, and the width is 4.5 mm. The
heights of the second and third teeth are approximately
7 mm and 8 mm, respectively, in labial view. The third peg-
shaped tooth is the best preserved in this specimen. The blunt
mesial and distal carinae extend lingually toward the tip,
unlike in Liaoceratops. As a result, the tip of the third pre-
maxillary tooth is situated on the lingual side of the crown in
occlusal view. Serrations are not present along the carina.
Only three premaxillary teeth are present in the right premax-
illa, represented by two alveoli and the root of the third pre-
maxillary tooth. The second alveolus is slightly labial to
the first alveolus and to the third tooth.
Fig. 8. Archaoeceratops oshimai (IVPP V11114). (A) Skull and
mandible in right lateral view. (B) Right premaxillary tooth row in
lingual view. (C) Left maxillary teeth in labial view. (D) Rostral
dentary teeth in lingual view. Numbers indicate tooth positions.
(E) Caudal dentary teeth in lingual view. dt, dentary teeth. See
Fig. 3 for other abbreviations. Scale bars = 5 mm in B–E.
432 Can. J. Earth Sci. Vol. 46, 2009
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The axes of the maxillary teeth slope ventrodistally
(Fig. 9C). Thirteen maxillary teeth form the 65 mm long
left tooth row. The right maxillary tooth row consists of 12
teeth. It is 65 mm long. Both the length and width of the
maxillary teeth increase distally, except for the distalmost
two or three teeth in both maxillary tooth rows. The ovate
crown of the left third tooth displays an unworn labial sur-
face. The primary ridge is situated distal to the midline
(Fig. 9C). It widens toward the tooth base and is basally
confluent with the cingulum. Mesial and distal to the pri-
mary ridge are shallow depressions. The concave surface of
the mesial lobe extends closer to the tooth base than does
the distal one. Three secondary ridges are located mesial
and two distal to the primary ridge. They stretch toward the
base from the occlusal margin of the crown but are only
about half the length of the primary ridge. Enamel covers
only the labial surfaces of the maxillary teeth, but this may
be due to preservation. Replacement teeth are not exposed in
lingual view. Although small spaces are found between adja-
cent roots, adjacent crowns are in contact and form a tightly
packed tooth row. The tooth height decreases mesially and
distally from tooth position 8 in lingual view. The wear
facet is nearly vertical.
The dentary teeth are poorly preserved. Twelve teeth form
the dentary tooth row on either side (Fig. 9D). The length
and the width of the teeth increase up to the seventh or
eighth tooth and then decrease distally in both tooth rows.
In labial view, the better preserved left tooth row is highest
at the middle, with the seventh and eighth teeth being the
highest. In dorsal view, the tooth rows are lingually convex
(Fig. 9D). No unworn teeth are present. The roots of the first
four teeth in the left tooth row are longer than they are wide.
The crown is asymmetrical in lingual view, with the primary
ridge developing mesial to the midline (Fig. 9E). The
primary ridge is confluent with the cingulum. Three secon-
dary ridges lie distal to the primary ridge in relatively well-
preserved teeth. The secondary ridges do not reach the
cingulum. Enamel is preserved only on the labial sides of
the dentary teeth. The presence of an enamel layer on the lin-
gual surface is uncertain because of poor preservation. The
wear facet appears to be vertical.
Two sockets are located mesial to the right dentary tooth
row (Fig. 9D). The diameters of these sockets are approxi-
mately 2 to 3 mm, with the interval between them of about
the same length. These may be alveoli for small mesial teeth
that were set off from each other and from the more distal
tooth row. They would have fit in the diastema between pre-
maxillary and maxillary teeth.
The description of the dentition of Leptoceratops is based
primarily on CMN 8889 (Fig. 10A). Isolated teeth of Lepto-
ceratops (CMN 8889, CMN 52781) are single-rooted and
among the largest in basal ceratopsians. Leptoceratops lacks
premaxillary teeth (Sternberg 1951).
Fig. 9. Auroraceratops rugosus (CAGS-2004-IG-VD-001).
(A) Skull in left lateral view. (B) Left premaxillary teeth in labial
view. (C) Left maxillary teeth in labial view. (D) Mandible in dorsal
view. (E) Left dentary teeth in lingual view. dt, dentary teeth; r, root;
s, sockets (possibly alveoli). See Fig. 3 for other abbreviations.
Tanoue et al. 433
Published by NRC Research Press
Both maxillary tooth rows of CMN 8889 consist of 17
teeth. The left tooth row is 157 mm long and the right tooth
row 160 mm. In occlusal view, the tooth rows are slightly
concave labially. The width of the tooth crown exceeds the
length (Sternberg 1951). The primary ridge is shifted distally
and is curved in some teeth (Fig. 10B). Some primary ridges
show striations parallel to the ridge axis. At least three sec-
ondary ridges are situated mesial to, and one distal to, the
primary ridge. The secondary ridges are subparallel to the
primary ridge, and some of them turn away from the pri-
mary ridge as they approach the tooth base. They are rela-
tively long in Leptoceratops, but do not reach the cingulum.
The cingulum is well developed. In some maxillary teeth,
the cingulum is notched in the middle (Fig. 10B). The base
of the primary ridge does not quite extend to the lingual
edge of the cingulum (Godefroit and Lambert 2007). In oc-
clusal view, the wear facet is steeply inclined in most teeth,
but vertical in a few teeth in the second quarter of the tooth
row. The left 14th and 16th and the right 14th maxillary
teeth exhibit narrow horizontal shelves at the base of the
subvertical wear facets.
Sixteen teeth compose both dentary tooth rows
(Fig. 10C). The tooth row is slightly concave labially in oc-
clusal view, as in the maxillary tooth row, but the tooth
rows of Leptoceratops are much less curved than those of
other basal neoceratopsians. The width of the dentary tooth
is greater than the length. The primary ridge is mesial to
the midline of the dentary tooth crown, as in other basal
neoceratopsians (Fig. 10D). It is more prominent than that
of a maxillary tooth and widens basally, just dorsal to the
junction with the cingulum. The primary ridge is confluent
with the cingulum, unlike the condition in maxillary teeth.
Up to four mesial and three distal secondary ridges are
present. The secondary ridges develop in a way that is
unique among the basal ceratopsians. Secondary ridges me-
sial to the primary ridge converge toward the primary ridge,
whereas those distal to the primary ridge are parallel to it.
These features of the junction between the primary ridge
and the cingulum and the secondary ridges are illustrated in
Brown (1914, fig. 2) as those of a maxillary tooth; however,
it appears to be a dentary tooth. Similarly, the dentary tooth
illustrated by Brown (1914, fig. 6) appears to be a maxillary
tooth. As on the maxillary teeth, wear facets are steeply in-
clined or vertical (Sternberg 1951). At the base of this wear
facet, there is a horizontal shelf, as is also seen in other
North American basal neoceratopsians and some Asian
forms, including Udanoceratops and Archaeoceratops
(Sternberg 1951; Kurzanov 1992; Chinnery and Weishampel
1998; Chinnery 2004; Chinnery and Horner 2007; You et al.
in press). It is difficult to confirm the distribution of enamel
in the teeth of CMN 8889 since the entire specimen is
stained black. However, isolated teeth of CMN 8888, whose
dentine portion is brown, show distinct enamel layers on
both labial and lingual surfaces of the crown.
Two peg-shaped teeth are present in each premaxilla in
Protoceratops (Figs. 11A, 11B; Gregory and Mook 1925;
Brown and Schlaikjer 1940). The second tooth is slightly la-
bial to the first tooth. There is little or no space in between
the two teeth (Fig. 11B). The long cylindrical root is nearly
round in horizontal section. Among the observed premaxil-
lary teeth, the right first tooth of AMNH 6433 is the largest
with a width of 7.6 mm.
The maxillary tooth row is labially concave mesially, but
is straight distally. There are up to 15 maxillary tooth posi-
tions (You and Dodson 2004). Each maxillary tooth crown
is oval in labial view. The primary ridge is prominent
(Fig. 11C). It is shifted distally from the midline. The pri-
mary ridge widens at the base and merges with the cingu-
lum. In some teeth, it is sinuous. The cingulum mesial to
Fig. 10. Leptoceratops gracilis (CMN 8889). (A) Skull in left lat-
eral view. (B) Left maxillary teeth in labial view. (C) Right man-
dibular ramus without predentary in dorsal view. (D) Left dentary
teeth in lingual view. dt, dentary teeth; n, notch. See Fig. 3 for
434 Can. J. Earth Sci. Vol. 46, 2009
Published by NRC Research Press
the primary ridge is often basal to the distal cingulum. The
indentations on both mesial and distal lobes of the crown are
deep. The secondary ridges on both lobes extend from the
occlusal margin toward the basal part of the primary ridge.
Isolated teeth are single-rooted and show longitudinal
grooves on mesial and distal surfaces of the root about one-
third the width of the root (Brown and Schlaikjer 1940).
These grooves would have accommodated the distal and me-
sial edges of preceding and subsequent teeth, respectively.
Enamel covers the labial surface of the crown and the apical
half of the lingual surface of the crown; the lingual enamel
band is only preserved on unworn teeth. The wear facet is
steeply inclined or vertical.
As in maxillary teeth, isolated dentary teeth are single-
rooted, with longitudinal grooves extending on mesial and
distal surfaces of the root. Unworn dentary tooth crowns are
oval in lingual view. The primary ridge of the crown is situ-
ated mesially (Fig. 11D). It is straight or slightly curved and
flares toward the base. The primary ridge merges with the
cingulum at the base. The bases of some primary ridges are
striated. The cingulum often extends distobasally. The inden-
tations mesial and distal to the primary ridge are shallower
than those of maxillary tooth crowns. At least four secondary
ridges are present both mesial and distal to the primary
ridge. These secondary ridges extend obliquely toward the
tooth base and toward the primary ridge. There is only one
replacement tooth in each tooth position. The lingual surfa-
ces, and the apical half of the labial surface of unworn tooth
crowns, are enameled. The wear facet is nearly vertical.
Premaxillary teeth of basalmost ceratopsians and some
basal neoceratopsians share some morphological features
with those of Pachycephalosauria, the sister-taxon of Cera-
topsia, including subconical crowns with oval cross-section
and enamel on both the labial and lingual sides of the crown
(Maryan´ska 1990). On the other hand, premaxillary tooth
crowns in pachycephalosaurians are set off from the roots,
whereas crowns of basal ceratopsian premaxillary teeth are
only slightly longer and wider than the root (Figs. 3B, 7B,
8B). Premaxillary tooth crowns of pachycephalosaurians are
also recurved apicodistally, but those of basal ceratopsians
are symmetrical in labial view.
Wear facets on the mesial surface of the apex of premax-
illary tooth crowns in Yinlong and Archaeoceratops oshimai
(Figs. 3C, 8B) indicate that they were used during feeding,
probably as they contacted against the lower beaks (F. Var-
riale, personal communication, 2008). Some premaxillary
teeth of Yinlong, Liaoceratops, Yamaceratops, and a new
species of Archaeoceratops (Figs. 3B, 7B; Makovicky and
Norell 2006; Xu et al. 2006; You et al. in press) show den-
ticles along the carina. In Yinlong and Liaoceratops, a longer
portion of the distal carina bears denticles than the mesial
carina (Figs. 3B, 7B), suggesting that the premaxillary teeth
were used to bite an object at an angle oblique to the vertical
axis of the tooth (D. D’Amore, personal communication,
2008), associated with the motion of pulling the lowered
head posterodorsally or the lifted head posteroventrally.
Maxillary and dentary dentition
Maxillary teeth of Yinlong resemble those of pachycepha-
losaurs in that the low primary ridge of maxillary tooth is
wide at the base (Figs. 3D, 3E). Crowns of unworn dentary
tooth of Yinlong and mesial dentary teeth of A. oshimai are
subtriangular as in pachycephalosaurians (Figs. 3F, 3G, 8D).
All other maxillary and dentary teeth of basal ceratopsians
examined, however, exhibit ovate crowns. Additionally, the
labial surface of pachycephalosaurian maxillary tooth
crowns is concave vertically (Sues and Galton 1987; Mar-
yan´ska et al. 2004), in contrast to the convex labial surface
of ceratopsian tooth crowns (Chinnery et al. 1998).
The evolutionary transition toward the ceratopsid dental
structure can also be observed in basal neoceratopsians. In
most basal ceratopsians, enamel covers both sides of maxil-
lary and dentary teeth. In Chaoyangsaurus, enamel layers on
both sides of maxillary and dentary teeth exhibit almost the
same thickness, but in other basal ceratopsians with tooth
crowns enameled on both sides, the enamel layer on the la-
bial side of the maxillary tooth crown and the lingual side of
the dentary tooth crown is thicker than that on the opposing
Fig. 11. Protoceratops andrewsi. (A) Skull and mandible in right
lateral view (AMNH6425). (B) Right premaxillary tooth row in lin-
gual view (AMNH 6433). (C) Right maxillary tooth row in labial
view (AMNH 6433). (D) Left dentary tooth row in lingual view
(AMNH 6460). dt, dentary teeth. See Fig. 3 for other abbreviations.
Scale bars = 5 mm in B–D.
Tanoue et al. 435
Published by NRC Research Press
side. Only the labial side of the maxillary and the lingual
side of the dentary teeth in Auroraceratops and Liaocera-
tops are enameled, but it may be due to poor preservation
of the known specimens. The thickness of enamel layers in
Leptoceratops is uncertain. In contrast, ceratopsid teeth de-
velop the enamel only on the non-occluding side. Having
the non-occluding side of the tooth crown covered with
thicker enamel layer to resist abrasion and preserve the
apex of the crown, while covering the occluding side with
less or no enamel would have facilitated effective shearing
In general, it is difficult to measure the inclination of
wear facets on maxillary and dentary teeth of specimens be-
cause of deformation during fossilization. However, among
basal ceratopsians, the wear facets for all teeth seem to be
steeper in Auroraceratops, Leptoceratops, and Protocera-
tops. It appears that an increase of the wear facet angle
took place in basal neoceratopsians, and it is possible that a
bite producing a vertical wear pattern typical of ceratopsids
may have evolved in basal Neoceratopsia.
Although the condition is unclear in Chaoyangsaurus, the
primary ridges of maxillary and dentary teeth in Yinlong and
most psittacosaurids are relatively wider than those of basal
neoceratopsians. This feature appears to have helped protect
the apex of the tooth crown from abrasion, especially on the
dentary teeth of psittacosaurids (Figs. 5E, 6D). However,
these primary ridges taper apically and many of the deeply
worn teeth with narrow or low primary ridges show nearly
horizontal apical margins (Fig. 5D). As for basal neocera-
topsians, in large individuals of Leptoceratops and Proto-
ceratops, the teeth show pointed apical edges, even when
worn, because of the presence of prominent primary ridges
(Figs. 11C, 11D). However, some maxillary teeth of Lepto-
ceratops, in which the primary ridge is the most prominent
of any seen among all the taxa studied, still can exhibit hor-
izontal apical margins (Fig. 10B). Overall, the primary
ridges of basal ceratopsians are less developed than those of
ceratopsids. The high primary ridges of ceratopsids remain
as high points along the cutting edge of each tooth as the
tooth is worn down. The mesial and distal lobes of each
tooth slope away from the high point supported by the pri-
mary ridge, to abut the distal or mesial lobe of the adjacent
tooth in the dental battery. The net effect is that the cutting
edge of the scissor-like occlusal surface is serrated; this ser-
ration might have been more effective at slicing through
vegetation than a horizontal occlusal edge would have been.
The maxillary teeth of basalmost ceratopsians and the
maxillary and dentary teeth of some basal neoceratopsians
show shallow indentations on mesial and distal sides of the
primary ridges (Figs. 3D, 4D, 7C, 8C). In contrast, the max-
illary and dentary teeth of derived basal neoceratopsians, in-
cluding Leptoceratops and Protoceratops, have deep
indentations on both sides of the primary ridges, as in cera-
topsids (Figs. 10B, 10D, 11C, 11D). Development of deep
indentations in neoceratopsians is associated with that of
prominent primary ridges. The function of deep indenta-
tions, however, is uncertain.
Shallow longitudinal grooves on the roots of maxillary
and dentary teeth in Protoceratops, which have also been re-
ported in Zuniceratops (Wolfe and Kirkland 1998), may be
precursor of ceratopsid bifid roots if they became deeper
and eventually split a root into two prongs. Replacement
tooth fit between the bifid roots of ceratopsids, allowing
each tooth position to accommodate more teeth and possibly
increasing the rate of tooth replacement compared with that
in basal ceratopsians (Ostrom 1966). However, dentition
with transitional morphology, such as tooth with deep longi-
tudinal grooves or single-rooted teeth with more than one re-
placement tooth at each position has not been discovered so
far (Ostrom 1966; You and Dodson 2004). Further examina-
tion of well-preserved teeth is required to understand the
evolution of double-rooted teeth in Ceratopsidae.
Tooth counts in maxillary and dentary tooth rows of adult
basal ceratopsians range from eight in Psittacosaurus xin-
jiangensis (Sereno and Chao 1988) to 17 in Leptoceratops
(Sternberg 1951). In ceratopsids, the number of functional
teeth forming tooth rows increased, and the maximum tooth
count is 40 in Triceratops (Ostrom 1966). Increase in tooth
size associated with that in absolute skull size also occurred
among Neoceratopsia. Derived basal neoceratopsians, in-
cluding Auroraceratops, Leptoceratops, and Protoceratops,
have larger maxillary and dentary teeth than more basal
forms, but the teeth are smaller than in ceratopsids. Increase
in the number of teeth and tooth size resulted in the progres-
sive elongation of tooth rows in neoceratopsians. Tooth rows
extended distally, even beyond the coronoid process in cera-
topsids (Ostrom 1964; Ostrom 1966).
Maxillary and dentary teeth of examined basal ceratop-
sians (except in Leptoceratops) clearly differ from those of
ceratopsids in that the crowns are longer than they are
wide; these proportions are the opposite in ceratopsids. Max-
illary and dentary teeth of basal ceratopsians occlude indi-
vidually with each other. Having a mesiodistally elongated
crown would have increased the surface area for contact,
presumably increasing the efficiency of mastication. In cera-
topsids, mesiodistally narrow crowns form a tightly packed
dental battery that functions as a single unit thus maximiz-
ing the area available to process food. There is no space be-
tween adjacent teeth in ceratopsids, whereas adjacent crown
bases and roots are not in contact with each other in basal
In some basal neoceratopsians observed, the first dentary
tooth is mesial to the first maxillary tooth when the skull
and mandible are articulated (Fig. 8A). This also seems to
be the case with Chaoyangsaurus. Archaeoceratops and pos-
sibly Auroraceratops possess dentary teeth mesial to and
apart from the packed tooth rows. These isolated mesial
teeth lack primary ridges and are structurally somewhat sim-
ilar to the premaxillary teeth with no wear facets (Fig. 8D).
When the mouth of the animal was closed, they fit in the
diastema between premaxillary and maxillary tooth row,
which is distal to the upper beak. Hence, they may have
been vestigial teeth.
Worn dentary teeth of North American basal neoceratop-
sians, as well as Udanoceratops and a new species of Ar-
chaeoceratops from Asia, exhibit horizontal shelves in
addition to vertical or subvertical wear facets (Sternberg
1951; Kurzanov 1992; Chinnery and Weishampel 1998;
Chinnery 2004; Chinnery and Horner 2007; You et al. in
press), which suggests that shearing and crushing functions
were combined in these taxa (Sternberg 1951; Ostrom
1966). However, the apex of the maxillary tooth bears no
436 Can. J. Earth Sci. Vol. 46, 2009
Published by NRC Research Press
horizontal wear facet to occlude with the horizontal shelf of
the dentary tooth. The horizontal shelf appears to be formed
by food-to-tooth occlusion (Varriale 2008).
Although isolated teeth are considered of little taxonomic
utility, dental characters, including the notch on cingulum of
maxillary teeth in Leptoceratops (Fig. 10B), V-shaped in-
dentations of the maxillary tooth crown in Bagaceratops
rather than U-shaped depressions in other basal ceratopsians
(Maryan´ska and Osmo´lska 1975), and a primary ridge on the
labial side of dentary tooth crown in Montanoceratops
(Chinnery and Weishampel 1998), can be utilized for identi-
fication. In addition, the various species of Psittacosaurus
differ in the relative sizes of the primary ridges in maxillary
and dentary tooth crowns (Sereno 1990). Some differences
have been observed in the maxillary and dentary teeth of
several other genera. In Hongshanosaurus, the primary
ridges of the dentary teeth are more prominent than those of
the maxillary teeth, as in Psittacosaurus (Figs. 6B, 6D). In
Leptoceratops, the orientation of the secondary ridge is dif-
ferent on the maxillary and dentary teeth. The secondary
ridges in maxillary teeth are subparallel to the primary ridge
(Fig. 10B). On the dentary teeth, however, secondary ridges
mesial to the primary ridge converge toward the primary
ridge, whereas those distal to the primary ridge extend paral-
lel to it (Fig. 10D). In Protoceratops, the indentations on
mesial and distal sides of the primary ridge are deeper on
the maxillary teeth than on the dentary teeth (Figs. 11C,
11D). These features can be used to distinguish isolated
maxillary and dentary teeth.
Some premaxillary teeth show wear facets and serrated
carina, which imply that they were utilized for feeding in
concert with the predentary beaks. Evolutionary trends in
maxillary and dentary teeth of basal ceratopsians include
(1) decrease and possible loss of enamel on the occluding
side of tooth crowns,
(2) increase in the angle of wear facets,
(3) development of a prominent primary ridge,
(4) development of deep indentations on the mesial and dis-
tal sides of the primary ridge, and
(5) increase in tooth size in neoceratopsians.
Overall, the dentitions of basal ceratopsians appear to be
less effective for cutting than those of ceratopsids because
of the shorter tooth rows and less developed primary ridges.
In ceratopsids, the prominent primary ridges contribute to
the serration of the dental battery when it is considered as a
single blade, retaining pointed apices on the individual
crowns. Basal ceratopsian dentition differs from ceratopsids
in that the teeth occluded individually in general unlike the
packed tooth row forming a dental battery in ceratopsids.
Horizontal shelves in dentary teeth, which imply crushing
function, are confined to some basal neoceratopsian genera.
Additionally, some dental characters can be utilized to iden-
tify isolated teeth of basal ceratopsians.
The authors are grateful to C. Mehling (AMNH), K.
Shepherd and M. Feuerstack (CMN), X. Xu (IVPP), and K.-
Q. Gao (Peking University, Beijing, China) for providing ac-
cess to their collections. B. Grandstaff (University of Penn-
sylvania, Philadelphia, Pa.) and R. Holmes (University of
Alberta, Edmonton, Alberta) kindly reviewed the early ver-
sion of the manuscript. Thanks are due to D. D’Amore
(Rutgers University, New Brunswick, N.J.) and F. Varriale
(Rowan University, Glassboro, N.J.) for valuable discus-
sions. The authors also appreciate Associate Editor H.-D.
Sues and reviewers A. Averianov, B. Chinnery-Allgeier,
and M. Ryan for helpful suggestions which greatly improved
the manuscript. K. Tanoue was funded by Summer Research
Stipends in Paleontology (University of Pennsylvania),
School of Arts and Sciences Dissertation Research Fellow-
ship (University of Pennsylvania), Jurassic Foundation Re-
search Grant, and Government of Canada Post-Doctoral
Research Fellowship. Funding was provided by the Basic
Outlay of Scientific Research Work and 973 Project from
Ministry of Science and Technology, the National Natural
Science Foundation of China (40672007), and Hundred Tal-
ents Project of Ministry of Land and Resources of China to
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